Atmospheres ALL

General circulation

Horizontal temperature differences  

• Pressure differences 

• PGF  

• Coriolis force modifies motion on larger spatial scales – perpendicular to the direction of motion  

• Geostrophic balance = between PGF and CF 

• Frictional force slows down motion  

Example of temperature driven horizontal movement

Land sea breeze

• Occur due to the unequal heating rates of land and water 

• During the day the land surface heats up faster than ocean water surface  

• This warms the air just above each surface differently  

• This causes the air above the land to rise 

• Rising air causes horizontal movement drawing cooler ocean air to replace the rising warm air  

• This is a sea breeze and it occurs during the day

At night

• The different relative heat capacities of the ocean and the land cause the reverse  

• Whereby the ocean surface is warmer than the land surface  

• And so air above the ocean becomes buoyant and rises  

• This causes the denser cool air over land to flow horizontal to replace the rising warmer air  

• This is a land breeze 

Vertical differences in heating

• cause unstable stratification in the atmosphere 

• Causes buoyancy forces – convergence and divergence 

Radiative convective equilibrium

• Radiation introduces instability as temperature falls with height so doesn’t the energy emitted at each layer of the atmosphere 

• No net horizontal transport of heat 

• Explains why warmer regions have a higher tropopause and why the tropopause expands under global warming 

Processes which heat the atmosphere 

• Latent heat release from convective clouds in tropics dominates heating in free troposphere and drives large scale mean circulation and teleconnections 

• Clouds affect climate  

• Through latent heat or interaction with radiation  

SVP 

• Saturation Vapour Pressure  

• If exceeded vapour condenses until SVP reached again  

• Depends on temperature  

Cloud formation mechanisms  

• Any process that causes a cooling in the atmosphere until supersaturation is reached  

• Radiative cooling or lifting/mixing of air masses 

Typical droplet sizes

• CCN – 0.0002 mm 

• Cloud droplet – 0.02 mm 

• Drizzle – 0.1 mm < d < 0.5 mm 

• Rain – 0.5 < d < 5mm 

Summary - general

• Clouds = crucial for the climate system 

• Influence weather and climate through latent heat effects and interaction with radiation  

• Precipitation determines the net heating of clouds through latent heat release 

• Precipitation formation = complex  

• Important information in the atmosphere and ocean propagated through waves  

Aerosol

liquid or solid particle suspended in the air  

• 0.01-100 micrometres 

• But size depends on type of aerosol and changes with time – why not all CNN and cloud droplets the same time  

• Aerosols are ubiquitous features of the atmosphere – sulphur dioxide, sea salt, desert dust 

• They have many health impacts but we care about them for their cloud and climate impacts 

Aerosol observations

• In situ – collect and analyse – sparse but accurate  

• Remote sensing – radiation measurements, ground based, satellite based  

• Infer radiative properties of aerosol particles  

• Assumptions needed for retrieval algorithm  

• Less accurate but global coverage possible

• Complex shapes and composition in contrast to assumptions of sphericality, and perfect mixture  

• This influences how the particles interact and CCN formation   

Aerosol characteristics and types

• Population highly variable in space, time, physical and chamical properties  

• Characterised by size distribution, composition, shape and optical properties  

• Primary – directly emitted – sea salt 

• Secondary – formed by gas-to-particle conversion  

• Sulphate aerosols produced through oxidation of SO2, H2S, DMS, COS and VOCs 

Aerosol sources

• Natural and anthropogenic  

• The estimates of global aerosol emissions vary hugely  

• Primary are less of a concern as they are bigger = lower residence time  

• Secondary smaller and so longer  

Aerosol sources Boucher, 2015

Marine Aerosols (Sea spray aerosols) 

• Wind friction = salt water into the atmosphere – some water evaporates = more salty  

• Therefore, sea salt particles are differentially hydrated based on local humidity  

• Size = 100 nanometers – tens of micrometers

Desert dust 

• Wind friction = soil suspended in atmosphere  

• Especially the case in desert or arid regions where there is little slowing down the wind, and where there is little moisture and so little cohesion between particles  

• Size = 100 nanometers – tens of micrometers  

• Emission dependent on local environmental and meteorological conditions

Volcanic aerosols

• Emission of ash during eruption  

• Size = micrometer – millimeter  

• Can be transported huge distances (1000s km)  

• But fall out of the atmosphere rapidly  

• Limited effect on climate  

Sulphur rich gasses (SO2 and H2S)

• Oxidised in the atmosphere to form submicronic sulphate aerosols  

• If in the troposphere = max residence time = weeks  

• If in the stratosphere = few months – more than a year (dep. Alt of injection and region) 

Spatial and temporal aerosol distributions

Boucher, 2015

• Once aerosols are in the atmosphere, they are transported but also removed 

• Removal pathways = atmospheric sinks 

• Globally, and over a long enough period sources and sinks should be in equilibrium 

• Sinks = dry deposition at the surface and wet deposition from precipitation  

• Aerosol properties also evolve during atmospheric transport  

• Aerosol concentrations and properties also vary in the vertical 

• Concentrations larger in the atmospheric boundary than the free troposphere  

• But can be lifted into the troposphere  

• And also, into the stratosphere (where sinks are less effective, and residence times are therefore longer) 

• Aerosol-Cloud-Radiation Interactions 

Aerosol size distribution

NOAA, n.d.

• Radius or diameter characterize size of one particles, but the particles may have complex shapes + radii vary by orders of magnitude => NOT one size but size distribution covering full spectrum of radius. 

• Aerosol distribution characterized by 3 modes: 

• fine mode (d < 2.5 m) and coarse mode (d > 2.5 m);  

• fine mode is divided on the nuclei mode (about 0.005 m < d < 0.1 m) accumulation mode (0.1m < d < 2.5 m). 

Modes

• Nucleation (Aitken) 

• Accumulation 

• Coarse 

(Alfarra, 2004)

Coarse

Alfarra, 2004

• mainly produced by mechanical processes and are introduced directly into the atmosphere from both natural and anthropogenic sources. 

• most significant source is the bursting of bubbles in the ocean, which creates coarse particles of sea salt 

Accumulation

Alfarra, 2004

• representing a region of particle growth mainly due to the coagulation of particles with diameters smaller than 0.1 µm and from condensation of vapours onto existing particles, causing them to grow into this size range 

• particle removal mechanisms are least efficient in this regime, causing particles to accumulate there until they are ultimately lost through rain or other forms of precipitation (wet deposition) 

Aitken mode

Alfarra, 2004

• Act as nuclei for condensation  

• Short lifetime  

• Account for the most particles by number  

• But due to their small size they account for a few percent of the total mass 

Processes which change the size/number

• Nucleation, condensation, coagulation, deposition  

• Determine the life/residence time  

Nucleation

• Formation of new particles from the gas phase without influence of existing particles  

• Homogenous nucleation of sulfate aerosols 

• Second organic carbon (SOC) aerosols believed to require the presence of sulfuric acid 

• Have to overcome nucleation barrier to push gad to particle phase 

• Rate is a function of temperature, WV and sulfuric acid – the colder, more EV and more sulfuric acid the better (Lohmann et al., 2016) 

Condensation

• Growth by uptake of gaseous species that condense on pre-existing particles  

• Preferential condensation > nucleation (due to nucleation barrier) 

Nucleation vs condensation

Friedlander, 2000

Types of nucleation  

• Homogenous – no foreign nuclei or surfaces 

• Heterogenous – nucleation on a foreign substance – like an aerosol particle

Condensation

• Growth of aerosol particles by uptake of vapours from the gas phase --> the most important mechanism of particle growth in the atmosphere 

Nucleation vs condensation

• Homogeneous nucleation = a new liquid / solid cluster is formed from vapor molecules in the absence of a surface  

• Heterogeneous nucleation = new liquid/solid cluster is formed on a preexisting surface  

• Condensation = vapor molecules go from the gas phase to existing liquid phase 

• In condensation there already exists a liquid phase of the condensing compound.  

• In heterogeneous nucleation there exists a surface, but it can be a cluster composed of some other compound or some planar surface (e.g. wall). In addition, heterogeneous nucleation also can occur between liquid and solid phases (e.g. ice nucleation). 

• Before condensation can occur, homogeneous/heterogeneous nucleation need to have happened. 

Coagulation, collision and coalescence

• Collisions in the atmosphere cause by Brownian motion, turbulent shear, differential fall velocity, electrostatic charges  

• Collision efficiency depends on size – more efficient bigger until air streams of other particles get involved  (Seinfeld and Pandis, 1998) 

Friedlander, 2000

• Collisions + coalescence  

• Agglomerations = collision and sticking (no coalescence) 

• In the atmosphere, coagulation of sub-micron particles is governed by the Brownian motion of particles (Brownian coagulation). 

• For super-micron particles and cloud droplets gravitation, turbulence and wind shear also influence the coagulation rate. 

Removal processes

Dry deposition  

• Removal in the absence of precipitation – function of size and density  

• Turbulent motion contributes to vertical transport – hit another particle and stick to it  

• 0.1 micrometre < r < 1 micrometer least efficient  

Wet deposition

• In cloud scavenging – incorporation of aerosols in cloud droplets  

• Below cloud – by precipitation  

• Snow is super-efficient 

Role of aerosols in cloud formation

• Clouds – small water droplets/ice crystals suspended in air  

• Clouds are NOT WV 

• To understand microphysics, you need to understand the movement of the cloud particles  

Microphysics

• Processes on micrometre scales – phase transfers between vapour, liquid and ice  

• Nucleation, condensation, coagulation, auto conversion, accretion, riming and freezing 

• These determine the size distribution, density and shape – which determine cloud properties  

• Herzog et al., 1998 

CDN - cloud droplet number

(Pringle et al., 2009) 

• For a given aerosol number concentration produces a wide range of cloud droplet concentrations due to variation in the shape of the aerosol size distribution  

Prediction of CDN 

• Number of cloud droplets in a rising air parcel is dependent on the number, size and chemical composition of the aerosol particles and the meteorological conditions (e.g. updraft velocity) 

• Calculation simplified in climate models 

• Empirical relations derived from regional measurements and extrapolated to the globe and for past and future  

• By bypass detailed microphysical processes which control CDN  

Cloud formation

• Air saturated at SVP – no cloud formation – stable balance between condensation and evaporation  

• Super-saturated – existing water droplets grow by condensation and new water droplets may form  

• Super-saturation necessary but not sufficient – need CCN  

• SVP different for water and ice  

• Need something to produce supersaturation – radiative processes, lifting air masses, mixing two subsaturated

Cloud physics - Houze, 2014

• Cloud physics = cloud microphysics and cloud dynamics  

Warm clouds  

• Liquid cloud droplets begin as CCN  

• And grow to become precipitation particles  

Nucleation - H, 2014

If the CCN happens to be composed of a material that is soluble in water, the efficacy of the nucleation process is further enhanced.

Since the saturation vapor pressure over the liquid solution is generally lower than that over a surface of pure water 

Condensation, H 2014

Rate of condensation determined by diffusional ratio between WV and latent heat

Fallspeeds, H14

• Cloud droplets are subject to downward gravitational force which leads to their fallout  

• As a particle accelerated down its motion is increasingly resisted by frictional force  

• Final speed = terminal fall velocity 

• Both cloud droplets and precipitation undergo sedimentation in this way  

• Larger rain drops become flattened into the shape of a disc as they fall – velocity increases nonlinearly 

Continous collection, H14

• Cloud drop growth by coalescence 

• Continuous collection – the mass of a falling particle increases continually as it falls thorough a cloud and coalesces with all particles in its cross-sectional area swept out – which is continually increasing 

• Early on stochastic collection leads to a broadening of the size distribution  

• Larger drops grow faster due to more frequnt collisions 

• Runaway growth process  

• Self reinforcing system 

• This is modified as larger particles break up due to aerodynamical instability and become a number of smaller drops 

Cold clouds H14

Homogenous nucleation of ice particles  

• In theory ice particle can be homogenously nucleated from the vapour phase but requires very low temperatures –65 and high supersaturations ~1000% 

• This doesn’t occur  

• Homogenous from the liquid phase could occur at temps –35 to –40 add to from lecture 

Heterogenous nucleation  

• Observations of ice crystals between 0 and –38 

• Homogenous doesn’t happen here so there must be a heterogenous process 

• The principal difficulty with the heterogeneous nucleation of the ice is that the molecules of the solid phase are arranged in a highly ordered crystal lattice 

• To allow the formation of an interfacial surface between the ice embryo and the foreign substance, the latter should have a lattice structure similar to that of ice 

Ice nucleation complexity H14

• An ice nucleus contained within a supercooled drop may initiate heterogeneous freezing when the temperature of the drop is lowered to the value at which the nucleus can be activated 

• If the CCN on which the drop forms is the ice nucleus, the process is called condensation nucleation 

• If the nucleation is caused by any other nucleus suspended in supercooled water, the process is referred to as immersion freezing 

• Drops may also be frozen if an ice nucleus in the air comes into contact with the drop; this process is called contact nucleation 

• the ice may be formed on a nucleus directly from the vapor phase, in which case the process is called deposition nucleation 

• The probability of ice-particle nucleation increases with decreasing temperature, and substances possessing a crystal lattice structure similar to ice provide best nucleating surface 

Aggregation and riming, H14

• If ice particles collect other ice particles, the process is called aggregation. 

• Temperature dependent 0 probability of adhesion is likely > -5 – ice crystal surfaces become sticky 

• If ice particles collect liquid drops, which freeze on contact, the process is called riming 

Fog

(Gultepe et al., 2007)

• Fog refers to a collection of suspended water droplets of ice crystals near the Earths surface that lead to a reduction in visibility below 1 km (NOAA, 1966) 

• There are many classifications – by formation  

• Radiation fog – forms over land due to nocturnal cooling  

• Advection fog – warm moist air over cold surfaces 

• And others (frontal, upslope) 

• Microphysics and nucelation processes 

• Formation occurs in aerosol laden surface air under high relative humidity conditions  

• Composed of haze (unactivated) particles 

• Fog droplets are generally smaller than cloud droplets due to the lack of updraft which means supersaturation remains low  

• Most important factors for fog formation (Duynkerke, 1991) 

• Cooling of moist air by radiative flux divergence  

• Mixing of heat and moisture  

• The presence of clouds increases the incoming longwave radiation at ground level and reduces longwave radiative cooling at the surface 

• Radiation fog occurs in sprint in coastal plains under the influence of advection inland of moist marine air during the previous afternoon  

• Advection fog tends to occur in the spring and summer months – when the occurrence of warm air flowing over the cold ocean is maximised  

Cloud formation

• Buoyance, convergence, topography, frontal lifting 

• Mixing  

• Due to convex shape of SVP line two subsaturated can mix to be supersaturated – e.g. at edge of frontal systems 

• Processes aren’t distinct 

• Mixing enhanced vertical motion and emission of latent heat adds energy and makes unstable – generated convection heating  

• The development of a particular cloud type depends on vertical temperature and humidity profiles  

Orographic uplift and foehn effect 

• Moist adiabatic rise and precipitation on the windward side

• dry adiabatic decent on the leeward side

• leeward size = warmer than windward side = foehn effect

Other formation mechanisms

• Gravity waves 

• Can form or change shape  

• Turbulent mixing  

• Boundary layer clouds  

• Stevens, 2005 

• Marine stratocumulus 

• Development  

• Dynamically evolving  

• Change forms  

Cloud droplet nucleation  

• Energy barrier must be overcome for phase change  

• Homogenous – droplet formed solely from WV 

• Heterogenous – CCN facilitates condensation 

Kelvin effect  

• If a droplet is in equilibrium with its environment at any time condensation and evaporation at the surface is equal  

• The ease of this dynamic equilibrium is dependent on the surface tension of the droplet which depends on size or curvature  

• SVP larger over curved surface 

• Curvature reduced the number of nearest neighbours (coordination number) which makes it easier for molecules to evaporate  

• Higher vapour pressure for smaller droplets – important for nucleation of new droplets and lifetime of small droplets  

• SVP over curved > over flat  

• Expresses as a ratio  

• If they grow past 0.12 micrometres then the ratio is essentially 1 and the Kelvin effect is less important (link to Koehler) 

Homogenous nucleation (liquid)

Homogenous nucleation requires RH>400% when observed supersaturation is typically smaller than 1% 

Raoult

Raoult’s law opposes the kelvin effect  

• The solution effect 

• Reduces SVP  

• Solution effect  

• If solute added, then liquid molecules at the surface are replaced by solute molecules and the SVP is reduced – easier for WV molecules to transfer from gas to liquid 

• For droplet 

• Smaller droplets have lower SVP for constant solute content  

• Rogers and Yau, 1989  

Heterogenous nucleation (liquid)

• Droplet larger than critical radius is said to be activated and continued to grow by condensation at successively lower supersaturation  

• Condensation on particles/aerosol surface = acting as CCN now  

• Activation depends on critical radius  

Koehler curve 

• Bringing together Kelvin and Raoult’s law  

• Supersaturation of droplet with given solute content as a function of radius  

• Increase in SVP due to dominance of Kelvin at small scales and then decrease as Raoult comes into play  

• The hump of the curve is the critical supersaturation which the droplet much reach to grow into a cloud droplet 

• If this critical environmental humidity was never reached, then the particle would never become activated into a cloud droplet 

• This critical diameter and supersaturation depend on the amount of solute  

• The more into supersaturation the environment, the greater range of particle sizes activated  

CCN

Aitken or nucleation mode  

• D<0.1 micrometer  

Accumulation mode 

• 01<d<1 

• Most important for cloud formation 

Coarse mode

• D<1 micrometer

Fog and cloud droplets

• D> 10 micrometres 

• Typical CN  

• Radii > 0.1 micrometres 

Supersaturation necessary but not sufficient – need CCN (aerosol particles – most CCN are soluble accumulation mode particles) 

Atmospheric moist convection

Stevens, 2005

• Unlike dry convection it involves phase changes of water leading to complex interactions with radiation, gravity waves and microphysical processes like precipitation 

• Two fluid problem – one fluid (unsaturated air) and transform itself into another (saturated air) simply through vertical displacement 

Stratocumulus convection  

• Low lying stratiform clouds driven by radiative cooling at the cloud top  

• Turbulent mixing in the boundary layer driven by surface heating, wind shear and radiative cooling  

• Lift moist air parcels upward, cooling them adiabatically until they reach saturation  

Cumulus cloud

Ahrens, 2006

• Warm air rises 

• And condenses forming a cloud  

• There will be downward movement down the sides of the cloud  

• Due to cool air descending to replace the warm air  

• And evaporation around the outer edge which cools the air and makes it heavier  

• Subsiding air inhibits the formation of clouds and so cumulus clouds have space between them  

• Shading of the surface also cuts of surface warming and therefore continued development of the cloud  

• Causing it to dissipate (as the water droplets evaporate) and the cycle of warming to start again 

Key idea of aerosl effect

Net cooling

Radiative forcing  

• Change in radiative flux (W/m2) at tropopause or TOA due to change in forcing agent  

• Climate systems not allowed to adjust to forcing  

• Net radiative flu = incoming – outgoing  

• Radiative effect 

• The net effect of a specified change in the system (cooling or warming without quantification) 

Aerosol radiative effects 

• Aerosol-radiation interaction  

• Aerosol-cloud interaction  

Effective radiative forcing  

If radiative forcing calculated at the tropopause, stratosphere allowed to adjust to changes --> thus radiative forcing in a theoretical concept not directly observable  

Climate effects of aerosols  

Direct – scatter and absorb --> cool 

Indirect

• Aerosols increase number concentration of cloud droplets and ice particles --> cool 

• Aerosols decrease precipitation efficiency --> cool  

• There are other indirect and semi-indirect effects which both warm and cool  

Effective radiative forcing

• Effective radiative forcing quantifies the energy gained or lost by the earth system following a perturbation  

• Fundamental driver of changes in earths TOA energy budget  

Bellouin et al., 2020

• Very likely that ERF is negative 

• Aerosol radiation interactions = –0.22 [–0.47 to 0.04] 

• Aerosol cloud interactions = –0.84 [–1.45 to –0.25] 

• Anthropogenic aerosol particles primarily affect water clouds by serving as additional cloud condensation nuclei (CCN) and thus increasing cloud drop number concentration (Twomey, 1959). 

Adjustment processes

For aerosol-radiation interactions = semi-direct effects (Johnson et al., 2004) 

• Semi-direct effect of absorbing aerosols on marine stratocumulus clouds – how their vertical distribution influences cloud properties and radiative forcing  

• used a large-eddy model, isolating the semi-direct effect by excluding microphysical (indirect) aerosol impacts 

Found

• Within the boundary layer 

• Heat the cloud layer, reducing low-cloud cover and liquid-water path (LWP) by ~10 g m⁻². 

• Cause a warming effect  

• Even mildly absorbing produce a semi-direct forcing 3x stronger and in the opposite sign than the direct aerosol forcing  

Above the boundary layer

• Increase LWP by 5–10 g m⁻², leading to a negative radiative forcing (cooling effect). 

• Strengthen the temperature inversion, reducing cloud-top entrainment and thickening the cloud layer. 

W/in and above the BL

• Result in a positive but weaker semi-direct forcing (half that of BL-only aerosols). 

• Since marine stratocumulus covers ~20% of the globe, the semi-direct effect of absorbing aerosols could significantly impact global radiative forcing. 

• Boundary layer = lowest part of the atmosphere – 1-2 km altitude  

• In the context of marine stratocumulus clouds (low sheet like couds over the ocean) 

• They form and persist within the BL 

• Aerosols within the BL are within the cloud layer  

• Aerosols above it cool the air above the cloud – strengthening the temperature inversion  

Lifetime effect

Rosenfeld, 2006

• The study challenges the traditional focus on the Twomey effect (aerosols increasing cloud albedo by making droplets smaller) and emphasizes the lifetime effect—where aerosols suppress precipitation, prolonging cloud lifetime and increasing cloud cover. This effect dominates aerosol-induced radiative forcing, particularly for marine stratocumulus and deep convective clouds. 

• Lifetime effect mechanisms  

• In shallow clouds – marine stratocumulus  

• Aerosols supress precipitation - More aerosols → smaller droplets → slower coalescence into raindrops → less cloud water loss 

• Cloud cover increases as a result  

• Deep convective clouds 

• Polluted clouds  

• Delayed precipitation – allows more water/ice to reach upper levels releasing latent heat higher up  

• Invigorating updrafts and generating taller clouds  

• Finds  

• Lifetime effect dwarfs albedo effect  

• The Twomey (albedo) effect contributes <20% of the total forcing; the rest is due to suppressed precipitation and increased cloud cover. 

Twomey effect (microphysics driven – droplet optics) 

• Aerosols act as cloud condensation nuclei (CCN), increasing the number of cloud droplets for the same amount of liquid water. 

• This makes droplets smaller and more numerous, enhancing cloud reflectivity (albedo). 

• Shortwave cooling: More sunlight is reflected back to space, reducing surface warming. 

• Confined to shallow clouds: Deep clouds already reflect most sunlight, so the effect is minimal. 

But lifetime effect (dynamics driven – precipitation and convection) 

• Smaller droplets (from aerosols) suppress precipitation because coalescence into raindrops slows. 

• Cloud water is not depleted by rain, prolonging cloud lifetime and increasing coverage/thickness. 

• Longwave and shortwave effects: Persisting clouds trap more thermal radiation (warming) while reflecting more sunlight (cooling). Net forcing depends on cloud type. 

Too cool to be true?

• When thinking about atmospheric forcing in general (applies to ghgs) assumption – forcing is related to GMST independently of what is causing the forcing – is that true for aerosols? No lol 

• Equivalence of surface and top of atmosphere 

• Purely scattering = if you know TOA you know BOA  

• Absorbing aerosols 

• You have less SW out  

• Through warming = more LW emissions  

• GH effect  

• Overall, with a net positive forcing – more radiation in than out  

• But not indicative of warming at the surface  

• The atmosphere warms but not the surface  

• Less reaches the surface as the warming as absorbed in the aerosol layer  

• So radiative forcing does not indicate surface temperature change 

• Indicative of wider atmospheric change 

Complexity in aerosol forcing

Aerosol concentrations vary in space and time and so forcing comes with associated patterns which alter pressure and temperature gradients and change circulation  

Winter warming

Kirchner et al., 1999 

Post 1991 pintubo eruption and aerosol emission

Stratospheric heating  

• Aerosols absorb solar near-IR and terrestrial LW radiation, warming the lower stratosphere by ~4 K (simulated; observed ~2 K after accounting for QBO and ozone depletion) 

• Heating peaks in the tropics, enhancing the meridional temperature gradient. 

Tropospheric cooling 

• Aerosols scatter solar radiation, reducing surface shortwave (SW) flux by ~3 W/m² (global average). 

• Causes summer cooling (direct effect). 

Winter warming  

• Enhanced polar vortex 

• Stronger tropical heating → increased equator-pole temperature gradient → strengthens the stratospheric polar vortex (westerly winds). 

• Observed: Zonal winds at 60°N intensify by ~4 m/s in winter. 

• S-T coupling 

• A stronger polar vortex reflects planetary waves back into the troposphere, altering circulation. 

• Favors a positive phase of the North Atlantic Oscillation (NAO): 

• High-pressure anomaly over the North Atlantic → warmer advection over NH continents (Eurasia, North America). 

• Low-pressure anomaly over Greenland. 

• Surface expression 

• Winter warming: Simulated warming over NH land (e.g., Europe, North America) matches observations  

• Delayed response: Warming peaks in late winter (February–April) due to vortex persistence. 

Semi-direct effect

• Absorbing aerosol warms the aerosol layer and cools the surface  

• Stabilises the atmosphere and a reduction of low-level cloud cover (need more moisture to reach SVP) 

• Semi-direct effect leads to warming  

• Can be several times larger than the direct effect  

Semi-direct effect case study

Koren et al., 2004

• Urban air pollution and smoke from fires modelled to reduce cloud formation by absorbing sunlight and cooling the surface (and heating the atmosphere)  

• Inhibiting convection and cloud formation 

• Satellite data over the Amazon region during the biomass burning season showed that scattered cumulus cloud cover was reduced from 38%in clean conditions to 0%for heavy smoke   

• Smoke does some scattering but not as much as clouds

• and so cannot make up for the cloud loss of scattering

• and it also causes some warming

Relationship between cloud droplet and aerosol number

Gultepe and Isaac, 1999

• Compare aerosol number and cloud droplet 

• Expect increase with CD as AN increases  

• But this does not align with measurements 

• There is a lot of noise as cloud rework – dynamic system  

• These is evolution and entrainment of air and feeding from moisture 

• Most increase in CD in clean atmosphere 

• Most vulnerable to aerosol perturbation  

• Polluted air less susceptible to the effects of aerosols on CDN  

• Also depends on the type of aerosol  

• Saturation effect  

• Large uncertainties even when the process (cloud brightening) is clear

ACE-2 

Raes et al., 2000 

• Experiment which looked at clouds and aerosols  

• In a clean marine environment – low pollution  

• Polluted air from African continent coming through which can be compared 

• Used model to investigate aerosol indirect effects  

• High resolution in both vertical and horizontal 

• Cyclic boundary to get long term development 

Twomey effect

• More aerosols (e.g., pollution) increase cloud droplet number but reduce droplet size, making clouds more reflective (brightening effect). 

Albrecht effect

• Smaller droplets suppress rain formation, potentially prolonging cloud lifetime (more cloud cover). 

ACE-2 Observed Both: Pollution plumes showed higher droplet concentrations and reduced drizzle, supporting these indirect effects. 

Marine Stratocumulus Clouds: 

• Polluted air masses had higher droplet concentrations but no evidence of reduced cloud cover 

• In some cases, suppressed precipitation (Albrecht effect) could maintain cloud layers longer.

Key idea precipitation in the liquid

need a mechanism which accounts for the production of precipitation on timescales observe in the atmosphere – 30 minutes  

Precip liquid growth

Condensation  

• Need supersaturation around the droplet  

• Difference (gradient) between near-field and far-field saturation  

• Growth dependent on the speed of diffusion from the far- to the near-field  

• Latent heat produced by condensation needs to diffuse away to maintain condensation  

Speed 

• The larger the radius the slower the growth (double size, half growth) 

• Too slow for precipitation formation (even discounting for competition between droplets for supersaturation) (Rogers and Yau, 1989)  

• To reach 50 micrometer (drizzle – smallest precip possible) you need ~12 hours  

PL - collision and coalescence

Collisions  

• Brownian motion, turbulent shear, differential fall velocity, electrostatic charges 

•  For cloud droplets DFV is the most important – due to non-monodisperse size distribution  

• Collisions only efficient in clouds for r>20 micrometres  

• 1mm raindrop needs collection of 100k droplets, need >20 micrometer to be efficient, only need a few of these large ones – only need one in 100k  

• 18 micrometres is a magic number – collisional growth becomes stable  

Coagulation

• Collision doesn’t guarantee coalescence  

• Bounce apart, coalesce temporarily 

• Liquid = usually permanent coalescence 

• Terminal fall velocity  

• Gravitational force balanced by drag force  

• Drag depends on flow regime which is dependent on droplet size  

• Large = turbulent flow  

Droplet spectrum

• Droplet growth extremely sensitive to initial size distribution  

• Berry, Reinhardt, 1974

Marshall Palmer, 1948 distribution 

Empirical study of the distribution of raindrop sizes  

• Exponential drop size distribution  

• Small drops are more common  

• The number of raindrops decreases rapidly as size decreases  

• And large drops are unstable  

• The shape of the distribution depends on rainfall rate 

• In light rain most drops are small 

• In heavy rain there are more medium and large, but still fewer large than small 

Warm rain

• Never homogenous formation  

• Initial cloud droplets (r~1micrometer) differ in size due to different properties of CCN, fluctuations in temperature and humidity  

• Condensational growth alone is too slow 

• Collision and coalescence drives warm rain process 

• Initial collisions determined by stochastic process  

• Then turbulence and condensational growth enhance initial collisions 

Key idea - precipitation in the ice phase

Cool rain forms in similar but not the same ways, warm and cool rain processes interact 

Freezing

• Nucleation barrier needs to be overcome  

• Homogenous freezing  

• Need T<-38 degrees c for spontaneous freezing of cloud droplets  

• Need to get liquid water molecules to organise in an ice lattice  

• Through rearranging  

• Probability of formation of ice nucleus dependent on temperature  

• Once nucleus forms then it all goes and rearranges rapidly  

• The only thing we know is that when it gets cold enough then cloud droplets spontaneously freeze 

• Temps reached in the free troposphere  

Homogenous deposition

None!

• Solid ice and gas phase water forming on it  

• Never happens under atmospheric conditions  

• You would always get liquid water  

• There can be no formation of ice phase particles from gas phase 

• Heterogenous freezing  

• Aided by the presence of foreign surfaces or suspended particles  

• Becomes significant at –15 degrees  

• Dependent on surface properties, shape and chemistry  

• Statistical process – freezing depends on exposure time  

• No closed theoretical descroption  

Ice Nuclei 

• Needed for heterogenous freezing  

• IN only small fration of aerosol population (1 in a billion 

• IN conc dependent on temp – only active if temps are low enough 

• Some INs are CCNs which reduces the INs available for freezing 

• Mineral dust = good IN bad CCN  

Ice in clouds  

• Unlikely for temps above –5  

• More common in decaying than newly forming cumulus  

• More common in stratiform than cumulus with same cloud top temp  

• Because slow formation allows stoachastic processes to have more of a chance to be active 

• Observed ice crystal number larger than IN concentration – ice multiplication  

• Fracture of ice crystals/shattering or splintering of freezing drops  

• Models 

• Low level polar clouds often in liquid phase but models bias towards ice which is a problem for climate modelling  

Hallet-Mossop/rime splintering  

• Secondary ice crystal formation process  

• Supercooled droplet of the right temp and size are captured by graupel (soft hail) then small droplets can be produced during freezing  

• -8<T<-3 and d<25 micrometre 

• Warm enough that freezing takes time to allow for layered growth, cold enough to freeze 

 (Atlas et al., 2022)

• In clouds containing both liquid and ice with temperatures between −3°C and −8°C, liquid droplets collide with large ice crystals, freeze, and shatter, producing a plethora of small ice splinters.   

• These splinters act as new ice nuclei  

• Accelerating cloud glaciation 

• Enhancing processes like aggregation (sticking together) and riming (ice particles growing by collecting liquid droplets)  

• Earlier dissipation – early precipitation which speeds to transition from mixed-phase to fully glaciated cloud (which dissipates more quickly) 

• Causing clouds to reflect less sunlight and have shorter lifetimes 

Diffusional growth of ice 

• Same diffusional balance as water (latent heat from WV to ice which is 10% larger release) 

• Ice crystals have non-spherical shape  

• Makes modelling v hard as cannot predict shape 

• Ventilation effect 

• Don’t have radial shape of temp and WV around the ice crystal  

• Means there is asymmetric flow of air around the particle  

• Enahned diffusion processes – poorly understood  

• Preferential growth on ledges and edges as more WV deposition than flat surface  

• Explains snowflake branches  

• Environmental conditions – shape depends on historical environmental conditions 

Saturation over water and ice 

• Ice crystals grow at the expense of water droplets  

• SVP for ice and water is different  

• SVP over liquid surface is always larger than an ice surface  

• The absolute difference is meaningless a v low temps, max difference at –12  

• Bang on ice saturation but subsaturated for water  

• Water droplet evaporates and the ice gets more deposition  

• If ice crystals and water droplets both present 

• Then ice grows at the expense of water droplets  

• But we have lower IN conc than CCN conc 

• Form more cloud droplets than ice crystals  

• So have many cool ice droplets and few ice crystals 

• So diffusional growth of ice crystals can form very large ice particles in liquid clouds 

• Diffusional growth is too slow in water to produce precipitation  

• For ice it is so much faster 

Ice vs water

Super cooled droplets w/o ice crystals

• WV conc close to saturation

When ice starts to form

• ice crystals form into supersaturated environments

• rapid growth by diffusions

  • reduced WV conc in atmosphere

  • water droplets in subsaturated conditions

  • evaporate and become available to ice crystals as WV

Bergerson-Findeisen process 

• A theoretical explanation of the process by which precipitation particles may form within a mixed cloud (composed of both ice crystals and liquid water drops). 

• “cold-rain process” 

• Subsaturated environment for liquid water but a supersaturated environment for ice, resulting in rapid evaporation of liquid water and rapid ice crystal growth through vapor deposition.  

• If the number density of ice is small compared to liquid water, the ice crystals can grow large enough to fall out of the cloud, melting into rain drops if lower level temperatures are warm enough. 

Accretion  

• Precipitation particle (in the ice-phase) captures a supercooled droplet 

• Contact freezing and riming  

• Aggregation = clumping of ice crystals to form snowflakes  

• Collision and capture 

• Similar to coagulation of droplets  

• Need to know fall velocities – shape and density plays an important role  

Size distribution  

• Collection efficiency of ice crystal aggregation is strongly temperature dependent – only significant for T>-10  

• Observed size distributions for hail, graupel and snow follow power law -> MP distribution  

• But no breakup for hail and graupel so size is only limited by fall velocity vs updraft velocity  

• No absolute limit as no collisional breakup or aerodynamical instability  

• Only limit is fall velocity vs updraft velocity – thunderstorms = huge crystals 

Comparison of warm vs cool rain  

Warm rain process (collision-coalescence) 

• Warm rain formation is generally faster because droplets grow through frequent collisions and can produce rain within tens of minutes in intense convection. 

• Can sustain clouds for longer if updrafts keep droplets suspended. 

Cool rain process (Bergeron-Findeisen process) 

• Cool rain formation is slower because it relies on ice crystal growth and secondary ice processes, often taking an hour or more to produce precipitation. 

• Can shorten cloud lifetime by enhancing ice crystal growth and precipitation. 

Circulation and rotation

KI: circulation and vorticity are different and important

KI: Rotation is key for predicting motion in the atmosphere (this links nicely to a discussion on waves and then teleconnections) 

Rotation general

• Atmospheric motion is never along straight lines and is never circular  

• Constant change in direction means there is always an element of rotation involved 

• Tropical cyclones and midlatitude cyclones represent rotational features of the atmosphere  

• The gulf stream consists of many isolated vortices and eddies 

General principles of atmospheric motion

• Differential heating of the earths surface causes temperature gradients  

• This generates pressure gradients  

• Which cause horizontal motion along the PGF  

• The Coriolis force (the effect of the earths rotation) deflects moving objects  

• Resulting in flow perpendicular to the PGF  

• This describes flow generation but not flow development  

Flow development

• Change in flow in time = advection of flow by flow itself + CF + PGF + Gravitational force + frictional force 

• Development = non-linear transport + generation + G in the vertical + friction (including turbulence) 

• This is a key framework for discussion and essay structure  

• Asymmetry created by non-linear term (advection) 

PGF

• Only force which leads to an acceleration and increase in horizontal windspeed  

• In the vertical PG is balanced by gravitational/buoyancy forces 

Friction

• Near surface friction (turbulence) reduces flow  

• New equilibrium between CF, PGF and frictional force  

• Causes slowdown of flow  

• Also ,when there is a reduction in flow speed the PGF becomes out of balance w CF and so there is motion to balance this again 

Cyclostrophic balance 

• Non-linear advection term includes centrifugal force  

• Perpendicular to radial component  

• Cyclostrophic balance between PGF, CF and centrifugal  

Low pressure system

• Centrifugal opposes PGF in the same direction as the CF  

• Lower flow velocity compared to geostrophic 

High pressure system

• Centrifugal in the same direction as the PGF and so higher flow velocity than geostrophic 

• Strength of high pressure system is limited – otherwise the sum of centrifugal plus PGF grows faster with increasing velocity than CF  

• Centrifugal force only changes direction and so has an indirect effect on flow speed (through generating a new balance) 

Scale asymmetry 

• Compact intense low pressure systems  

• Large weak high pressure systems  

• Due to this asymmetry wind speeds are higher in high pressure systems than low pressure systems at the same pressure gradient  

• High pressure systems are more uncommon than low pressure systems  

Concept of circulation  

• Describes rotation in a fluid without reference to a rotational axis  

• Positive for counterclockwise (cyclonic) rotation  

Development of circulation

•  Changes with time if the integral flow along a closed contour changes  

• Due to transport, CF, pressure gradient and friction  

• In the absence of friction circulation can only change due to the PGF if density is not just a function of pressure (baroclinicity) 

• Baroclinic conditions = temperature not constant along surfaces of constant pressure  

• Temperature changes independently to pressure  

• Baroclinicity generates circulation through converting potential energy from temperature gradients into kinetic energy  

• Important for mid-latitudes  

• Circulation will change under baroclinic conditions even if no rotational component existed in the flow before  

• Thus, baroclinic instabilities are at the heart of fronts and cyclones in midlatitudes  

• Frontal systems indicate baroclinic flow  

Baroclinicity 

• When the gradient of pressure is misaligned from the gradient of density in a fluid  

• This means that density is dependent on both temperature and pressure  

• Baroclinicity is what drives the land sea breeze 

Circulation lynch and cassano, 2006

• Circulation does not require an axis of rotation because it is a measure of the total rotational motion of a fluid along a closed path, rather than around a specific axis 

• Convenient when an axis of rotation is hard to identify  

• Circulation is a path-dependent property of fluid motion and does not require a specific axis, whereas rigid-body rotation does. 

• C is positive for counterclockwise flow  

Kelvins circulation theorem

L&C 06

• Circulation around a closed curve will be a function of both time and space  

• Circulation is constant in a barotropic, inviscid fluid 

• The theorem implies that if a fluid initially has no circulation, it will remain circulation-free unless external influences act. 

• The theorem implies that if a fluid initially has no circulation, it will remain circulation-free unless external influences act. 

• In the absence of viscosity, fluid elements cannot spontaneously develop rotation (vorticity), meaning rotational motion must be inherited from initial conditions or external forcing. 

• Another way of describing the curved motion of fluid parcels without reference to a centre of rotation  

• Simplifies analysis  

• No need to define a specific closed circuit  

• Relative vorticity = positive for counterclockwise rotation  

• Flow around a low-pressure centre in the NH – relative vorticity = positive  

• Temporal changes in vorticity tell us about cyclone development  

• Vorticity increases as cyclones spin up and decreases as they die  

• Spatial changes in vorticity can indicate topographic features or temperature gradients  

• Which can lead to the generation of circulation according to Bjerknes’ theorem 

• One way to address the spatial variation or vorticity in isolation is to develop a quantity which is conserved  

• Potential vorticity  

Conservation of potential vorticity  

L&C 06

• Kelvin’s theorem  

• Inviscid barotropic flow  

• Circulation is conserved  

• In real cases circulation (or vortex strength) changes due to the presence of baroclinicity or friction  

• We can simplify  

• Remove friction by assuming we are far from the surface  

• And that motion is adiabatic (potential temperature is constant) 

• Potentiotropic – the density is a function of pressure alone  

Vorticity gen

• Circulation is tricky because it depends on a closed contour – it is a property of an area that can change even if circulation doesn’t 

• So, more intuitive to study circulation per unit are = vorticity  

• Circulation per unit area 

• Property of a flow at a given location  

• Circulation around a closed contour divided by the area enclosed by this contour  

• Vorticity can change even if circulation remains constant 

• To account for this = concepts of relative and absolute vorticity 

• Relative = vorticity for an observer on earth – change in wind speed perpendicular to the direction of motion  

• Absolute = relative plus Coriolis parameter  

Vorticity due to horizontal wind shear  

• Any rotation in the same direction as the earth rotation in that hemisphere is termed positive vorticity  

• So counterclockwise in NH  

• Wind shear  

• For example, where westerly (flow from W – E) flow is increasing in meridional direction (N-S direction) causing cyclonic rotation  

Vorticity in flow with curvature

• A flow with curvature has change in meridional and latitudinal direction at the same time  

• For example, a low pressure system moving within a mean weaterly flow which meanders as a planetary RW 

• Ridge = clockwise anti-cyclonic/negative vorticity, minimum vorticity at the peak 

• Trough = counterclockwise positive vorticity, maximum vorticity at the peak 

• If a cyclone is moving through these troughs and ridges 

• It has positive vorticity  

• It will weaken through the ridge and strengthen through the trough 

Development of vorticity 

• In divergent flow circulation will not change but vorticity will – why? 

• Divergence (convergence) reduces (increases) absolute vorticity  

• Producing anti-cyclonic (cyclonic) rotation even if relative vorticity is initially zero  

• Barotropic vorticity equation 

• Equivalent to Kelvins circulation theorem  

• In a barotropic, frictionless, divergence free fluid absolute vorticity is conserved 

• Fluid moving N-S direction  

• Coriolis parameter changes  

• So relative vorticity changes accordingly  

Barotropic vorticity equation  

• Absolute vorticity is conserved in a barotropic, inviscid, divergence free fluid  

• Explains why low-pressure systems weaken (strengthen) when moving poleward (equatorward) 

• Vorticities interact with mean flow  

• A low-pressure system travelling with the meandering winds of the midlatitude westerlies will intensify when it travels south easterly towards a trough and will weaken when it travels north easterly towards a ridge  

• The opposite happens to a high pressure system which is characterised by negative vorticity – anticyclonic