VO5 - Parameterization 3

What are the main tasks of microphysics?

• description of formation/growth/sedimentation of hydrometeors

• phase transitions with accompanying release of latent heat (Thermodynamics)

• Determine quantitity and nature of hydrometeors for electromagnetic calculations (radiation, radar, satellite comparisons)

• Type and amount of precipiation (rain, snow, hail,..)

What are the theoretical foundations for the description of hydrometeors in parameterizations?

• no universally valid theory

• scales mostly well below 1cm

• difficult to gain insight from high-resolution simulations

What are important approaches to describe hydrometeors in parameterizations?

• one and two-bulk schemes

• bin-schemes

Regarding the phases of water, what must be decided in the model?

how a total amount of H₂O is divided into water vapor, water and ice

What is the simplest assumption for thermodynamics in the model?

• Saturation water vapor depends only on T and p (Clausius Clapeyron)

• T>0 water

• T<0 ice

• T=0 water and ice

Why do simple assumptions regarding thermodynamics in models not hold?

• In reality saturation water vapor depends on droplet radius, ice crystal shape and CCN/IN

• mixed-phase clouds exist for T<0, no thermal equilibrium

What does the Kessler parameterization consider?

only:

• cloud water

• rain

processes:

• condensation

• evaporation

• autoconversion - cloud droplets collide → raindrops

• accretion - falling raindrops collect cloud droplets while falling

• evaporation

Explain Autoconversion.

as a function, with M rain and m cloud water,

• has 2 tuning parameters

  • a… at what amount of cloud water autoconversion begins, m<a term turned of

  • k₁… how quickly cloud water is converted into rain

Explain Accretion.

another therm added to autoconversion function,

adds another tuning parameter:

• k₂… collection efficiency, how efficiently raindrops collect cloud droplets

Explain 2 properties of bulk schemes.

• more often used

• assume an analytical distribution

Explain properties of a One-moment bulk scheme.

• assume a fixed distribution

• only the mass of each hydrometeor category is calculated, e.g. 1kg cloud water

• essentially the Kessler parameterization

Explain properties of a Two-moment bulk scheme.

• distribution not assumed fixed, shape the same but stretch possible

• mass and number of hydrometeors are determined, e.g. 1kg cloud water, 10^6 droplets

• adds self collection as well

Explain properties of Bin microphysics schemes.

• not that often used

• approximate the distribution through several discrete “bins”

• are also called “spectral”

• usually, 30-45 bins → expensive

How do microphysics parameterization depend on other parameterizations?

• vertical updrafts critical, they determine how long and quickly precipitation objects can grow in the cloud before they fall → often from convection parameterizations

• turbulent mixing leads to evaporation at cloud edges

• aerosols from BL act as CCN/IN

What does this plot show?

bulk-two-moment scheme with 6 hydrometeors

• 12 prognostic variables

• many processes that need to be formulated with help of tuning parameters and various closures

• many coupled PDEs need to be solved

• all hydrometeors always > 0

To which other parameterizations are microphysics closely linked?

• radiation

• convection

• aerosols

How do microphysics directly influence the dynamics of the model?

through latent heat (e.g. moist adiabatic processes)

Define radiation in general.

• driving force of our atmosphere, controls daily and annual cycles

• 1% more outgoing radiation → -3°C

What aspects of radiation are particularly interesting in NWP models?

• warming / cooling of surface (driver daily cycle)

• waming / cooling of the air layers directly (O₃, Clouds in LW, etc…)

• forecasts in PV performance

On what does scattering and absorption in gases depend on?

• wavelength

• angle

On what does the interaction of radiation and hydrometeors depend?

• geometry

• properties of the microscopic objects

Which geometric property has a strong influence on interaction between radiation and hydrometeors?

radius of cloud droplet → has enormous effect (RADAR)
same amount of water will have different effects if there are many small or few large objects

What does this plot show?

Example of mie scattering on a sphere

Extent of surface corresponds to intensity of scattering in that direction

How does cloud geometry influence radiation?

• in 1D column, different overlap with same cloud fraction lead to different cloud cover

Name the challenges of parameterizing radiation.

• wavelength dependent

• 3D

• angle dependent

• sensitive to number, size, type of hydrometeors

• affected by aerosols

• cloud geometry plays important role

• non-trivial interactions with earths surface

Name important approaches to parameterize radiation.

• Spectral bands

• 2-stream approach

• Effective radius of cloud droplets

• cloud generators

• stochastic cloud generators

Explain the sprectral bands approach.

• instead of calculating each wavelength individually, closely correlated wavelengths are calculated together

• number of groups is conceptually closely related to grid resolution

• individual groups can be calculated independent of each other

Explain the 2-stream approach.

• no horizontal processes

• for each band/group, only one upward and one downward calculation is performed

• each direction represents an integral over all angles in a hemisphere

• each column can be calculated separately as no horizontal exchange occurs

Explain the approach of Effective radius of cloud droplets.

• instead of considering the full droplet distribution, a single radius is used that is supposed to achieve the same result

How much of the total ECMWF computation time is taken by radiation?

~5%, so very expensive → rest parameterizations ~25%

Explain how Cloud generators work.

• despite 1D simplifications there are still many different cloud profiles in a column due to varying overlaps

• There are 10 cloud sub profiles and on clear sky sub profile, each has a different radiation profile (in the example plot)

Explain how stochastic cloud generators work.

• randomly generates several subprofiles from average profiles of cloud cover, cloud water, ice

• distinguishes not only between cloudy and clear but also varies the amount of moisture

• assumptions about overlap probability and assumed distributions of water and ice in each layer are take into account

• in expl. 25 cloud profiles generated, 5 with no clouds at all, some other identical

How can computing time be reduced for radiation parameterizations?

• calculate solutions for limited number of spectral bands

• for each band, only 1 up/down flux calculated, no horizontal communication between columns (2-stream)

• calculate radiation on coarser grid and longer time step

• determine average radiation properties for all hydrometeors (effective radius, spherical)

What are important input variables for radiation parameterizations?

• Temperature

• Gas composition

• aerosols

• hydrometeors

• cloud fraction

What are important uncertainties in radiation parameterizations?

• geometric arrangement of clouds and distribution

• characteristics of hydrometeors