VO6 - Parameterizations 4

What is convection?

• circulation driven by differences in density / buoyancy

• caused by radiation, surface warming, cooling of the atmosphere

In which 3 categories can convection be divided in parameterizations?

• turbulent convection in the BL

• shallow convection

• deep convection

On which scales and in which models is turbulent convection parameterized?

• up to the BL top

• 10min

• Climate, NWP, partially also in LES (dx~10-100m)

On which scales and in which models is shallow convection parameterized?

• about height of BL top + 1km

• 20 - 60min

• Climate, NWP (regional and global)

On which scales and in which models is deep convection parameterized?

• up to the tropopause

• 1 - 3h

• Climate, Global NWP

How does convection influence the core variables?

vertical transport of:

• moisture

• energy

• momentum

For which parameterized values is convection responsible?

• convective precipitation

• convective cloudiness

• aerosol dispersion / lifting

• subgrid vertical winds

• TKE

What would happen if convection were not parameterized in NWP models?

the circulation would at least be as large as the effective resolution

Explain this plot.

• shows maritime shallow convection in the trade winds region

• walker circulation leads to subsidence inversion at ~2.7km

• cloud layer then is located between the well-mixed and shallow maritime BL and the subsidence inversion due to descending part of the walker circulation

Which models can be used to study shallow convection?

LES with dx~10-100m and domains up to 50×50km, today even over whole countries

Why is deep convection challenging to simulate in LES?

stronger winds, more complicated microphysics, much larger required model domain

How are rising and sinking air due to convection reffered to?

• “convective” rising air

• “environmental” sinking air

Which method can be used to see the dispersion due to convection over time?

ground released radioactive trace → decay gives time dependency

a normal tracer would just get mixed with no time description

Explain this plot.

shows continental shallow convection in LES

horzontal 25×25m grid

1024×1024 grid cells

25.6×25.6km area

What does this plot show?

From LES:

Average humidity and potential temperature show typical inversion with high continental BL, neutral stratification below cloud, then stable

What does this plot show?

From LES:

10% of the BL classified as convective seen in the object area fraction (how much of the domain is convective), proportion decreases in the cloud layer

What does this plot show?

From LES:

convective air transports more moisture upward than surrounding air downward

What does this plot show?

From LES:

humidity anomaly transport (q’ = q - q_mean) is highest in the cloud

What does this plot show?

From LES:

cloud cover up to 10%, gray background marks cloud cover > 0

Can individual convection be regarded as bubbles or plumes?

no complete description as either one → chaotic 3D fluid-dynamic process

In general for convection, bubbles or plumes?

When averaged over many clouds, a plume-like structure emerges

Explain this plot.

• composite of 10⁶ shallow cumulus cross sections from LES

• normalized in height with cloud base

• normalized in x with cloud length

• updrafts strongest in middle under cloud at ~0.7 _^x cloud base

• narrow strong downdraft at cloud edges (subsiding shell)

• weak downdraft between the clouds in the BL

What does EDMF stand for and what is it?

• Eddy Diffusivity Mass Flux

• possible parameterization of convection

What is the basic assumption in the EDMF?

convective vertical flux can be divided in 2 components

• Eddy diffusivity

• Mass flux

Explain the Eddy diffusivity component of the EDMF parameterization in general.

unorganized turbulent flows that only act locally between adjacent layers and have a diffusive effect

Explain the Mass flux component in the EDMF parameterization in general.

Organized strong updrafts with inertia, which allow transport across multiple layers and can transport variables against the gradient

How much of the domain of the EDMF does the Eddy Diffusivity component make up?

~90% of the domain

How much of the domain of the EDMF does the Mass Flux component make up?

~10%

How are fluxes in the environment in the EDMF approximated?

K-approach / Boussinesq closure

How is the mass flux in the updraft in the EDMF approximated?

with the mass flux definition M = a_u (w_u - w)

How does the flux approximation in the EDMF look like and what has been done to get to this form?

• Environmental fluxes (diffusive) are approximated by the K-approach / Boussinesq closure

• Fluxes in the updraft are approximated with the mass flux definition

What are properties of K?

• K >= 0

• K small at BL top

• K = 0 at ground

• K increases with thermal instability

• K decreases with thermal stability

• K increases with rising TKE

What is the most common parameterization of convection?

Mass flux

On what is the mass flux parameterization based?

idealized steady-state plume, which continuously moves air from lower BL to the inversion

What is a important assumption in the mass flux parameterization?

mostly assumed properties in plume are horizontally homogeneous (BL-approximation)

What are the 3 most important processes acting on a cylindrical steady state mass flux?

• Entrainment / Detrainment

• positive/negative buoyancy

• vertical pressure gradients

Explain how entrainment / detrainment affects a cylindrical steady state mass flux.

Mixing of environmental air into the updraft slows the ascent

Explain how buoyancy affects a cylindrical steady state mass flux.

vertical acceleration due to positive or negative buoyancy, latent heat relevant as release increases buoyancy

Explain how vertical pressure gradients affect a cylindrical steady state mass flux.

the faster the ascent, the more air in front of parcel while ascending → creates positive pressure pertubation at parcel top and negative below

leads to downward pressure gradient slowing the parcel

On what does the rate of entrainment / detrainment depend?

distribution of a variable inside the updraft

What are counter-gradient fluxes?

in comparison to eddy diffusivity, the mass flux component can also transport against the gradient

Give an example for a counter-gradient flux.

At 2500m eddy diffusivity transports moisture from the BL to the free atmosphere, and potential temperature in the opposite direction.

The mass flux can transport heat from the “cold” BL across the inversion (inertia)

Explain this plot.

left: strong entrainment

right: weak entrainment

with sufficient entrainment condensation height is never reached

for weak entrainment condensation height is reached → release of latent heat leads to second acceleration above cloud base