Boundary Layer of shallow clouds and Boundary Layer of shallow clouds and deep convectiondeep convection

Review of last lectureReview of last lecture Vertical structure of the atmosphere and Vertical structure of the atmosphere and

definition of the boundary layerdefinition of the boundary layer Vertical structure of the boundary layer Vertical structure of the boundary layer Definition of turbulence and forcings Definition of turbulence and forcings

generating turbulencegenerating turbulence Static stability and vertical profile of virtual Static stability and vertical profile of virtual

potential temperature: 3 casespotential temperature: 3 cases. Richardson . Richardson numbernumber

Boundary layer over oceanBoundary layer over ocean Boundary layer over land: diurnal variationBoundary layer over land: diurnal variation

Shallow clouds Shallow clouds Include stratus, stratocumulus and cumulus cloudsInclude stratus, stratocumulus and cumulus clouds Cloud top height generally below 4 kmCloud top height generally below 4 km Sometimes associated with light rain, sometimes notSometimes associated with light rain, sometimes not

Global distribution of shallow cloudsGlobal distribution of shallow clouds

Do we notice any patterns?Do we notice any patterns?Klein and Hartman, 1993

Vertical structure of shallow cloudsVertical structure of shallow clouds

Intense longwave Intense longwave radiative cooling radiative cooling at cloud top at cloud top drives eddies in drives eddies in BLBL

Eddies pick up Eddies pick up moisture and moisture and maintain cloudmaintain cloud

Eddies also Eddies also entrain warm, dry entrain warm, dry air from above air from above the inversionthe inversion

Entrainment lifts Entrainment lifts the cloud, the cloud, subsidence subsidence lowers itlowers it

Bretherton et al. 2004

Boundary layer of shallow clouds: Boundary layer of shallow clouds: well-definedwell-defined

(From Barnes 1982)

Deep convectionDeep convection Cloud top height above 9 kmCloud top height above 9 km Generally associated with rainGenerally associated with rain Sometimes organized into mesoscale convective systemsSometimes organized into mesoscale convective systems

Global distribution of deep convectionGlobal distribution of deep convection

Zipser (1977), modified by Houze (1993)

Convective updrafts (controlled by lower troposphere moisture)

Mesoscale updrafts

Convective downdrafts

Mesoscale downdrafts

High θe

Low θe

High θe

Vertical structure of deep convection: Vertical structure of deep convection: Four componentsFour components

Boundary layer of deep convection: Boundary layer of deep convection: Not well-definedNot well-defined

Differences between shallow clouds Differences between shallow clouds and deep convection: Boundary layer T and deep convection: Boundary layer T

and qand q

Differences between shallow clouds Differences between shallow clouds and deep convection: Change of and deep convection: Change of

boundary layer Tboundary layer T

Shallow/midtop convection: T decreases

Deep convection: T decreases

Differences between shallow clouds Differences between shallow clouds and deep convection: Change of and deep convection: Change of

boundary layer qboundary layer q

Shallow/midtop convection: q increases

Deep convection: q decreases

Differences between shallow clouds and deep Differences between shallow clouds and deep convection: convection:

Change of boundary layer moist static energy hChange of boundary layer moist static energy h

Moist static energy

h = Cp T + L q + g z

Shallow/midtop convection: h keeps constant

Deep convection: h decreases

Self-suppression processes in deep convectionSelf-suppression processes in deep convection

Zipser (1977), modified by Houze (1993)

Convective updrafts (controlled by lower troposphere temperature and moisture)

Mesoscale updrafts

Convective downdrafts

Mesoscale downdrafts

Missing physics I

Missing physics II

Missing physics III, IV

High θe

Low θe

High θe

Over-stabilized state: A state Over-stabilized state: A state far awayfar away from quasi-equilibriumfrom quasi-equilibrium

Model atmosphere fluctuates around the quasi-equilibrium state (always drizzle)

The observed atmosphere swings far away from the quasi-equilibrium state (switch between heavy shower and clear sky)

SummarySummary Global distribution and vertical structure of Global distribution and vertical structure of

shallow cloudsshallow clouds Global distribution and vertical structure of deep Global distribution and vertical structure of deep

convection. Four components: convective convection. Four components: convective updraft, convective downdraft, mesoscale updraft, convective downdraft, mesoscale updraft, mesoscale downdraftupdraft, mesoscale downdraft

Differences between shallow clouds and deep Differences between shallow clouds and deep convection: change of T, q and h in the boundary convection: change of T, q and h in the boundary layer layer

Self-suppression processes in deep convection: Self-suppression processes in deep convection: Overly stabilized state after deep convectionOverly stabilized state after deep convection

Problems in current global climate models: lack Problems in current global climate models: lack of self-suppression processesof self-suppression processes