Boundary Layer of shallow clouds and deep convection

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Boundary Layer of shallow clouds and deep
convection
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Review of last lecture

• Vertical structure of the atmosphere and
  definition of the boundary layer
• Vertical structure of the boundary layer
• Definition of turbulence and forcings generating
  turbulence
• Static stability and vertical profile of virtual
  potential temperature 3 cases. Richardson number
• Boundary layer over ocean
• Boundary layer over land diurnal variation

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Shallow clouds

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

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Global distribution of shallow clouds

• Do we notice any patterns?

Klein and Hartman, 1993
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Vertical structure of shallow clouds

• Intense longwave radiative cooling at cloud top
  drives eddies in BL
• Eddies pick up moisture and maintain cloud
• Eddies also entrain warm, dry air from above the
  inversion
• Entrainment lifts the cloud, subsidence lowers it

Bretherton et al. 2004
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Boundary layer of shallow clouds well-defined
(From Barnes 1982)
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Deep convection

• Cloud top height above 9 km
• Generally associated with rain
• Sometimes organized into mesoscale convective
  systems

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Global distribution of deep convection
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Vertical structure of deep convection Four
components
Convective updrafts (controlled by lower
troposphere moisture)
High ?e
Mesoscale updrafts
Low ?e
Mesoscale downdrafts
High ?e
Convective downdrafts
Zipser (1977), modified by Houze (1993)
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Boundary layer of deep convection Not
well-defined
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Differences between shallow clouds and deep
convection Boundary layer T and q
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Differences between shallow clouds and deep
convection Change of boundary layer T
Shallow/midtop convection T decreases Deep
convection T decreases
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Differences between shallow clouds and deep
convection Change of boundary layer q
Shallow/midtop convection q increases Deep
convection q decreases
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Differences between shallow clouds and deep
convection Change 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
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Self-suppression processes in deep convection
Missing physics III, IV
Convective updrafts (controlled by lower
troposphere temperature and moisture)
High ?e
Mesoscale updrafts
Low ?e
Missing physics II
Mesoscale downdrafts
High ?e
Convective downdrafts
Missing physics I
Zipser (1977), modified by Houze (1993)
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Over-stabilized state A state far away from
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)
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Summary

• Global distribution and vertical structure of
  shallow clouds
• Global distribution and vertical structure of
  deep convection. Four components convective
  updraft, convective downdraft, mesoscale updraft,
  mesoscale downdraft
• Differences between shallow clouds and deep
  convection change of T, q and h in the boundary
  layer
• Self-suppression processes in deep convection
  Overly stabilized state after deep convection
• Problems in current global climate models lack
  of self-suppression processes