Isolating the Impact of Height Dependence onCumulus Entrainment

Walter Hannah

May 25th, 2011

EAPSI

• 8 week visit (10 weeks or Japan)• Myong-In Lee at Ulsan National Institute of

Science and Technology in South Korea

Relaxed Arakawa-SchubertAssume form of normalized mass flux

Calculate λ based on MSE profile

Estimate the cloud work-function (i.e. CAPE)

Calculate the cloud-base mass flux

Calculate large-scale tendencies

Problems• Most cumulus schemes based on the “entraining plume”

are not sensitive enough to the environmental conditions• Precipitation occurs too often• Precipitation occurs too early• Tropical wave and MJO variability is unrealistic

• These problems are typically addressed with arbitrary trigger conditions

• Environmental Humidity (Wang and Schlesinger, 1999)• Entrainment (Tokioka et al, 1988)• Large scale vertical velocity (Kain and Fritsch, 1992)• Cloud base height (Lee et al, 2008)• Precipitation evaporation (Sud and Walker, 1993)• Convective inhibition (Kain and Fritsch, 1992)

Chikira and Sugiyama (2010)• Closure based on kinetic energy instead of mass flux• Entrainment rate is assumed to be inversely related to

vertical velocity• Improved performance is credited to having large

entrainment at low levels

Gregory (2001) Neggers et al. (2002) Pan and Randall (1998)

Height Dependent Entrainment

• RAS constant entrainment

• RAS linear entrainment

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Chikira’s Test Case

• Input profile calculated from TOGA-COARE– Precipitation rate > 20 mm/day

SCM Results: GATE

SCM Results: TOGA-COARE

Diurnal Cycle Application• Lee et al. (2007) showed that only certain

trigger mechanisms can significantly effect the diurnal timing of precipitation– Height of starting level

• Defined as level of maximum MSE within 300 hPa from the surface

– Convective inhibition• The LFC is constrained to be within 150 hPa from the

starting level for convection to activate

• Can modifying the cloud base entrainment have the same effect?

Lee et al. (2007)

Diurnal Cycle Application

Lee et al. (2007)

Is Height Dependence Realistic?• Romps (2010) presented a

method for a direct measure of entrainment rate in LES models

• Entrainment and detrainment are sources and sinks of cloud air

Romps (2010)• Entrainment rate was found not to vary

significantly with height

• Entrainment did not scale with vertical velocity or buoyancy

Romps (2010)Cloud air defined by

moisture onlyCloud air defined by

moisture and vertical velocity

Questions?

Suggestions?