Evaluating mass flux closures using cloud resolving model simulations of deep convection Jennifer Fletcher and Chris Bretherton COGS talk May 7, 2009 Evaluating Mass Flux Closures Using Cloud Resolving Model Simulations of Deep Convection Jennifer Fletcher and Chris Bretherton COGS presentation May 7, 2009
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Evaluating mass flux closures using cloud resolving model simulations of deep convection
Evaluating Mass Flux Closures Using Cloud Resolving Model Simulations of Deep Convection. Evaluating mass flux closures using cloud resolving model simulations of deep convection. Jennifer Fletcher and Chris Bretherton COGS talk May 7, 2009. - PowerPoint PPT Presentation
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Evaluating mass flux closures using cloud resolving model
simulations of deep convection
Jennifer Fletcher and
Chris Bretherton
COGS talk May 7, 2009
Evaluating Mass Flux Closures Using Cloud
Resolving Model Simulations of Deep Convection
Jennifer Fletcher and Chris Bretherton COGS presentation
May 7, 2009
What does cumulus convection do?
• Makes precipitation and severe weather• Changes stability• Generates and redistributes heat and moisture• Transports tracers• Makes clouds that affect radiation
Cumulus parameterizations must represent some or all of these effects
Two basic types of cumulus parameterization: adjustment and
mass flux• Adjustment: temperature and moisture adjust
to pre-specified profiles over a finite timescale.
• Mass flux: a cloud model explicitly calculates profiles of cumulus mass flux and thermodynamic variables.
• Mass flux more widely used because it can provide an internally consistent representation of turbulent mixing, updraft dynamics, microphysics, and tracer transport
Three components to a mass flux parameterization
• Trigger: will convection occur?
• Closure: how much convection?
• Model: how will convection distribute heat, moisture, etc?
The trigger and the closure may be considered two parts of the same thing
Mass flux parameterization• The closure relates the cloud base mass flux
to resolved-scale variables• A cloud model
predicts the vertical structure of mass flux and thermodynamic quantities above cloud base
• My research focuses on a type of mass flux closure
mcb = f(large scale)
Cloud model predicts cumulus properties per cloud base mass flux
What I am going to show you
• Mass flux closure based on convective inhibition (CIN) is well suited for both shallow and deep convection.
• This closure takes the form mcb=c1Wexp(-c2CIN/TKE)
• W is a vertical velocity scale
• TKE = mean turbulent kinetic energy in the boundary layer
Mass flux closure: fundamental assumptions
• Cumulus cloud base mass flux is a function of large scale variables
• Cloud base mass flux has no memory, i.e., it is determined by instantaneous forcings.
Mass flux closure types
• Moisture convergence
• CAPE
• Boundary layer based
Mass flux closure type 1: moisture convergence
• Mass flux determined so that precipitation balances moisture convergence, as observed over the tropical oceans.
• Lacking in causality: convection can’t see moisture convergence & must ultimately respond to local thermodynamic profile
• Performs poorly if the storage term in the moisture budget is significant on convective time scales (e.g., continental convection).
• Examples: Kuo, Anthes• Popular in the 80’s but has largely fallen out
of favor.
Mass flux closure type 2: CAPE• Cumulus base mass flux depends on the vertically-
integrated parcel buoyancy (adiabatic or entraining)• Causality issue: how can cloud base mass flux feel the deep updraft buoyancy profile?
• CAPE and cloud base mass flux are inconsistently correlated in tropical oceanic convection.