Influence of the Subcloud Layer on the Development of a Deep Convective Ensemble

Boing et al., 2012

Depiction of shallow-deep convection transition

Rio et al., 2009

Thermals drive shallow convection in the morning; key role in preconditioning

Wakes (cold-pools) reinforce deep-convection in the afternoon

Events preceding rainfall Onset Evaporation-----Negative

buoyant downdrafts Spread over surface as

density currents away from rainfall

Characteristics

Circular patterns Outflow boundaries: High

RH Cold and Dry at center

Shallow-Deep Convection Transition (Previous work)

Rapid growth of deep convection could be prevented when evaporation of rainfall is suppressed (Khairaoutdinov and Randall (2006); 3D CRM)

Rapid transition in cloud height precedes the formation of cold pools; Mean cloud buoyancy is crucial (Wu et al; 2D CRM)

Growth of length scales in the subcloud layer is strongly correlated with the presence of precipitation (Martins 2011; CRM and Satellite)

Moisture and moist static energy in homogeneities good indicator: Increase of Boundary layer Inhomegeneity precides formation of precipitation (Zhang and Klein 2010; Observations)

Forced uplift at the outflow boundary (edge of cold pools) where gust front occurs

Forced uplift at the outflow boundary (edge of cold pools) where gust front occurs; Initiation of new convection(Over ocean: Rauber et al., 2007; Zuedema et al., 2012; Over Land: Weckwerth and Wakimoto 1992)

Convective triggering is preferential at the interface of colloiding outflow boundaries occurred during afternoon (Lima and Wilson)

Rio et al (2009) parameterized the dynamical effects of wake; realistic timing of parameterized deep convection

Tomkins (2001a) argues deep convection over the ocean is thermodynamic rather than dynamic; Enhanced updraft activity occurs because of a change in the thermodynamic properties of subcloud layer (surface fluxes/contrast to mean state)

Cold pool mechanisms (Dynamic/Thermodynamic effects)

Other effects

RH of mid-troposphere crucial for the onset of congestus clouds(Derbyshie et al., 2006. Wu et al., 2009)

Large-scale motion helps premoisten the atmosphere and enhance convective activityWind shear may organize the convection (Rotunno et al. 1998) under certain conditions

Purpose of this paper

Testing the Positive Loop

Evaporation in subcloud layer leads to formation of cold pools because of thermodynamic/and dynamic properties of the subcloud layer; wider clouds with lower entrainment, and larger excess of thermodynamics at outflow of boundaries These clouds reach higher into troposphere and large fraction of t heir moisture excess converted into precip

To verify the new implementation of new scale to damp the variance in the subcloud layer; disentangle feedback loop and only focus on subcloud (thermo/dyna) with cloud layer

Model and Assumptions

DALES model:Dutch Atmos.LESDomain and Resolution: 57.6x 57.6 km ,150 m horizontal; streched grid 250 pts increasesing from 40 m at surfaceDouble periodic boundary conditionsSub-filter scale TKE equationSingle moment ice microphysics (Grabowski 1998)

LES is initialized with random l perturbations with a uniform distribution between +/- 0.1 θk (to avoid absence of initial large scale structure may delay the develop. Of deep convection)

Prognostic variables: total non-precip water spec. hum., total hydrometeor. spec.hum., linear liquid potential temperature

Latent of freezing neglectedMelting layer dynamics also neglected (to avoid downdrafts)No large scale vertical wind shearSurface fluxes held constant: SH: 161w/m2, LH: 343 w/m2; (morning transition values)(Influence of land surface and its properties no considered !!)

Experiments

1. Reference case: No modification to subcloud layer2. Evoporation of rain removed altogether (modifies both cloud

layer (downdrafts) and subcloud layer (CP form.); All rain reaches surface and removed from simulatons

3. Only cold-pool formation suppressed4. T and q tendencies due to evaporation horizont.homo. in

subcloud layer5. Microphy. scheme at each step to determine tendencies due

to evaporation6. Apply hori.hom. tendencies in subcloud;