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This study presents preliminary results on the diurnal cycle and vertical velocity observations of different convection regimes in shallow cumulus clouds at the ARM Southern Great Plains site. The goal is to assess convection theories, construct composite cases for LES or CRM studies, and provide feedback for GCM parameterization improvements.
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Observed Updraft & Mass Flux in Shallow Cumulus at ARM Southern Great Plains sitePreliminary results Yunyan Zhang, Steve Klein & Pavlos Kollias CFMIP/GCSS Boundary Layer WG June 11, 2009, Vancouver, Canada
Motivation • There are decade-long comprehensive observations at ARM • Southern Great Plains site • To document diurnal cycle of different convection regimes • e.g. fair-weather non-precipitating shallow cumulus • e.g. late-afternoon precipitating deep convection • To assess convection theories • To construct composite case for LES or CRM studies • To feedback on improvement of GCM parameterization • Outline • Diurnal cycle study (2 slides) • The vertical velocity observational study (11 slides)
Diurnal Cycles in Warm Seasons ABRFC Precipitation CMBE ARSCL Cloud Fraction Clear-sky Fair-weather Shallow Cumulus Late afternoon Deep Convection
Use ARM data to assess convection theories The impact of boundary layer inhomogeneity on deep convection On the preconditioning of free troposphere humidity for deep convection Precip Leads MSE STD leads Data from LSSONDE Data from SMOS and OK Mesonet
Vertical Velocity Observation • Vertical pointing Doppler Radar • Measure the movement of • scattering targets. • In non-precipitating shallow cumulus, • the target is the liquid water cloud droplet • Usually the terminal velocity of liquid cloud droplet is about ~cm/s, this is much smaller compared to air motion velocity ~ m/s • Thus the vertical velocity of cloud droplet is representative of air motion Pavlos Kollias ARM SGP Millimeter Wavelength Cloud Radar (MMCR)
vertical: 45 m horizontal: 10 m frequency: 10 s Vertical Velocity Observation • There is no retrieval in clear air nor the precipitating part of the cloud; it is particularly good for non-precipitating liquid-phase shallow cumulus • Pavlos has retrieved data and made hourly averages, from 1999 to present • Data detail: • Hydrometeor fraction / low-dBZ fraction • In-cloud Updraft / downdraft • In-cloud updraft / downdraft fraction Pavlos Kollias Updraft fraction = cloud fraction * in-cloud updraft fraction Updraft mass flux = updraft fraction * updraft velocity
Methodology • Make diurnal composites for different convection regimes • Fair-weather non-precipitating shallow cumulus • Shallow cumulus before late-afternoon deep convection • How to average clouds with different cloud base heights and preserve the intrinsic shape of the vertical profiles? e.g. Mass Flux Average cloud base height
Non-Precipitating Shallow Cumulus • What do we know from LES? (BOMEX, an ocean case) • Cloud fraction, updraft fraction updraft mass flux peak above cloud base and then decrease with height • Updraft velocity increases with height • Updraft fraction dominates in cloud fraction • Updraft mass flux dominates in net mass flux • Brown et al, 2002 (SGP, 06/21/1997, a land case) • Similar profile shapes are shown for updraft fraction and updraft mass flux • This one day simulation will serve as a qualitative comparison in the following Figures from Siebesma and Cuijpers 1995; Siebesma et al, 2003
Updraft Mass Flux Updraft mass flux = updraft fraction * updraft velocity • vertical shape • diurnal variation • comparable to LES study of Brown et al, 2002
Updraft Fraction Updraft fraction = cloud fraction * in-cloud updraft fraction
In-Cloud Updraft Updraft velocity decreases with height. Such behavior is different from that of updraft in shallow cumulus over ocean.
Remarks on updraft velocity Buoyancy diurnal cycle based on composite sounding for non-precipitating shallow cu Composite average CIN = 67 J/kg
Downdraft & Net Mass Flux The symmetry between updraft and downdraft might be explained by an overshooting of non-entraining plume. But this is not consistent with highly entraining plume for shallow convection.
Remarks on symmetry • the radar might only see the undilute convective core, but not cloud edges • the contribution to mass flux from eddies, whose scale is smaller than the radar observational resolution, is not included in the data • to resolve these issues, we will need higher resolution radar data (e.g. WCAR data) and radar simulator studies combined with large eddy simulations
Remarks on Shallow Cumulus • Observed composite updraft fraction and updraft mass flux are comparable to LES results in both magnitudes and the shapes of vertical profile. • Observed composite updraft has magnitude below 1m/s and decreases with height. The composite updraft mass flux does not dominate in the composite net mass flux; this is because observed downdraft and updraft have very similar statistics. • These observations are different from we knew according to BOMEX, an ocean case. Some tentative explanation provided. • Will LES help if we try similar sampling and compositing as observations?
