Transcript Use ARM data to access convection theories

Observed Updraft & Mass Flux in
Shallow Cumulus
at ARM Southern Great Plains site
Preliminary 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
Clear-sky
Fair-weather
Shallow
Cumulus
Late
afternoon
Deep
Convection
CMBE ARSCL Cloud Fraction
Use ARM data to assess convection theories
On the preconditioning of free troposphere
humidity for deep convection
Data from LSSONDE
The impact of boundary layer
inhomogeneity on deep convection
Data from SMOS and OK Mesonet
Vertical Velocity Observation
ARM SGP Millimeter
Wavelength Cloud
Radar (MMCR)
•
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
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:
•
vertical: 45 m
horizontal: 10 m
frequency: 10 s
•
•
Hydrometeor fraction / low-dBZ
fraction
In-cloud Updraft / downdraft
In-cloud updraft / downdraft fraction
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)
Figures from Siebesma and Cuijpers 1995; Siebesma et al, 2003
– 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
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?
Fair-weather vs. before Deep Convection