Monash Radar Workshop
19-20 November 2015
Using a long term tropical radar dataset to inform a cumulus
parameterization development
www.bom.gov.au
A. Protat (BOM), C. Jakob (Monash),
B. Moebis (Monash), V. Kumar (BOM – now a space weather scientist)
The Centre for Australian Weather and Climate Research
A partnership between CSIRO and the Bureau of Meteorology
Motivation : all models struggle with tropical rainfall !
Stephens et al. (2010): “dreary state of precip in global models …”
Global ocean accumulated rainfall ~ OK
BUT …
slightly overestimated in the tropics,
slightly underestimated in the midlatitudes
BUT …
FREQUENCY
Raining too often
ACCUMULATION
INTENSITY
Raining too little
(latitude dependent)
What about :
Our very own model (ACCESS) ?
The Centre for Australian Weather and Climate Research
A partnership between CSIRO and the Bureau of Meteorology
Darwin CPOL rainfall vs ACCESS-G2,-R1, -C1, C2
Nguyen et al. 2014 (QJRMS) : older ACCESS-A "raining too little too often"
Diurnal variability not OK (12 LT peak). Hourly rainfall PDF highlights overestimation of
R < 4 mmh-1, underestimation 4 < R < 40 mmh-1, grid point storms > 40 mmh-1.
What about recent versions of ACCESS and how does that change with resolution ?
CPOL
ACCESS-R1 (12 km)
ACCESS-C1 (4 km) results = ACCESS-R1
Land / ocean contrast. Rain does not get inland
Convective scheme is the culprit.
ACCESS-C2 much
improved PDF but …
CPOL
ACCESS-C2 (1.5 km)
The Centre for Australian Weather and Climate Research
A partnership between CSIRO and the Bureau of Meteorology
New cumulus parameterization concept at Monash
A stochastic cloud model provides the framework for a new parametrization
States considered originally:
•
•
•
•
Clear
Congestus
Deep Convection
Stratiform Clouds
Khouider et al (2010),
Peters et al, (2013)
GCM grid box
Needs information to relate the large-scale state
To the convective-scale state : Weather Radar !
Strategy
1. Characterize statistical properties of tropical convection using dual-polarization radar
(dual-pol used for particle ID and accurate rainfall)  we have one in the Tropics !
CPOL C-band dual-polarization Doppler research radar : 8 wet seasons
Convective cloud top statistics, DSD parameters, Transition from shallow to congestus to deep to
overshooting convection, convective vs stratiform rainfall, convective mass flux and
components (updraft / downdraft and area fraction), diurnal cycle etc …
2. Characterize what the dominant large-scale ”regimes” in the region are
Darwin “Pope” regimes, MJO phase, and ISCCP cloud regimes
3. Characterize how these statistical properties vary as a function of the regimes,
the surface type (land / coastal / ocean), and the diurnal cycle
The Centre for Australian Weather and Climate Research
A partnership between CSIRO and the Bureau of Meteorology
Regime and diurnal variability of rainfall
Polarimetric radar = accurate rainfall estimates (using Zh, Zdr, Kdp)
Rainfall Properties
STRAT
Rainfall Microphysics
CONV
Diurnal Variability of Convection
STRAT
CONV
The Centre
for Australian Weather
and Climate
Take home message: the regime dependence of rainfall
properties
is large
! Research
A partnership between CSIRO and the Bureau of Meteorology
The four tropical cumulus modes
ODC
DC
Cg
Microphysics at 2.5 km is related to convective top heights:
Cg top height increase with Z at 2.5 km (increase in Nw)
DC top height does not depend much on 2.5 km microphysics
ODC happens only for high Z conditions
(due to high Nw and large Do at 2.5km)
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Australian Weather
Weather and
Research
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andClimate
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Research
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betweenCSIRO
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and the
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Composite life cycle of tropical convection
(from 144 heavy rainfall events within CPOL coverage)
number of cells
cell area
cell rain rate
Transition during AM events (Oceanic)
faster than PM events (continental).
Timing of peak rainfall is determined by increase
in mean area, not rain intensity.
Build-up phase (5-10h before peak), growth phase (-5 to 1h)
and decay phase (from 1h after peak rainfall)
Question : how does that look like in a model ?
The
Centre
Australian Weather
Weather and
Research
The
Centre
forforAustralian
andClimate
Climate
Research
A partnership
betweenCSIRO
CSIRO and
and the
Meteorology
A partnership
between
theBureau
Bureauofof
Meteorology
Convective-scale Dynamics and Mass Flux
Convective-scale dynamics = 3D wind (including convective updrafts / downdrafts) and firstorder derivatives (convergence, vorticity)
Convective mass flux = a crucial quantity for models, main component of cumulus
parameterization schemes. Two components : convective area fraction CAF and vertical
velocity (updraft / downdraft) in convective cores Wconv :
Mass flux = r . < CAF > . < Wconv >
Convective mass flux formulation used in GCMs comes from results from large-eddy
simulations and cloud-resolving model outputs, not from observations.
Scanning Doppler weather radars almost directly measure < CAF > (need a convective –
stratiform classification technique, either from reflectivity-only or dual-pol variables). ~ easy bit !
Vertical velocity in convection is more tricky to characterize from observations:
•
•
•
dual-frequency profiler observations : most direct, but only vertically-pointing
Dual – Doppler (DD) radar network : less direct (assumptions), but scanning (3D !)
Single – Doppler radar : much less direct (dodgy assumptions) but larger area than DD
We are exploring all options …
The Centre for Australian Weather and Climate Research
A partnership between CSIRO and the Bureau of Meteorology
Dual – Frequency Wind Profiler Retrieval of
Convective Updraft / Downdraft (Williams 2012, JAOT)
The Centre for Australian Weather and Climate Research
A partnership between CSIRO and the Bureau of Meteorology
(Kumar et al. 2015, JAS) : Convective Mass Flux
from dual-frequency wind profiler
Statistical Properties of Mass flux and its Components
Mass flux as a function of the LS environment
Contributions of cumulus
modes to mass flux
RH
CAPE
The Centre for Australian Weather and Climate Research
A partnership between CSIRO and the Bureau of Meteorology
Single – Doppler radar : Vertical Velocity and Mass Flux
(Kumar et al. 2015, JAOT)
Kumar et al. 2015 JAS : convective area fraction
is more important than vertical velocity
A proxy of vertical velocity is sufficient for mass flux
Developed a parametric approximation of w as a
function of radar reflectivity indices :
• 0-dBZ ETH for the mean vertical profile
• Height-weighted sum of Z for variability
The Centre for Australian Weather and Climate Research
A partnership between CSIRO and the Bureau of Meteorology
Single – Doppler results (Kumar et al. 2015, JAOT)
The Centre for Australian Weather and Climate Research
A partnership between CSIRO and the Bureau of Meteorology
Dual – Doppler 3D wind retrieval (Collis et al. 2013, JAOT)
Variational 3D wind retrieval
(Protat and Zawadzki 1999, JAOT; Collis et al. 2013, JAOT)
Constraints :
•2 non-colinear Doppler measurements (difference in LOS > 15°)
•The anelastic approximation of the airmass continuity equation div (r V) = 0
We use a weighted combination of upward and downward integrations
•W = 0 at ground and at cloud top height
•Smoothness constraints on the second-order derivatives of wind components
•Cost function minimized using conjugate-gradient technique
•Code runs in 30 secs – 1 min for each volumetric scan
A full wet season is available (2005-2006)
Convective Storms: 3D wind retrieval
Wu, Del Genio, 2009, JGR
Verification using
UHF/VHF profiler retrievals
Stats
(monsoon)
NASA GISS
WRF CRM
The Centre for Australian Weather and Climate Research
A partnership between CSIRO and the Bureau of Meteorology
From 3D wind to convective mass flux
Red boxes are the two 80-km
GCM grids for mass flux
The Centre for Australian Weather and Climate Research
A partnership between CSIRO and the Bureau of Meteorology
Dual – Doppler Stats : Convective Mass Flux
From one month of data (~4300 volumetric scans !)
Shaded: 25th and 75th percentiles
The Centre for Australian Weather and Climate Research
A partnership between CSIRO and the Bureau of Meteorology
Alain Protat
Convective-Scale Dynamics and Mass Flux over Darwin
Tel : +613 9669 8128
Thank you
Thank you
@: [email protected]