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On the Role of Cloud Adjustment Time Scale In Simulating Precipitation with Relaxed Arakawa­Schubert Convection Scheme


We find the role of cumulus parameterization closure assumption on mean Indian monsoon precipitation prediction
Role of Cloud Relaxation parameters on the
simulation of Indian Summer Monsoon
Deepesh Jain
Centre for Atmospheric and Oceanic Sciences,
Indian Institute of Science
In a General Circulation Model, precipitation by Relaxed Arakawa Schubert cumulus parameterization is sensitive to the choice of specified cloud adjustment time scale. We examine the sensitivity of simulated
precipitation using National Center for Environmental Prediction (NCEP) Seasonal Forecast Model (SFM) over different parts of tropics during the period of Indian summer monsoon. The results show that a
single specified value of cloud adjustment time scale or relaxation parameter performs best only over a particular region and different values are preferred over different parts over the tropics. To find a relation
between cloud adjustment time scale and cloud depth (convective activity) we choose six regions over the tropics. Based on the observed relation between out-going long wave radiation and cloud adjustment time,
we propose a linear cloud type dependent relaxation parameter to be used in the model. The simulations improve over most part of the tropics. e impact of this parameter on the
intraseasonal and interannual variation of ISMR will be studied.
Comparison of SFM simulated JJAS mean precipitation (mm/day) with observed TRMM and GPCP for the year 2008. SC is spatial correlation
and RMSE is root mean square error.
The cloud adjustment time in Relaxed Arakawa Schubert cumulus parameterization is varied by varying the relaxation parameter from 0.10 to 0.50.
Different values give better simulations over different regions over the tropics.
As can be seen from the spatial mean over the
tropics, the simulated precipitation is
overestimated compared to the observations For the same sea surface
temperature, the
simulated precipitation is
For the same sea surface
temperature, the simulated
precipitation has
significantly higher
magnitude than TRMM
(especially the peak
The model simulated precipitation is overestimated
over most parts of the tropics
Model time step is fixed to 30 minutes. In the
control simulations, is set to 0.30
Relaxation parameter,
The choice of relaxation parameter
(and hence cloud adjustment time
scale) has significant influence on
the simulated precipitation over
Renno and Ingersolls (1996) definition of Cloud
adjustment time scale
Outgoing longwave radiation (OLR) was averaged for the period of Indian summer monsoon
season over six different regions of the tropics. The OLR available as daily output was
averaged only for the days showing precipitation more than 0.1 mm/day over the selected
region. A low OLR here represents dominant high and deep clouds. It can be seen that over
the regions where high and deep clouds are dominant, a lower value of relaxation parameter
gives better simulations.
The magnitude of precipitation with
cloud type dependent relaxation
parameter is comparable to TRMM
over Indian region
Spatial mean precipitation over the tropics remains
almost same for all the relaxation parameter values.
0.10 comes closest to TRMM. The
peak precipitation produced by 0.50 is
highest. The cloud type dependent
relaxation parameter gives intermediate
values of peak precipitation
Full-text available
Interdecadal Variability of the Indian Monsoon in an Atmospheric General Circulation Model
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