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Impact of convective parameterization on high ressolution precipitation
climatology using WRF for Indian summer monsoon
Sourav Taraphdar, P. Mukhopadhyay, B. N. Goswami and K. Krishnakumar
Indian Institute of Tropical Meteorology, Pune – 411008, India. sourav@tropmet.res.in
1. Introduction
The Indian monsoon rainfall shows significant variability in
spatial and temporal scale. The temporal variability causes
intraseasonal, interannual and multi-decadal scale
oscillations and spatial variability causes the heterogeneous
precipitation distribution from north to south and east to
west. These variabilities are caused by two types of forcing
namely slowly varying large scale surface boundary
conditions e. g. sea surface temperature (SST), snow cover
etc. and the regional forcing from topography e. g.
Himalayas and Western Ghats etc. Thus resolving the
regional heterogeneity is one of the key to improve the
precipitation distribution over the region (Giorgi and Mearns
1991). It is also reported that higher resolution AGCM helps
in improving the monsoon simulation, however very high
resolution AGCM requires large computational resources.
Thus the computationally less expensive strategy of running
regional models embedded in AGCM gain popularity in
simulating regional climate at higher resolution since mid
90s.
To evaluate the sensitivity of convective parameterization
schemes on monsoon simulation, several attempts were
made. Ratnam and Kumar (2005) simulated two contrasting
years of monsoon 1987 and 1988 by a mesoscale model
(MM5) at 45 km resolution with variety of cumulus
parameterization schemes. They found Betts-Miller-Janjic
(BMJ) and Kain-Fritsch (KF) has certain merits compared to
Grell. At this resolution, large scale monsoonal features can
be captured but to resolve the physiographical heterogeneity
due to topography, ocean etc. of the region, higher
resolution regional climate models (RCM) are required (Im
et al. 2006). Thus, it remains to be seen whether the mean
monsoon rainfall bias improves with a high resolution (Less
than 20 km) mesoscale model. Therefore, the first objective
of the present paper is to produce robust daily monsoon
climatology over Indian region at high resolution by various
cumulus parameterization schemes and its validation with
available observed rainfall data. Along with this attempt will
also be made to identify the models ability to correctly
capture the underlying spatio-temporal variability of
precipitation. The detail investigation of this kind only can
help to decipher the weakness in the formulation of moist
convection which can be improved further. To gain
sufficient insight for further development that would reduce
the uncertainties in the simulations, the third goal of the
paper is to investigate and identify the possible sources of
deviations in simulation arising from different convective
closures.
2. Model, data used and experimental design
The non-hydrostatic, fully compressible with a terrain
following sigma mass co-ordinate mesoscale model WRF-
ARW version 2.2 developed by National Center for
Atmospheric Research (NCAR) has been used for the
present study. The model is used with two nested domains
with horizontal resolutions of 45 and 15 km and 31 sigma
levels with model top at 10 hPa. The model mother domain
covers the large scale Indian monsoon region where the
nested domain focuses mainly on the Indian land masses.
The physical parameterizations schemes used in the model
is with Lin microphysics, Monin-Obukhov similarity
scheme for surface layer, Yonsei university scheme for
PBL, RRTM scheme for long wave and Dudhia scheme
for short wave in all the numerical experiments. The
experiments are differing only by three convective
parameterization schemes namely KF (Kain and Fritsch
1993), BMJ (Betts-Miller (1986); Janjic 1994) and Grell-
Devenyi (GD; Grell and Devenyi 2002).
In this study, the mother domain simulations are driven by
the National Center for Environmental Prediction (NCEP)/
NCAR reanalysis data at a resolution of 2.5 o . The 6-hourly
SST was obtained by linearly interpolating the daily SSTs
of RTG and used as the slowly varying lower boundary
condition for the model. The model simulation spans from
1 May to 31 October for the year 2001 to 2007. The daily
precipitation simulated by the model is compared with the
daily gridded rainfall data of India Meteorological
Department (IMD) at 1 o x1 o resolutions for the land areas
and for the land-ocean area the GPCP and TRMM rainfall
are used.
3. Results and Discussion
a. The monsoon circulation pattern
It is important to examine whether the driving field in the
mother domain are adequate for the nested domain
particularly in connection with synoptic scale climatic
features of Indian summer monsoon. So the JJAS mean
wind (2001-2007) at 850 hPa for each of the three
schemes and from NCEP/NCAR (NNRP) reanalysis are
showed in Fig. 1. The large scale southwesterly flow over
Arabian Sea and Bay of Bengal (BOB) and a cyclonic
vorticity in the north of BOB are reasonably captured by
all the three cumulus schemes (Figs. 1b-d) as compared to
the NNRP (Fig. 1a).
Figure 1. JJAS averaged mean 850 hPa wind (m s -1 )
from (a) NNRP, (b) BMJ, (c) KF and (d) GD for the
year 2001-2007.
However the wind field by KF (Fig. 1c) over the Arabian
Sea and BOB appears to be stronger than the observation
(Fig. 1a). The BMJ is found to have (Fig. 1b) simulated
the most realistic wind field in both the oceanic basin
where as the GD (Fig. 1d) has produced a weaker wind
field over BOB although the Arabian Sea branch is
reasonably reproduced. The upper air (200 hpa) easterly
and the Tibetan anticyclone are captured by all the three
experiments with BMJ, KF and GD (Figures not shown)
but with varied intensity. The center of the anticyclone is