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Ann. Geophys., 26, 1877-1887, 2008
www.ann-geophys.net/26/1877/2008/
© European Geosciences Union 2008
Impact of convective downdrafts on model simulations: results from aqua-planet integrations
S. Sahany and R. S. Nanjundiah
Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore-560012, India
Abstract. The role of convective scale downdrafts has been examined, using the
NCAR-CAM3.0 aqua-planet configuration. We find that, convective downdrafts
make the atmosphere more unstable thus increasing the convective available
potential energy (CAPE) of the atmosphere. It is noticed that, although the
rate of CAPE consumption increases with the incorporation of downdrafts,
the generation of CAPE increases with a higher rate. Also, it is noted
that there is a reduction in the deep convective rainfall, with the
inclusion of downdrafts, which is primarily due to the re-evaporation of
precipitation within the downdrafts. There is a large increase in the low
cloud fraction and the shortwave cloud forcing with the inclusion of
convective scale downdrafts in the cumulus scheme, which along with the
evaporation within the downdraft causes cooling in the troposphere.
This is in contrast to previous studies on the impact of downdrafts using single
column models. In Zhang and McFarlane (1995), using a single column model, it
was shown that with the increase in the strength of the downdrafts, there
was a reduction in CAPE. In the present study, using an aqua-planet
framework,
it is shown that CAPE is actually found to increase when the downdrafts are
incorporated into the cumulus scheme, as compared to the case when there are
no downdrafts. The rate of CAPE consumption which, is the same as the rate of
stabilization of the atmosphere, is found to increase, but the mean CAPE as
such is higher compared to the case when there are no downdrafts.
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