Sahany, S., and S. K. Mishra
Theoretical and Applied Climatology, 107, Nos. 3-4, 547-562.
Abstract
In this work, an attempt is made to systematically evaluate the effect of convective scale downdrafts on the
model-simulated rainfall, in both aqua- and actual-planet frameworks, using the NCAR CAM3. From the aqua-planet
simulations, it was found that there is a reduction in the total rainfall (TRF) with increase in the intensity of
downdrafts, which is primarily attributed to the reduction in the deep convective component (DRF). However, with
stronger downdrafts, the shallow convective and the large-scale components (SRF and LRF, respectively) are found to
increase. The reduction in DRF is due to the increased evaporation of convective precipitation within the down-
drafts. It is found that, with intense downdrafts, there is an increase in relative humidity throughout the troposphere,
due to the combined effect of both moisture and temperature. There is an overall increase in specific humidity of the
atmosphere with stronger downdrafts, excepting at around the 900-hPa level. In addition, there is a reduction in
temperature throughout the troposphere, primarily due to the reduction in the overall temperature tendency due to
moist processes and that due to the radiative processes. The changes in the radiative forcing are found to be primarily
due to a significant increase in the low cloud fraction with strong downdrafts. In the actual-planet framework, it is seen
that, with strong convective downdrafts, there is a reduction in TRF and DRF and a corresponding increase in SRF and
LRF, similar to the results obtained from the aqua-planet simulations. The vertical structures of the thermodynamic
variables (RH, q, and T) show similar sensitivity to the downdraft intensity as that seen in the aqua-planet
framework. Sensitivity of frequency and intensity of model-simulated rainfall to the downdraft intensity was
also analyzed, and it was seen that there were significant differences in the frequency distribution of rainfall. It was
seen that, with an increase in downdraft intensity, there is an increase in the frequency of light rain (1–10 mm/day) for
TRF with a corresponding reduction in all other rainfall bins. A similar behavior was seen for the DRF as well,
while the SRF and LRF components showed an increase in rainfall accumulation in all the bins. In addition, the impact
of convective downdrafts on the mean spatial pattern of rainfall is also analyzed, for the DJF and JJA periods
(boreal winter and summer, respectively). For the DJF period, with strong downdrafts, it was seen that grossly
over the whole domain, there were a reduction in DRF and an increase in SRF and LRF. In contrast, during JJA,
although a major part of the domain showed a reduction in DRF, there were regions like western Arabian Sea and the
Somali coast with increase in DRF with intense downdrafts. The SRF and LRF components, however, show a spatially
homogeneous increase over almost the entire domain with increase in downdraft intensity.
Citation Sahany, S., and S. K. Mishra, 2011: Effects of Convective Scale Downdrafts on the Rainfall Simulation in NCAR-CAM3. Theoretical and Applied Climatology, Vol. 107, Nos. 3-4, 547-562.