Impact of convective downdrafts on model simulations: results from aqua-planet integrations

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.