The effects of upper tropospheric cloud radiative forcing (CRF) on the atmosphere have been examined using a recent version of the atmospheric general circulation model (AGCM) developed by the Max Planck Institute of Meteorology and the University of Hamburg. This model reproduces satellite‐observed radiative forcing of clouds well overall, except that model maxima somewhat exceed those of observations. Three simulations have been performed where the clouds above 600 mbar have been rendered transparent to all radiation: first, throughout the tropics in the “NC” experiment; then only over oceans warmer than 25°C in the “NCW” experiment; and finally, only over the western Pacific warm oceans in the “NCWP” experiment. The local radiative effects of these clouds when they are present in the model are radiative heating of the middle to upper troposphere due to convergence of longwave and solar radiation; radiative cooling of the tropical atmosphere near and above the tropopause; a large reduction of solar radiation (50 to 100 W/m2) reaching the surface; and a slight increase (5 to 20 W/m2) in the downward longwave radiation at the surface. The removal of cloud radiative forcing significantly alters the circulation of the model atmosphere, as in previous AGCM studies, showing that a seemingly moderate heat source such as CRF is nonetheless capable of widespread influence over the global circulation and precipitation. The experiment responses include a significant weakening (in NCW) or rearrangement (in NCWP) of the Walker circulation. Zonal mean cloud cover, rainfall, and low‐level convergence change modestly in the experiments, while zonal departures of these from their tropical means shift considerably. Regions over the warmest oceans which lose CRF become much less cloudy, indicating a positive local feedback to convection. The experiment circulation changes are diagnosed in terms of simple energy budget arguments, which suggest that the importance of CRF is enabled by the small magnitude of the atmospheric moist energy transport in the tropics. They also suggest that the response of the zonal mean atmosphere may be strongly dependent on the response of zonal eddies and on interactions between surface fluxes and tropospheric lapse rates. The response of the zonal eddies itself should be relatively independent of these interactions.

No abstract

Abstract. Any theory of water vapor in the tropical tropopause layer (TTL) must explain both the abundance and isotopic composition of water there. In a previous paper, we presented a model of the TTL that simulated the abundance of water vapor as well as the details of the vertical profile. That model included the effects of  "overshooting" convection, which injects dry air directly into the TTL. Here, we present results for the model after modifying it to include water's stable isotopologue HDO (where D represents deuterium, 2H). We find that the model predicts a nearly uniform HDO depletion throughout the TTL, in agreement with recent measurements. This occurs because the model dehydrates by dilution, which does not fractionate, instead of by condensation. Our model shows that this dehydration by dilution is consistent with other physical constraints on the system. We also show the key role that lofted ice plays in determining the abundance of HDO in the TTL. Such lofted ice requires a complementary source of dry air in the TTL; without that, the TTL will rapidly saturate and the lofted ice will not evaporate.

Idealized simulations of the tropical atmosphere have predicted that clouds can spontaneously clump together in space, despite perfectly homogeneous settings. This phenomenon has been called self-aggregation, and it results in a state where a moist cloudy region with intense deep convective storms is surrounded by extremely dry subsiding air devoid of deep clouds. We review here the main findings from theoretical work and idealized models of this phenomenon, highlighting the physical processes believed to play a key role in convective self-aggregation. We also review the growing literature on the importance and implications of this phenomenon for the tropical atmosphere, notably, for the hydrological cycle and for precipitation extremes, in our current and in a warming climate. Expected final online publication date for the Annual Review of Fluid Mechanics, Volume 54 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.