The elements of the thermodynamic structure of the tropical atmosphere

Deep convection plays an important role in driving the large-scale circulation and the complex interaction between moist convection and the large-scale circulation regulates the thermodynamic structure of the tropical atmosphere. 

The convectional thoughts of the thermodynamic structure of the tropical atmosphere are that the horizontal temperature in the free troposphere is homogeneous, which is referred to as weak temperature gradient (WTG), while the vertical structure follows a moist-adiabatic lapse rate. However, it is not known how accurate WTG holds and which moist- adiabatic process the tropical atmosphere indeed experiences. This study centers around the horizontal and vertical structure of the tropical atmosphere and uses the global storm resolving simulations (GSRMs) from ICON at 2.5 km to investigate them

The virtual effect or the vapor buoyancy effect arises from that the molecular weight of water vapor is much smaller than that of dry air. With the same pressure and temperature, this virtual effect makes moist air lighter than dry air. As the horizontal buoyancy differences are eliminated by convection gravity waves, virtual temperature, a temperature variable including the moisture conditions, is expected to be homogeneous. Then, to obtain a homogeneous virtual temperature horizontally, the absolute temperature has to change to accommodate the horizontal moisture difference. The model results show that virtual temperature is relatively homogeneous at mid- and lower troposphere. Therefore, the virtual effect plays a very important role in the horizontal temperature structure, making the absolute temperature colder in moist regions and warmer in dry regions. However, in the upper troposphere, both the absolute temperature and the virtual temperature are not homogeneous, and vary as a function of moisture, indicating a weakening influence of convection gravity waves there.

We use saturation equivalent potential temperature (theta-es) to explore the vertical structure of the tropical atmosphere. Theta-es is expected to be conserved above the lifting condensation level (LCL) if calculated following the exact moist-adiabatic process that tropical atmosphere undergoes. The pseudo-adiabat and the reversible-adiabat with the effect of condensate loading are compared. To minimize the horizontal differences in theta-es due to moisture, we also define theta-es to account for the virtual effect and the condensate loading effect. The model results suggest that the actual moist-adiabatic process that tropical atmosphere experiences is between the pseudo-adiabat and the reversible-adiabat with the effect of condensate loading assuming air parcels originating from 972 hPa. 

The above results are broadly consistent with the results from ERA5 reanalysis.