Modeling the diurnal cycle using the weak temperature gradient approximation

The weak temperature gradient (WTG) approximation represents a parameterization of the large scale environment in limited domain simulations. Characteristics of convection in WTG simulations are sensitive to changes in the reference thermodynamic profiles which represent the environment outside the domain. Raymond and Sessions (2007) found that precipitation rates in a cloud system resolving model increase with either an increase in reference moisture, or an increase in atmospheric stability.

Given the sensitivity of convection to changes in the reference profiles, the WTG approximation provides a unique opportunity to investigate properties of convection in reference profiles which incorporate observed time dependent changes in potential temperature and mixing ratio profiles. In recent work, Wang et al. (2013) used observed profiles from TOGA COARE to determine the extent to which the observed variability could be accounted for in the thermodynamic profiles. They concluded that the majority of the intraseasonal variability during TOGA COARE was attributed to changes in surface fluxes (not the thermodynamic profiles).

We present another example of incorporating time-dependent observations into WTG reference profiles by imposing the observed diurnal variability in the thermodynamic environment during EPIC2001 to determine the extent to which the diurnal cycle over open oceans is modulated by the thermodynamic environment. In contrast to the Wang et al. results, we find that the diurnal variability in precipitation rate is largely accounted for by changes in the thermodynamic environment.

In all of these studies, an important consideration is the treatment of horizontal moisture advection. Variations include horizontal advection of moisture by mass continuity in the enforcement of WTG, imposing a moisture relaxation independent of the WTG potential temperature relaxation, or absence of moisture advection altogether. We find that details in modeling the diurnal cycle are sensitive to the choice of moisture advection, and that this choice may lead to different conclusions in WTG simulations. Experiments of this type provide opportunities to not only investigate the mechanisms controlling tropical convection in different environmental conditions, but also to improve our understanding of the WTG approximation as a tool.