The role of atmospheric stability in deep tropical convection

To a good approximation, the horizontal temperature in the tropical atmosphere is homogeneous. This observation forms the basis of the weak temperature gradient (WTG) approximation often used as a parameterization of the large-scale flows in limited domain simulations. Various tropical waves, such as easterly waves, can perturb the potential temperature profile in a region, resulting either in an increase or decrease in atmospheric stability. As shown in Raymond and Sessions (2007; hereafter RS07), this change in stability can have a substantial effect the precipitation efficiency of the convection: Increased stability decreases the level of maximum vertical mass flux, which concentrates the inflow at low levels thus enhancing surface fluxes and increasing tropospheric moisture and precipitation. This modification in the vertical mass flux also results in a decrease in the normalized gross moist stability (NGMS; ratio of moist entropy export to moisture import).

Expanding on the ideas in Raymond and Sessions (2007), we further investigate the role of stability in convection using a cloud resolving model (CRM) in the WTG approximation. To implement WTG, we specify a reference profile of potential temperature which is representative of the environment surrounding the domain. This reference profile is maintained in the model by imposing a hypothetical vertical velocity which counters buoyancy anomalies produced by heating. In an ``unperturbed'' state, the reference profile is generated by running the CRM to radiative convective equilibrium (RCE) in non-WTG mode. In RS07, the RCE reference profile was perturbed to mimic the effect of an easterly wave by imposing a cooling in the lower troposphere and warming aloft, thus increasing atmospheric stability.

The current work expands on RS07 in two ways: 1) Rather than prescribing a specific perturbation to the reference profile, we incorporate observational soundings from the TPARC-TCS08 field program; 2) In addition to considering the effect of increased stability on NGMS and precipitation rate, we also investigate the relationship between stability, saturation fraction (precipitable water divided by saturated precipitable water), and precipitation rate.

In order to incorporate observational temperature and moisture profiles in our WTG simulations, we subtract profiles taken during a non-developing disturbance (analogous to RCE) from those taken during various stages of developing typhoon Nuri. The difference is then added to the RCE reference profile to represent the local environment in the genesis stages in several WTG simulations. The modeled results compare quite well to the observations.

An important extension to the analysis portion of RS07 is the consideration of the sensitivity of precipitation to saturation fraction, and specifically how this is affected by changes in the environmental stability. To quantify this relationship, we introduce an ``instability index'', defined as the average saturated moist entropy in the lower troposphere minus the average mid-tropospheric saturated moist entropy. Changes in the vertical profile of potential temperature are quantified by the instability index and compared to the saturation fraction, NGMS, and precipitation rate.