900–700-hPa Static Stability Controls on Tropical Convection in Moist Environments

A moist tropical troposphere is a necessary but insufficient condition for sustenance of deep, widespread convection. When column-integrated relative humidity exceeds 80%, satellite and ground-based observations indicate that a wide range of rain rate occurs—from near zero to ~50 mm day^-1. This study used radar and rawinsonde data from five ground-based observing stations in the tropical Indo-Pacific warm pool; the two datasets quantified rain rate and column-integrated relative humidity, respectively. At each station, the 25^th and 75^th percentiles of mean rain rate within the radar domain were computed, then atmospheric profiles of wind, temperature, and humidity were compared between times during which domain-mean rain rate was in the upper and lower quartiles. The results indicate that 900–700 hPa lapse rate was, on average, about 0.25 K km^-1 steeper during times of upper quartile rain rate. No significant differences in zonal or meridional wind were observed, and reanalysis-derived sea surface temperature and its gradient also were not different between upper and lower quartile times. Analysis of radar data concluded that the domain-mean rain rate was highly sensitive to the areal coverage of intense, convective rainfall that occurs. The areal coverage of convective echo was largely determined by the number of individual convective echoes rather than their sizes, consistent with prior studies.

Model simulations of cumuliform elements in Cloud Model 1 (CM1) show a similar sensitivity to 900–700 lapse rate. Several experiments were run with the same initial humidity profile and constant SST. In the initial conditions, only the 900–700 hPa lapse rate differed. Model-domain mean rainfall is very sensitive to the 900–700 lapse rate, and observed variability in rainfall rate can be simulated by altering this quantity by O(0.1) K km^-1.