ABSTRACT - DRUM - University of Maryland

in overall, the sensitivity of the cooling amount to both thresholds is not significant,
especially in the lower fractional threshold. For higher fractional thresholds, over 50%,
the number of available cases decreases rapidly, so results are mostly scattered. This
sensitivity test is consistent with Sherwood and Wahrlich [2003], who also showed
that the results were not sensitive to the lower fractional threshold. The sensitivity test
of the cooling rate also shows no significance, with values between -5 and -10 K/day
in the lower fractional threshold range. This can be seen in Fig. 2.5b. A similar
sensitivity test was performed by changing the box size to 2°x2°, and no significant
difference was found.
2.3.4 Regional distribution
The regional distribution of the 100-hPa cooling during convective events
(stages 2 through 4) is shown in Fig. 2.6. Significant tropopause cooling occurs in the
southern Indian Ocean and central Pacific (February), and in the Asian Monsoon area
(July). Superimposed solid lines are the percentage of deep convection – the fraction of
pixels with brightness temperature below 208 K, measured by the Visible and Infrared
Scanner (VIRS) [Kummerow et al., 1998] onboard the Tropical Rainfall Measuring
Mission (TRMM) [Simpson et al., 1996]. Plotted lines are 1, 3, and 5 % levels. The
brightness temperature of 208K is a typical temperature for the bottom of the TTL, so
these contours indicate the locations of where clouds penetrate the TTL. The
distribution of the tropopause cooling amount generally matches well with the
35