Quantifying environmental control on tropical cyclone intensity change

Composite analysis is used to examine characteristics of tropical cyclones (TCs) undergoing different intensity changes in the Western North Pacific (WPAC) and North Atlantic (ATL) ocean basins. Using the cumulative distribution functions of 24 h intensity changes from the 2003–2008 best track data, four intensity change bins are defined: rapidly intensifying (RI), intensifying, neutral, and weakening. The Navy Operational Global Atmospheric Prediction System (NOGAPS) global analysis and TRMM Microwave Imager (TMI) data are then used as proxies for the real atmosphere, and composites of various environmental fields believed relevant to TC intensity change are made in the vicinity of the TCs. These composites give the average characteristics near the TC, prior to undergoing a given intensity change episode.

Statistically significant differences are examined between RI storms and the other groups. In comparing RI with weakening TCs, there are found to be significant environmental differences, as expected. RI storms had weaker deep-layer vertical wind shear, warmer sea surface temperatures (SST), and higher mid-level relative humidity in both basins. However, the large-scale low level convergence and relative vorticity were similar. In the WPAC, RI events occurred in environments with higher instability, while in the ATL the instability was similar. In comparing the environment between RI with intensifying TCs, very few differences were found. In the ATL, RI storms had weaker vertical wind shear, while the instability was similar. In the WPAC, RI storms had higher instability, while the vertical wind shear was similar. In both basins, the SSTs, low and mid-level relative humidity, low level relative vorticity, and low and upper level divergence patterns were similar. Since the SSTs were similar for intensifying and RI storms, an important finding of this work is that the rate at which TCs intensify is not critically dependent on SST.

TCs in both basins were more intense prior to undergoing an RI episode than an intensifying or neutral episode. In the WPAC, the two groups had a similar translational speed and heading, and initial position. In the ATL, RI storms were located farther south than intensifying storms, moved significantly faster, and had more westward component to the heading.