Reanalysis of West Pacific tropical cyclone intensity 1966-1987

Recently published studies (Emanuel 2005; Webster et al. 2005) have found upward trends in tropical cyclone intensity in the western North Pacific. These studies, however, were based on historical best track intensities, which have many shortcomings due to operational procedures and the quality and frequency of intensity estimation available (i.e., those reported in Chu et al 2002).

Prior to 1988, aircraft reconnaissance was available in this region and reliable minimum sea level pressure observations were often available to aid the Joint Typhoon Warning Center in intensity assignment. During this period, however, the operational methodology used to assign maximum wind speed given the MSLP (i.e., the pressure wind relationship) was not constant. The use of different pressure vs. wind relationships in this region has likely resulted in variations in TC intensity that are of the same order as the recently reported upward trends.

This study makes use of a recently developed pressure wind relationship that estimates maximum surface winds based on MSLP and accounts for variations of latitude, environmental pressure and TC size. This newly developed pressure wind relationship is based on 15 years of aircraft reconnaissance estimates of MSLP in the Atlantic and East Pacific, best track estimates of maximum surface winds and storm location, and information about the environmental pressure and tropical cyclone size from the NCEP reanalysis. Independent testing of this pressure wind relationship during the 2005 Atlantic Hurricane season resulted in nearly normally distributed errors with a biases of 1.7 kt, mean absolute errors of 6.0 kt, and standard deviations of 7.9 kt.

The purpose of this study is to determine how much of the reported trends in West Pacific tropical cyclone intensity are potentially due to the differing operational pressure vs. wind relationships. To this end, this study will homogeneously reanalyze the maximum surface winds associated with West Pacific tropical cyclones when aircraft based MSLP estimates are available (i.e., 1966-1987) using this new technique. The resulting TC climatology and temporal trends will be discussed.