Global Tropical Cyclone Storm Size and Environmental Pressure Climatology using CFSR

A global climatology of the size and environmental pressure of tropical cyclones (TC) occurring in all global ocean basins between 1979 and 2011 is constructed using 0.5° Climate Forecasting System Reanalysis (CFSR) 850 hPa relative vorticity and sea level pressure (SLP) and Regional Special Meteorological Center (RSMC) six hour location information. Only tropical storms and stronger were considered. TC size is defined as the average radius at which the azimuthally averaged relative vorticity decreases to 1x10-5 s-1 while the environmental pressure is defined as the azimuthally averaged SLP at the average radius. Such an approach assumes circular symmetry and to minimize contamination, all 6 hourly points where the 900-600 hPa storm-relative thickness symmetry (Hart, 2003) was greater than (less than) 10 (-10), were excluded from the analysis. This removed 19% and 25% of all 6 hourly points in the Western Pacific and North Atlantic, respectively, predominantly at higher latitudes.

Previous studies using scatterometer, reanalysis, synoptic weather maps, and operational best track data support the notion that Western North Pacific TCs are approximately 100-150 km larger than their North Atlantic counterparts and that environmental pressure differs by 2-9 hPa.

Globally, the mean TC size and environmental pressure are found to range between 300 and 450 km and 1005 and 1013 hPa, respectively, with statistically significant variation across ocean basins. The Western North Pacific and South Pacific have the largest size and lowest environmental pressure while the opposite is true of the North Atlantic and East Pacific. This work agrees with those from previous studies using different methods and measures of TC size and environmental pressure.

Seasonally, most Northern and Southern Hemisphere basins experience the smallest storm sizes and lowest environmental pressures during the highest SST months. In addition, TC size (environmental pressure) is found to generally increase (decrease) poleward while there is a very weak correlation between TC intensity and storm size or environmental pressure.

The basin mean TC size and basin mean TC size at genesis are similar indicating that larger initial disturbances may contribute to larger overall basin mean TC size, such as in the Western North Pacific, but also that storm size, on average, doesn't fluctuate considerably from genesis to lysis but can vary considerably from event to event. This agrees with the findings of Chavas and Emanuel (2010).