Relationship of Environmental Relative Humidity with Tropical Cyclone Intensity and Intensification Rate

The importance of environmental relative humidity (ERH) to tropical cyclone genesis and intensification has long been recognized by theoretical, modeling and observational studies. Yet a quantitative relation between ERH and TC intensity and intensification rate is not known largely due to the lack of observations over open oceans. The Atmospheric Infrared Sounder (AIRS) on board Aqua satellite provides global coverage of atmospheric temperature and humidity profiles and thus presents us an unprecedented opportunity to examine the relationship of large-scale environmental parameters with tropical cyclone development. We have analyzed 198 tropical cyclones from September 2002 to November 2010 and sorted the ERH profiles by radial distances to storm center, four quadrants relative to store movement, storm intensity and intensification rate. We find that the ERH generally decreases with time as storms evolve. Intense TCs (Category 3 and above) have higher ERH than weaker storms, but the relation of ERH with storm intensity is not monotonic. Instead, the ERH increases nearly monotonically with the increasing storm intensification rate. Azimuthal asymmetry in the ETH and intensification rate relation appears small in the near environment (200-600 km from the storm center) with slight higher ERH in the quadrants following the storm movement. However, the azimuthal asymmetry is evident in the far environment (>600 km from the storm center), with the moistest quadrant to the left of the storm movement following the storms where RH is clearly positively correlated with the intensification rate. Comparison of the ERH between the TCs in the North Atlantic and Western Pacific oceans is also conducted.