Using Dropsonde and Satellite Measurements to Evaluate Mid-Tropospheric Humidity and its Importance in Tropical Cyclogenesis

In an attempt to further unravel the process of tropical cyclogenesis, mid-tropospheric relative humidity is isolated and examined to weigh its importance in African Easterly Wave (AEW) genesis. To prove its proficiency in remotely-sensing mid-level relative humidity, we examine brightness temperature data from channel 19 of the AMSU-B microwave satellite matched with dropsonde point measurements from the NOAA G-IV hurricane hunter aircraft. A radiative transfer model is used to produce a simulated brightness temperature output from the dropsonde data, then brightness temperatures are converted into column-averaged relative humidity between 400-700mb and the satellite and dropsonde values are directly compared. Comparisons prove the AMSU-B mid-tropospheric humidity (MTH) values very accurate.

An analysis of the storm-centered mid-tropospheric relative humidity is carried out using a database of all NHC tropical systems including invests, both developing and non-developing, in the Atlantic Basin from 2001-2009. Storm-centered MTH is shown to be directly correlated to the strength of the system, with more intense storms having a higher average MTH.

To address causality of the observed trend, storm MTH is examined preceding storm intensification or weakening. Higher water vapor concentrations in the mid-troposphere in the storm's environment are proven to be conducive for intensification – particularly in cyclogenesis of NHC invests, and within a 12-hour window preceding intensification. Conversely, low water vapor concentrations are shown to support weakening. Trends are most pronounced when isolating MTH within a 400km radius of the storm center, although they are still evident when examining an 800km radius.

This analysis shows that real-time storm-centered MTH monitoring in a developing or existing tropical cyclone could aid intensity forecasts.