A satellite analysis of mid-tropopsheric humidity and its impact on tropical cyclone genesis and intensification

We use microwave satellite observations of mid-tropospheric humidity (MTH) to examine the relationship between environmental humidity and tropical cyclone (TC) activity in the North Atlantic basin over the period 2002–2013. Our analysis reveals spatially coherent changes in MTH over the tropical North Atlantic that are highly correlated with variations in accumulated cyclone energy (ACE). Fewer storms are generated in dry years and those that do form tend to originate outside of the driest regions. In contrast, genesis locations in humid years are more evenly distributed across the Atlantic main development region (MDR) with a substantially higher percentage of storms developing into hurricanes. By compositing MTH measurements as a function of future storm intensity change, we show that Atlantic Easterly Waves (AEWs) with higher MTH environments are more likely to develop into tropical depressions and that this signal is evident at least 24 hours prior to genesis. Similarly, TCs of all categories are more likely to intensify (weaken) over the subsequent 24 hour period when the mid-troposphere is more (less) humid. We also demonstrate that this signal is primarily evident near the storm center rather than in the larger scale environment, emphasizing the importance of microwave measurements for its detection and quantification. The MTH signal is also shown to be temporally and spatially distinct from microwave-based retrievals of total precipitable water (TPW) which underscores the distinction between low-level and mid-level water vapor. The potential applications of these measurements for real-time forecasting and model evaluation will be highlighted.