A Wet-Season Rainfall Climatology To Support Airline Arrivals At Key West

The island of Key West is served by an airport with a single runway, which has a length that is very near to the operational minima for modern narrow-body aircraft such as the Boeing 737. As such, various airline carriers have developed approach and landing procedures specific to Key West, all of which are highly dependent upon the current weather. Critical forecast elements to these airlines include low level wind shear and rainfall probabilities.

As part of a comprehensive revision of the aviation weather program at WFO Key West, Southwest Airlines requested a precipitation climatology for the Key West International Airport covering the convective season, defined for this study as June through September. Over 68 percent of the annual rainfall occurs in the Florida Keys during this four month period, when psuedo-barotropic conditions prevail and synoptic scale influences are minimal. As a consequence of the barotropic nature of the regional environment during this season, the primary driver of convection is assumed to be mass convergence resulting from boundary interactions in the lower troposphere. These boundaries primarily move with the prevailing low level flow which, to a good approximation, is represented by the 1000 to 850 mb mean layer wind.

Hourly rainfall data from the National Climatic Data Center from 1981-2010 were examined. As a baseline, the relative frequency of measurable rainfall at Key West showed near equal chances of rain across an average 24-hour diurnal cycle. This is in stark contrast to the Florida mainland, where wet season rainfall frequencies show a pronounced mid-afternoon maximum. Additionally, comparisons were made between this rainfall climatology and a previously published hourly lightning climatology for the vicinity of Key West Airport.

Twice-daily low level winds were obtained from a corresponding 30-year dataset of Key West rawindsonde soundings. The diurnal rainfall climatology was stratified by boundary layer flow regime (separated into the eight cardinal directions), with the goal of developing a systematic prediction scheme of probable rain chances based on the direction of the boundary layer flow. The diurnal rain probabilities for each flow regime were compared to climatology to determine any influences the time of day or wind direction may have on convective initiation. While easterly and southeasterly flow regimes constituted the majority of the cases from the 30 year analysis, striking results were revealed from other flow regimes as well. Included in this study was an examination of which physical mechanisms were of importance during various convective regimes (e.g., boundary generation, translation, and interaction). As verification of these physical processes, an additional analysis of spatial hourly lightning patterns, also stratified by flow regime and time of day, will also be presented.