Moisture Variability in a developing cyclone as measured by airborne GPS radio occultation

Spaceborne GPS radio occultation has generally been found to provide good information on the vertical structure of the upper troposphere and lower stratosphere. This has resulted in an overall impact in numerical weather prediction for synoptic scale weather systems. Although there has been some work showing a potential impact of GPS radio occultation profiles in forecasts of tropical cyclones, there has not been enough data collected to date in the near storm environment to determine whether the GPS radio occultation data is useful in such mesoscale convective environments. For the first time, dense GPS radio occultation observations have been collected near developing cyclones using an airborne system, with concurrent dense sampling by dropsondes. These observations were planned to investigate the genesis phase within tropical waves in the Pre-Depression Investigation of Cloud systems in the Tropics (PREDICT) campaign in 2010. The horizontally sampling rays provide an integrated measure of moisture over a relatively long path, however because most of the influence is local to the tangent point, the retrieved profiles do show the atmospheric variations at smaller scales, sufficient to distinguish lateral variations in moisture within the tropical wave. Observations made with tangent points inside the central region have more moisture in the 3.5 to 7 km height range than at greater distances, indicating preconditioning of moisture necessary for development, consistent with the dropsonde profiles. Occultation refractivity profiles are shown to agree closely with dropsonde profiles that are in close proximity to the occultation profile tangent point locations. The dropsonde data have been assimilated into the WRF model for the 5 day period over which hurricane Karl developed. We simulate radio occultation observations using model profiles extracted at different locations relative to the center of circulation and compare them to the recorded data in order to demonstrate to first order the sensitivity of the technique to mesoscale structure. This GPS radio occultation experiment paves the way for assimilation of this type of data with greater confidence in the forecasting of tropical cyclones.