Advances in Airborne Radio Occultation Profiling in the Tropical Environment of Pre-Hurricane Karl

A large set of airborne radio occultation (ARO) data has been collected by GISMOS, the GNSS Instrument System for Multi-static and Occultation Sensing, while deployed during the PRE-Depression Investigation of Cloud systems in the Tropics (PREDICT) experiment in 2010 to study developing tropical storms in the southern Atlantic and Caribbean region. GISMOS is designed to be operated onboard an aircraft and apply the radio occultation technique to remotely sense the atmosphere by measuring the phase and amplitude of received GPS radio signals. An airborne system makes a dense sampling of a targeted system possible and the deployment of GISMOS during PREDICT was the first large scale use of ARO in a research campaign. High resolution vertical profiles of atmospheric refractivity below the aircraft altitude are obtained from the ARO data. Both geodetic GPS receivers using conventional phase-lock loop tracking and a high speed 10 MHz sample rate GPS recording system were used to collect GPS signal data over twenty-six missions into tropical systems during the development stage. The number of profiles and their depth below the aircraft obtained from the geodetic receivers was limited by the conventional phase-lock loop tracking which does not perform optimally in the lower tropical atmosphere where moisture levels result in sharp changes in refractivity causing rapid changes in GPS signal phase. A much larger set of profiles is obtained from the 10 MHz recorded GPS signal data using an open loop tracking algorithm, which is implemented in a software receiver. The open loop method substitutes modeled Doppler shifts in carrier signal phase for the real time feedback used in phase-lock loop tracking and the open loop method generally results in extending the profiles 2 - 4 km farther below aircraft height. The latest advances have resulted in a four to five times increase in the number of profiles, bringing it closer to the number of dropsonde profiles available during the flights. Open loop refractivity profiles are found to agree well with profiles calculated using European Center for Medium-range Weather Forecasts ERA interim reanalysis and co-located dropsonde data. The tropical disturbance which evolved eventually into hurricane Karl was studied during PREDICT over five days leading to its genesis into a tropical depression on 14 September 2010. Refractivity generally increases in the troposphere with increasing moisture. We use the refractivity changes shown in a subset of ARO profiles obtained in the vicinity of the pre-Karl disturbance over this period to assess the moisture environment of the mesoscale area around the storm center. The refractivity profiles are directly related to pressure, temperature and humidity and will be assimilated into the Weather Research and Forecasting (WRF) model using a Three-Dimensional Variational (3DVAR) Data Assimilation (DA) system to assess the impact of ARO data on the forecast of the developing pre-Karl system.