Mid-tropospheric Moisture Variations During the Development of Hurricane Karl as Resolved by Airborne GPS Radio Occultation with Open Loop Tracking

Abstract:

The development of hurricane Karl in 2010 was investigated with dropsonde and airborne radio occultation (ARO) measurements from the stage of tropical disturbance within an easterly wave through to genesis of the tropical storm. Infrared imagery showed deep convection with extensive cold cloud tops on 11 September however the storm failed to develop until 3 days later. One possible explanation is the horizontal offset of the mid and lower level circulation centers. We illustrate with airborne radio occultation measurements additional information on the moisture distribution during this stage of development that indicates that average mid-level moisture was lower the following day and then increased again over the next two days prior to development. High sample rate RF data recorded by the GNSS instrument system for multistatic and occultation sensing (GISMOS) was analyzed with a version of the Purdue Software Receiver that has open-loop tracking implemented. Open loop tracking eliminates the feedback loop of conventional receivers that fails in the complex signal propagation environment typical of atmosphere with sharp moisture gradients. The open-loop excess phase profiles routinely sample below 4 km, with half of the profiles extending below 2 km. We retrieve slanted vertical profiles of atmospheric refractivity that can be considered a proxy for moisture in this tropical environment. We illustrate that in the mid to upper troposphere, ARO refractivity profiles sampling different areas within the tropical wave showed characteristics that were consistent with (~150 to 200 km scale) horizontal moisture gradients present in the NWP model representation of the developing tropical storm. Variation in refractivity preceding the development of the pre-Karl system is consistent with increasing moisture near the storm center. The ARO observations almost double the amount of thermodynamic data over that provided by the dropsondes. They provide interesting complementary measurements that sample regions off the flight path, which in some cases could not be sampled by dropsondes because of unsafe flying conditions due to deep convection. We discuss current challenges with geometric optics retrieval methods, which can produce biases in the lower troposphere, and some possible improvements.