Sunday, July 10, 2005

Today's flights consisted of two research missions. N43RF was scheduled to take off at 15 UTC for a pre-landfall mission into Hurricane Dennis, which was a weakening Category 4 expected to make landfall sometime during the late afternoon along the Florida panhandle.

N42RF was scheduled to takeoff at 19 UTC for a post-landfall mission into Dennis. Landfall occurred at 1925 UTC, so N42RF reached the IP just as the southern eyewall had reached the shore. The storm had weakened to a borderline Category 2/3 just prior to landfall. The plan called for a coastal survey, dropping sondes just upstream of tower locations that had been coordinated with ground teams. After the coastal survey, a series of figure-4's would be executed through the core of the storm as it was inland. Wind fields derived from the TA radar would be used to gain a better understanding of how the surface wind field contracts and weakens upon landfall. This will be useful in understanding how the boundary layer of landfalling hurricanes evolves and in designing parametric inland decay models.

The storm looked highly asymmetric on the LF radar, with an open southern eyewall and a maximum in reflectivity on the NW side of the storm, in the offshore flow. On the inbound leg, comparisons of flight-level and SFMR wind speeds showed that the flight-level winds were about 10-20% higher than the surface winds, a typical value for tropical cyclones. However, on the west side of the storm the flight-level winds decreased dramatically and the surface winds were slightly stronger than the inbound leg. This resulted in surface winds that were stronger than the flight-level winds, sometimes significantly so (e.g., on some portions of the leg the surface winds were twice the value of the flight-level winds). The first thought was that there was something wrong with the SFMR winds, since the aircraft was within about 5-10 nm of the shoreline and perhaps something in the offshore flow was giving an artificially high reading for the SFMR. Two drops were released in this section, and both roughly confirmed the SFMR winds, indicating that this likely was a real phenomenon. A similar relationship was observed on N43RF before the storm had made landfall, providing further support to the contention that this was a real phenomenon and not (at least entirely) an artifact of the SFMR. After the coastal survey was completed, N42RF turned inland to make a penetration into the storm. We completed a series of legs in all quadrants of the storm. The ride was relatively smooth. Most of the active convection was on the north side of the storm, though toward the end of the mission the eyewall (or the remants of the eyewall) showed a wavenumber-2 pattern to the reflectivity field.

One notable aspect of this flight was that the pilot allowed us to drop sondes over land. We dropped several sondes over land, mostly in the north and eastern sections of the storm to map out the decay of the low-level and surface wind field in the portion of the storm where the wind field was the strongest and broadest at landfall. Unfortunately, winds were not obtained from most of the sondes below about 900 mb. However, temperature, pressure, and humidity were provided. These sondes should provide an excellent opportunity to document the changes in the lower- tropospheric and boundary layer thermodynamic and dynamic fields after landfall. The NASA ER-2 aircraft did not fly.

The forecast situation indicated the possible development of a tropical depression about 1200 nm east of the lesser Antilles. There was also a tropical wave in the eastern Caribbean, which was forecasted to be in the vicinity of IFEX/TCSP operations by around Tuesday or Wednesday. N42RF was scheduled to return to Costa Rica on Monday, while N43RF had to remain behind for some aircraft maintenance necessitated by encountering graupel in an eyewall downdraft on the pre-landfall mission. The plan calls for N43RF to return to Costa Rica as soon as possible, hopefully by Tuesday. Both planes will have to undergo 50-hr mandatory maintenance, followed by a hard down day for the crew. The plan is to have both aircraft ready to go by Thursday.

Rob Rogers
HRD Field Program director