Mobile Ground-Based Observations of Landfalling Hurricanes: Current Capabilities and Future Plans

1. Mobile Ground-Based Observations of Landfalling Hurricanes: Current Capabilities and Future Plans Kevin Knupp, Walt Peterson and Dan Cecil Department of Atmospheric Science and Earth System Science Center University of Alabama in Huntsville

2. Outline • Goals (previous deployments) • Instruments • Past research activities – some highlights • Future research possibilities

3. General Research Goals Topics that have been considered at least superficially • Atmospheric Boundary Layer (ABL) Processes • ABL transition: land-to-water and water-to-land • Wind profiles • Structure of ABL eddies, damaging wind streaks, etc. • Intensity change around the landfall time • What factors, such as cold air production over land, control intensity? Topics not yet addressed • QPE (radar) and raindrop size distribution variability • Horizontal and vertical variability; factors controlling variability • Related mesoscale phenomena • Rainband/stratiform region kinematics and vertical transports • Tornadoes, gravity waves, fronts (and their interaction)

4. UAH deployments to date • Hurricane Earl (1998), Tallahassee, FL Adjacent to TLH WSR-88D, east of circulation center • Hurricane Georges (1998), N of Gulfport, MS Close to two DOW radars, remained within west eyewall for several hours • Hurricane Irene (1999), NC Irene turned to the right; no landfall • Tropical Storm Helene (2000), Panama City, FL Modest rain, east of circulation center • Tropical Storm (Hurricane) Gabrielle (2001), Venice, FL Center passed over MIPS, co-located with SMART-R • Hurricane (Tropical Storm) Isidore (2003), Gulfport, MS Sampled the right side, co-located with TTU towers and SMART-R • Hurricane Ivan (2004), 5 km N of Orange Beach, AL Sampled right eyewall, co-located with DOW

5. Instrumentation • Mobile Integrated Profiling System (MIPS) • 915 MHz Doppler wind profiler • X-band Profiling Radar (under construction) • 12-channel microwave profiling radiometer • Lidar ceilometer • Surface instrumentation • Mobile Meteorological Measurement Vehicle (M3V) • Surface measurements • Mobile Alabama X-band (MAX) radar • Dual polarization capability • Under construction, will be ready for the 2007 season. Future

6. Components of the Mobile Integrated Profiling System (MIPS) • 1) 915 MHz Doppler wind profiler • 2) 12 channel microwave profiling radiometer • 3) Lidar Ceilometer • Doppler sodar • Electric field mill • Surface instrumentation • 7) Satellite communication • 8) Parcival disdrometers (new, not shown) Future: X-band Profiling Radar (XPR) will replace the sodar 6 7 5 4 1 3 2 http://vortex.nsstc.uah.edu/mips

7. An ideal MIPS location on 23 June 2003 (BAMEX) • MIPS van and instrument trailer 3. Mobile Meteorological Measurement Vehicle (M3V) 2. Pickup & power trailer 6/23/03

8. Past research topics • Examination of wind profiles and their variability • Boundary layer properties • Fronts – in hurricanes? – yes • Cooling over land can be prominent and important • Cool air + convergence can produce frontogenesis • Intensity change? • Gravity waves • Prominence (Gabrielle) • A noteworthy gravity wave event in Hurricane Earl • The most active weather was associated with a gravity wave • Impact – unknown

9. Analysis of TS Gabrielle (NASA CAMEX-4 campaign) Reflectivity from Tampa Bay WSR-88D • Boundary layer transition for both on-shore and off-shore flow. (The circulation center passed very close to the MIPS) • Generation of shallow warm and cold fronts • Production of cold air over land MIPS & SMART-R Leading Rainband Intense deep convection x

10. 915 MHz profiler moments Regions in terms of precipitation characteristics: • Leading stratiform • Convective region • Inner core region • West flank stratiform Core Flow regimes Off-shore: 0400-1100 UTC On-shore: after 1200 UTC The stable off-shore regime does not exhibit enhanced spectral width, but the on-shore regime does.

11. 915 MHz wind profiles at 30 min intervals TS Gabrielle off-shore, veering with height cool surface air, stable BL on-shore, backing with height warmer surface air, neutral to stable ABL

12. Spatial variability across the coastal zone Surface fluxes produced from cool air flowing over warm water  production of temperature gradient along the coastal waters Analysis based on Doppler radar + profiler MIPS profile Isotachs (m s-1) Descending jet Surface fluxes coastline Cool air, 22 C Warm water, 28 C land water Off-shore flow

13. Spatial variability across the coastal zone Near surface winds are slowed by increase in surface roughness • Flow deceleration is apparent at flow levels over land. • Flow deceleration begins over water, prior to air reaching the land surface. Implication: perturbation pressure gradient force from dynamic forcing. • BL height is not well defined by the isotachs. • Vertical shear above the jet over water is greater than that over land.

14. Temporal variability in the 915 MHz wind profiles, 1330-1430 UTC • Systematic wind variation during this period: • Rapid changes in the wind profile close to the core region • General backing below 1 km • Sharp backing of wind with height is large initially. • Wind speed profile evolves from maximum at 200 m to a jet profile in which the maximum ascends with time. Thus, the jet is initially within the turbulent BL (consistent with Kepert 2001). Jet location near the surface

15. Fronts 0853 0553 0553 Surface Analysis Cool air (T≤23 C) covers a large area (50,000 km2). Stratiform rainfall is widespread downshear of the core of Gabrielle. A warm front formed as Gabrielle approached. This front was associated with tornadoes near 0800 UTC Cool air MIPS location Tornadoes formed along a warm front

16. Fronts 1153 1453 Surface Analysis A cold front appeared after 1200 UTC; occlusion by 1453 UTC. The fronts were shallow, confined to the ABL. The strongest frontal signature occurred at the MIPS site (near the center of the storm) Cold front

17. With regard to cold air production over land, the observations suggest the following hypothesis:

18. Hypothesis: Landfalling hurricanes weaken at an accelerated rate upon ingesting cold (low-valued qe) continental air from the ABL • The presence of cold air within the ABL will produce a rapid weakening in the storm if entrained into the core region. • The cold air is produced by rainfall evaporation within mesoscale downdrafts whose characteristics (relative distribution relative to the center, minimum qe) are predictable. • This hypothesis is consistent with observed rapid weakening over cool water. In fact, the weakening from cold air intrusion may be even more dramatic.

19. Cooling within stratiform precipitation Significant mesoscale downdrafts provided downward transport of low-valued qe air, and thereby produced appreciable cooling at low levels. w derived from EVAD analyses Gabrielle

20. Is this process observed in other storms?The case of Ivan:Cold air from the continent may have produced weakening in the NW flank just before landfall

21. Surface 0043 16 September 2004 Extensive stratiform precipitation is present to the north of the circ center. Cold feeder flow qe = 343 K Ivan appeared to weaken rapidly around the time of landfall, more so than was forecast.

22. Future work in this area: • Further examine the “cold air” hypothesis. • Relate the production of cold air to: • Thermodynamic vertical profiles in advance of the hurricane • Rainband kinematics, i.e., a more detailed description of the mesoscale downdrafts within rainbands and stratiform precipitation • Precipitation properties

23. Future plans More of the same, plus: • QPE (Quantitative Precipitation Estimation) • Disdrometers  profiler  radar calibration and improved Z-R (and eventually polarimetric) relations • Improved real-time QPE with the WSR-88D network • Precipitation growth processes (and drop breakup) • Mesoscale dynamics • Rainband and stratiform kinematics • Inner core processes • Thermodynamics – cooling within stratiform precipitation

24. Rainband Kinematics: A future research thrust Kinematic structure of rainbands in TS Gabrielle (dual Doppler analysis using SMART-R and TBW WSR-88D) Vertical E-W section TS Gabrielle Peak values of updrafts and reflectivity in the rainband : 20 m s-1 and >50 dBZ

25. Raindrop spectrum and size distributions Doppler spectra Size distributions Time: 060226 UTC Ht : 3.578km Vobs: 8.06m/s Dm: 1.67mm Spectral width: 2.52m/s  = 3 Observed distribution Convolved distribution Hydrometeor size distribution Develop a better understanding of precipitation processes, and drop break-up in the turbulent boundary layer.

26. Summary • Mobile instruments have been used to examine: • Flow transition within the ABL across the coastal zone • Gravity waves in the hurricane environment • Cold air production over land, which appears to exert a large impact on hurricane intensity around the time of landfall • Future work should continue to investigate the above plus • Investigations of rainband & stratiform kinematics and thermodynamics • Precipitation physics and improved Z-R relations for accurate QPE, with applications to the future dual-pol capability of the WSR-88D

27. But, this will require a dedicated multi-year field campaign utilizing other resources Towers (~10) Atmospheric & BL profiling (wind, T, rv, cloud, precipitation) Mobile Doppler radars WC-130 NOAA P-3 (2) Doppler radar radar In situ, Dropsondes Radars, Cloud physics In situ, dropsondes, radar

28. Questions? • E-mail: kevin@nsstc.uah.edu • Web site: http://vortex.nsstc.uah.edu/mips

29. Hurricane Earl (1998) • Cold air at the surface can provide an environment conducive to gravity waves • Doppler profiler observations of a gravity wave associated with Hurricane Earl at landfall (M.S. thesis by Barry Roberts)

30. Combined analysis Maximum updraft and downdraft of +13 and -9 m s-1 near the 0.8 km AGL level Low uniform cloud base within the updraft

31. FutureX-band Profiling Radar (XPR) • 9.4 GHz (l = 3.3 cm) • Peak power: 50 kW • Min detectable Z at 4 km: -20 dBZ • Time resolution 1-20 s • PRF ~ 2500 s-1 • Minimum gate spacing: ~30 m • Profiles of Z, W, Doppler spectra • Precipitation & clouds • Boundary Layer Properties 915 XPR

32. Surface instrumentation • T • RH • p • Wind • Solar radiation • Rainfall rate • Electric field • DSD (Parcival optical disdrometers)

33. Conclusions (ABL transition) • Significant temporal variability in airflow at the MIPS site related to gravity waves, large eddies (and boundaries). • Stable off-shore flow exhibited a jet profile that descended from land to water. Wind shear below the jet decreased over water. • On shore flow produced a more unstable BL. • In both cases, flow adjustment within the BL occurred within about 5 km of the coast line • A transition in flow occurs in the onshore (off-shore) cases as deceleration (acceleration) is observed to occur before air passes over the coast line.