Hurricane Boundary Layer Structure

1. Hurricane Boundary Layer Structure Ralph Foster, U. WA Robb Contreras, U. Mass CBLAST Workshop 2005

2. Motivation • Hurricane track forecasts have improved • Hurricane intensity forecasts have not improved at the same rate • Simplest model suggests that the ratio of PBL momentum flux to enthalpy flux is a key parameter in hurricane intensity (e.g. Emanuel, 1995: )

3. Science Question • Presently most PBL structure analyses separate the flow into Mean + turbulence • New data suggest that the hurricane PBL flow frequently divides into Mean + “lineal organization” + turbulence • Does the “lineal organization” represent an unparameterized contribution to the hurricane momentum and enthalpy fluxes?

4. Present Results • Wurman and Winslow (1998) found intense sub-km rolls inside RMW in hurricane landfall • Morrison et al. (2005) found convincing evidence of rolls in WSR88-D radar in four landfalls • Characterized wavelength, aspect ratio, strength • 30 to 60% of volumes sampled had rolls • Crude estimate: 2x’s surface stress when rolls are present • Various SAR studies frequently show lineal patterns at the scale and orientation expected for PBL rolls. • John Schroeder SMART radar scans in hurricane landfalls frequently find a variety of lineal features • Rolls, Streaks, Internal Waves? • Foster (2005) Theory: Mean + Rolls + turbulence is expected hurricane basic state

5. Refine Science Question • We assume that rolls enhance all fluxes • Morrison et al. (2005) suggest 2x’s stress compared to standard parameterization • Foster (2005) suggests 30% in mid-PBL • We must characterize when, where and how often rolls form • Can we predict roll formation? • Is tau_rolls/H_rolls = 1? • If Yes, then just “bump up” turbulence (done). • If No, then must be parameterized and added to forecast models. • Do rolls organize sea spray?

6. CBLAST Data • IWRAP (U. Mass) • High-resolution profiles of Wind vectors and Rain reflectivity • Surface wind vectors O(100 m) • SFMR (U. Mass) • Wind speed and rain rate O(km) • “Drennan” aircraft data • SMART Radar (TX Tech.) • Single and dual Doppler wind scans • SAR acquistions

7. Constraints • Roll momentum flux effects are addressable • Roll enthalpy fluxes are more challenging

8. New Analyses • To present, IWRAP profiles & surface wind vectors have been validated • High resolution ought to resolve roll effects • E.g. Small-scale wind variability & stress (u*) • Precipitation organization • SMART Radar • Separate various lineal features (streaks, rolls, g-waves) • Compare to model of Foster • Drennan aircraft analyses • (We left space for Will) • Theory • Better characterization of surface flux effects (momentum and enthalpy) • Validation against more data

9. IWRAP • Process data into highest possible resolution • Characterize when, where, how often lineal features are present • Characterize the strength of the roll circulations • Look for organization in the backscatter (surface, spray, rain)

10. Building Momentum • Special section on Coherent Structures in Hurricane Boundary Layers at next AMS Air-Sea Interaction Meeting, 2006 in Atlanta.

11. Useful New Data Sets • High-resolution LOS Doppler wind lidar can resolve lineal features • Combine ET/BATS fluxes with spatial (Lidar and Radar) scans. • Coincident enthalpy & momentum fluxes during roll events

12. MODIS 250 m Radiance hPBL ~800 m, Coupled l ~ 1.7-2.0 km Cloud Streets

13. Leg 1: Near-Surface Along Wind Mean VLOS subtracted Theory: Roll VLOS Primarily Along-Roll

15. Leg 2: Near-Surface Across Wind Mean VLOS subtracted Theory: Roll VLOS Primarily Cross-Roll

16. Leg 1 Spectra • Apparent lLOS peaks at 1.7 km and 300-900 m • Possibly Near-Surface Streaks (Foster, 1997, Drobinski et al., 2004)