Jun A. Zhang

Air–Sea Exchange in Hurricanes: Synthesis of Observations from the Coupled Boundary Layer Air–Sea Transfer Experiment

The Coupled Boundary Layer Air–Sea Transfer (CBLAST) field program, conducted from 2002 to 2004, has provided a wealth of new air–sea interaction observations in hurricanes. The wind speed range for which turbulent momentum and moisture exchange coefficients have been derived based upon direct flux measurements has been extended by 30% and 60%, respectively, from airborne observations in Hurricanes Fabian and Isabel in 2003. The drag coefficient (CD) values derived from CBLAST momentum flux measurements show CD becoming invariant with wind speed near a 23 m s−1 threshold rather than a hurricane-force threshold near 33 m s−1 . Values above 23 m s−1 are lower than previous open-ocean measurements. The Dalton number estimates (CE) derived from CBLAST moisture flux measurements are shown to be invariant with wind speeds up to 30 m s −1 which is in approximate agreement with previous measurements at lower winds. These observations imply a CE/CD ratio of approximately 0.7, suggesting that additional energy sour...

Turbulent fluxes in the hurricane boundary layer. Part I: Momentum flux

Abstract An important outcome from the ONR-sponsored Coupled Boundary Layer Air–Sea Transfer (CBLAST) Hurricane Program is the first-ever direct measurements of momentum flux from within hurricane boundary layers. In 2003, a specially instrumented NOAA P3 aircraft obtained measurements suitable for computing surface wind stress and ultimately estimating drag coefficients in regions with surface wind between 18 and 30 m s−1. Analyses of data are presented from 48 flux legs flown within 400 m of the surface in two storms. Results suggest a roll-off in the drag coefficient at higher wind speeds, in qualitative agreement with laboratory and modeling studies and inferences of drag coefficients using a log-profile method. However, the amount of roll-off and the wind speed at which the roll-off occurs remains uncertain, underscoring the need for additional measurements.

Turbulent Fluxes in the Hurricane Boundary Layer. Part II: Latent Heat Flux

Abstract As part of the recent ONR-sponsored Coupled Boundary Layer Air–Sea Transfer (CBLAST) Departmental Research Initiative, an aircraft was instrumented to carry out direct turbulent flux measurements in the high wind boundary layer of a hurricane. During the 2003 field season flux measurements were made during Hurricanes Fabian and Isabel. Here the first direct measurements of latent heat fluxes measured in the hurricane boundary layer are reported. The previous wind speed range for humidity fluxes and Dalton numbers has been extended by over 50%. Up to 30 m s−1, the highest 10-m winds measured, the Dalton number is not significantly different from the Humidity Exchange over the Sea (HEXOS) result, with no evidence of an increase with wind speed.

First direct measurements of enthalpy flux in the hurricane boundary layer: The CBLAST results

[1]xa0Hurricanes extract energy from the warm ocean through enthalpy fluxes. As part of the Coupled Boundary Layer Air-Sea Transfer (CBLAST) experiment, flights were conducted to measure turbulent fluxes in the high-wind boundary layer of hurricanes. Here we present the first field observations of sensible heat and enthalpy flux for 10m wind speeds to 30 ms−1. The analyses indicate no statistically significant dependence of these bulk exchange coefficients on wind speed. As a measure of hurricane development potential, we compute the mean ratio of the exchange coefficient for enthalpy to that for momentum and find it to be significantly below the lowest threshold estimated by previous investigators. This suggests that the enthalpy flux required for hurricane development may come from sources other than turbulent fluxes, such as lateral fluxes from the vortex warm core, or sea spray. Alternatively, it demands a re-evaluation of the theoretical models used to derive the threshold.

Turbulence Structure of the Hurricane Boundary Layer between the Outer Rainbands

Abstract As part of the Coupled Boundary Layers Air–Sea Transfer (CBLAST)-Hurricane program, flights were conducted to directly measure turbulent fluxes and turbulence properties in the high-wind boundary layer of hurricanes between the outer rainbands. For the first time, vertical profiles of normalized momentum fluxes, sensible heat and humidity fluxes, and variances of three-dimensional wind velocities and specific humidity are presented for the hurricane boundary layer with surface wind speeds ranging from 20 to 30 m s−1. The turbulent kinetic energy budget is estimated, indicating that the shear production and dissipation are the major source and sink terms, respectively. The imbalance in the turbulent kinetic energy budget indicates that the unmeasured terms, such as horizontal advection, may be important in hurricane boundary layer structure and dynamics. Finally, the thermodynamic boundary layer height, estimated based on the virtual potential temperature profiles, is roughly half of the boundary ...

A Study of the Impacts of Vertical Diffusion on the Structure and Intensity of the Tropical Cyclones Using the High-Resolution HWRF System

AbstractThe Hurricane Weather Research and Forecasting (HWRF) system was used in an idealized framework to gain a fundamental understanding of the variability in tropical cyclone (TC) structure and intensity prediction that may arise due to vertical diffusion. The modeling system uses the Medium-Range Forecast parameterization scheme. Flight-level data collected by a NOAA WP-3D research aircraft during the eyewall penetration of category 5 Hurricane Hugo (1989) at an altitude of about 450–500 m and Hurricane Allen (1980) were used as the basis to best match the modeled eddy diffusivities with wind speed. While reduction of the eddy diffusivity to a quarter of its original value produced the best match with the observations, such a reduction revealed a significant decrease in the height of the inflow layer as well which, in turn, drastically affected the size and intensity changes in the modeled TC. The cross-isobaric flow (inflow) was observed to be stronger with the decrease in the inflow depth. Stronger...

Multiscale Structure and Evolution of Hurricane Earl (2010) during Rapid Intensification

AbstractThe structure and evolution of Hurricane Earl (2010) during its rapid intensification as sampled by aircraft is studied here. Rapid intensification occurs in two stages. During the early stage, covering ~24 h, Earl was a tropical storm experiencing moderate northeasterly shear with an asymmetric distribution of convection, and the symmetric structure was shallow, broad, and diffuse. The upper-level circulation center was significantly displaced from the lower-level circulation at the beginning of this stage. Deep, vigorous convection—termed convective bursts—was located on the east side of the storm and appeared to play a role in positioning the upper-level cyclonic circulation center above the low-level center. By the end of this stage the vortex was aligned and extended over a deep layer, and rapid intensification began. During the late stage, rapid intensification continued as Earl intensified ~20 m s−1 during the next 24 h. The vortex remained aligned in the presence of weaker vertical shear, ...

Hurricane Sea Surface Inflow Angle and an Observation-Based Parametric Model

AbstractThis study presents an analysis of near-surface (10 m) inflow angles using wind vector data from over 1600 quality-controlled global positioning system dropwindsondes deployed by aircraft on 187 flights into 18 hurricanes. The mean inflow angle in hurricanes is found to be −22.6° ± 2.2° (95% confidence). Composite analysis results indicate little dependence of storm-relative axisymmetric inflow angle on local surface wind speed, and a weak but statistically significant dependence on the radial distance from the storm center. A small, but statistically significant dependence of the axisymmetric inflow angle on storm intensity is also found, especially well outside the eyewall. By compositing observations according to radial and azimuthal location relative to storm motion direction, significant inflow angle asymmetries are found to depend on storm motion speed, although a large amount of unexplained variability remains. Generally, the largest storm-relative inflow angles (<−50°) are found in the fas...

Asymmetric Hurricane Boundary Layer Structure from Dropsonde Composites in Relation to the Environmental Vertical Wind Shear

This study investigates the asymmetric structure of the hurricane boundary layer in relation to the environmentalverticalwind shearin theinner core region.Data from1878 GPS dropsondesdeployed by research aircraft in 19 hurricanes are analyzed in a composite framework. Kinematic structure analyses based on Doppler radar data from 75 flights are compared with the dropsonde composites. Shear-relative quadrantmeancomposite analysesshow that boththekinematicandthermodynamic boundarylayerheight scalestend to decrease with decreasing radius, consistent with previous axisymmetric analyses. There is still a clear separation between the kinematic and thermodynamic boundary layer heights. Both the thermodynamic mixed layer and height of maximum tangential wind speed are within the inflow layer. The inflow layer depth is found to be deeper in quadrants downshear, with the downshear right (DR) quadrant being the deepest. The mixed layer depth and height of maximum tangential wind speed are alike at the eyewall, but are deeper outside in quadrants left of the shear. The results also suggest that air parcels acquire equivalent potential temperature ue from surface fluxes as they rotate through the upshear right (UR) quadrant from the upshear left (UL) quadrant. Convection is triggered in the DR quadrant in the presence of asymmetric mesoscale lifting coincident with a maximum in ue. Energy is then released by latent heating in the downshear left (DL) quadrant. Convective downdrafts bring down cool and dry air to the surface and lower ue again in the DL and UL quadrants. This cycling process may be directly tied to shear-induced asymmetry of convection in hurricanes.

Observations of Air-Sea Interaction and Intensity Change in Hurricanes

Recent enhancements to the tropical cyclone-buoy database (TCBD) have incorporated data from the Extended Best Track (EBT) and the Statistical Hurricane Intensity Prediction Scheme (SHIPS) archive for tropical cyclones between 1975 and 2007. This information is used to analyze the relationships between large-scale atmospheric parameters, radial and shear-relative air‐sea structure, and intensity change in strengthening and weakening hurricanes. Observations from this research illustrate that the direction of the large-scale vertical wind shear at mid- to low levels can impact atmospheric moisture conditions found near the surface. Drier low-level environments were associated with northerly shear conditions. In a separate analysis comparing strengthening and weakening hurricanes, drier surface conditions were also found for the intensifying sample. Since SST conditions were similar for both groups of storms, it is likely that the atmospherewasprimarilyresponsibleformodifyingthenear-surfacethermodynamicenvironment(andultimately surface moisture flux conditions) for this particular analysis.