Subcloud Layer Inflow Dynamics Experiment

This document is divided into 3 sections:

Program Significance

The interaction of a rainband with a tropical cyclone is greatly affected by the intensity, location, and propagation of the rainband. Convectively active rainbands modify the kinematic and thermodynamic properties of the subcloud air entering the eyewa ll (the inflow air) and, in hurricanes with concentric structure, the rainband may contract and evolve into a new eyewall. Further observation is needed to determine the variety of rainband structures and to provide a more complete understanding of the m echanisms of rainband-hurricane interaction.

Recent observations of landfalling and open ocean hurricanes have indicated that the wind field can vary greatly, both horizontally and vertically, in the lowest level of the storm (less than 5000 ft), especially near rainbands. Because most hurricane re connaissance missions are flown at or above this level, detailed observations using remote-sensing capability and in-situ probes (airborne Doppler radar, C-SCAT, radome gust probe, SFMR, ODWs, airborne expendable bathythermographs (AXBTs), and minidriftin g buoys) below 5000 ft are required to specify better the vertical and horizontal wind distribution in the vicinity of hurricane rainbands. The boundary-layer stability , determined by the air-sea temperature difference, accounts for a significant portio n of the variability in the vertical wind profile and determines the rate of recovery of the boundary layer from rainband perturbations. Knowledge of the rainband location relative to the storm-induced, right side SST cooling is important to our understa nding of this interaction. A better understanding of the mesoscale variability of the hurricane planetary boundary layer and ocean mixed layer will lead to improved specification of surface wind distributions for input to forecasts and warnings.

A better understanding of the surface wind distribution is essential for improved input to hurricane forecasts and warnings as well as for input to storm surge and wave models. Surface wave spectra and boundary layer flux measurements are crucial to the improved understanding of heat moisture and momentum fluxes across the air-sea interface and for parameterization strategies in numerical models.

Occasionally, tropical cyclones move across SST fronts from warm to cold water and experience dramatic decreases in convective activity, as observed by satellite, especially in the eastern Pacific region. Presumably, a spin-down and filling of the vortex follows. It is of considerable interest from a forecast and warning viewpoint to determine the boundary-layer response to this type of transition. Specifically, it is important to understand the relation of surface wind changes to those at standard rec onnaissance flight levels and to the pressure field, as well as to understand the time scales over which significant changes occur.

Data acquired during this experiment will provide the means for improving our understanding of the role of the boundary layer (particularly under the influence of rainbands) in hurricane dynamics and for improving forecasts of storm-related surface winds.

Nominally detection of mesoscale air motion events of up to 10 to 20 nmi (~20-40 km) is expected. Ideally, even smaller scale activity may be resolved. We will construct composite kinematic field and thermodynamic analyses that are representative of the rainband features observed.


  1. Determine the mean vertical profiles of wind, temperature, and moisture in the PBL in the vicinity of hurricane rainbands and in decaying tropical cyclones.

  2. Determine the horizontal SST distribution and wind field at the surface and at other levels in the PBL, including mesoscale modifications induced by major rainbands.

  3. Determine the vertical profile of heat, moisture, and momentum fluxes in the hurricane PBL between the eyewall and principal rainband.

  4. Determine the horizontal distribution of surface wind speed and direction, surface stress, and directional surface wave properties.

  5. Determine the contributions of secondary flows and low-level wind maxima to the vertical fluxes of heat, moisture, and momentum in the PBL.

  6. Determine the radial distribution of cloud-base and mixed-layer depth and the relationship between these quantities along a mean PBL trajectory; i.e., investigate the nature and degree of boundary layer recovery in the moat region between the eyewall and a convectively active, outer rainband.

  7. Document the dependence of the rainband-hurricane interaction upon rainband location, structure, intensity, and propagation relative to the SST cold pool. To carry out this objective, we must study rainbands in different locations relative to storm c enter and in storms of varied structure and intensity.

  8. Determine the relationship of the surface winds to the gradient level winds and the pressure field for decaying tropical cyclones.

Mission Description

Operations will be executed in a tropical cyclone in the Atlantic or Eastern Pacific Ocean, the Caribbean Sea, or the Gulf of Mexico, preferably within 1 h (300 nmi/555 km) of the base of operations.

The primary atmospheric experiment requires profile patterns adjacent to the eyewall and convective rainbands at altitudes as low as 500 ft (150 m). If this requirement is to be met, most or all of each flight must occur in daylight.

To conduct this experiment, both aircraft should have working lower fuselage and tail Doppler radars. The standard flight-level instrumentation, side and down-looking radiometers, the SFMR on N42RF, and the ODW, AXBT, and minidrifting buoy instrumentatio n should be operational. ODWs, AXBTs, and minidrifting buoys must be carried to perform the drops critical to the experiment. If possible, specific humidity and equivalent potential temperature should be on the on-board CRT display and the 10-s printout . The airborne Doppler radar on each aircraft should be set in the F/AST scan mode when on non-radial legs. A requirement for this experiment is for the radome mounted gust probe system, the C-SCAT, and the SFMR to be on N42RF.

There are FOUR options associated with this experiment:

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