Subcloud Layer Inflow Dynamics Experiment

This document is divided into FOUR sections:

Summary

This multi-option, dual-aircraft experiment is designed to study the planetary boundary-layer (PBL) structure of the inflowing air of tropical cyclones (TC). The inflow layer or boundary layer delivers the energy needed to maintain the TC but may interact with rainband and eyewall convection in way that leads to intensification or decay. This experiment explores this interaction by examining the storm scale and rainband scale kinematic and thermodynamic structure within the inflow layer. These observations are extended to the smaller PBL scales by detailed turbulent scale measurements in the cloud free regions between rainbands and the areas where inflow feeds rainbands.

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Program Significance

Observational studies of Hurricanes Allen (1980), Floyd (1981), Irene (1981), Earl (1986), and Gilbert (1988) have shown that hurricane rainbands are strong modifiers of the thermodynamic fields in and near them. These few cases also suggest that the rainband's location relative to the storm's circulation center and the motion of the storm affect the intensity of the rainband and how it interacts with the TC. The more intensely convective rainbands in Floyd and Earl were in the forward portion of the storm, where inflow may be enhanced and where the inflowing air may be unmodified environmental air. Strongly convective rainbands, such as the ones observed in Floyd, may be responsible for the significant asymmetries in the eyewall reflectivity pattern if cool dry downdraft air fails to replenish heat and moisture on the way to the eyewall. The weaker dissipating rainband in Irene was more toward the rear of the storm where, apparently, it was affected by outflow from stronger convection in the eyewall. Hurricane Gilbert's left rear rainbands contained relatively moist and shallow outflow layers that allowed significant entrainment of warm moist air back into the atmospheric planetary boundary layer (PBL), resulting in very rapid increases of equivalent potential energy as the air approached the bands.

Theory and simple models present an emerging picture of hurricane rainband distribution as a function of hurricane strength, direction of motion, and environmental flow. These studies suggest that axisymmetric hurricanes with concentric rings of convection are stronger than asymmetric ones, which often have a stationary band complex surrounding the eye. The location of the stationary band complex, often to the right and forward of the storm motion vector, is most likely due to the convergence resulting from the effects of surface drag on the hurricane circulation. Further observations of hurricane rainbands in different locations relative to the storm circulation could extend our understanding of how position in the hurricane affects both the direction and intensity of energy/momentum interactions between band and hurricane. For example in a hurricane with concentric eyewalls this experiment could help to determine how rainbands become outer eyewalls and subsequently cut off the inner eyewall. In an asymmetric hurricane these studies could confirm, modify, or replace current hypotheses explaining the placement, orientation, and propagation of rainbands relative to the hurricane circulation.

Further observations of hurricane rainbands in different locations relative to the storm circulation will generalize and increase our understanding of the role that rainbands play in storm intensity and intensity change. A detailed study of the structure of rainbands in Hurricane Earl indicated that the inflow was modified by cool downdrafts as it crossed the ban. The modified air, if it arrived virtually unchanged at the eyewall, would be much more stable than environmental air and would be expected to cause some of the eyewall asymmetries that were noted in Floyd. Unfortunately, the rate of recovery of this rainband outflow could not be documented directly. The degree of modification and the rate of recovery are, however, of extreme importance, since they may affect eyewall intensity, as well as the surface-induced shear that produces frictional convergence and turbulent fluxes of heat and moisture from the sea surface.

In 1996, new instrumentation and techniques will substantially improve the capabilities of the WP-3Ds and motivate the collection of boundary layer datasets. With the new GPS-sondes it will be possible to get wind and thermodynamic profiles in the boundary layer with ~5-m vertical resolution. Instrumentation capable of measuring turbulent fluxes is now available with the WP-3D radome gustprobe. For the first time, this instrumentation will allow measurement of the entrainment of momentum, heat, and moisture at the top of the PBL. Entrainment is believed to be critical in maintaining the observed increase in PBL energy with decreasing distance from the eye.

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Objectives

The anticipated impact of a set of successful experiments in determining the atmospheric structure of the hurricane PBL over the next few years will be:
  1. The role of the PBL in the dynamics of the storm, including its importance in the development and maintenance of rainbands and storm intensification.

  2. Mesoscale and convective-scale influences that can act to modify the PBL, thereby affecting turbulent fluxes of heat, moisture, and momentum at the surface and above.

  3. The extent to which PBL maintenance is controlled by the sea surface temperature (SST) distribution, mesoscale and convective-scale downdrafts, rainfall evaporation, and between-band subsidence.

  4. A conceptual model of the three-dimensional structure of the PBL with symmetric and asymmetric rainband features and their effects on the low-level wind field.

  5. Improvements of existing boundary-layer parameterizations in numerical hurricane models that are being developed for forecast applications.

  6. Aid to forecasters in determining wind damage potential for input to public forecasts and warnings before hurricane landfall.

  7. Input to storm surge modeling and forecast efforts.

  8. Assistance to planners, designers, and engineers in developing safe, hurricane resistant, structures.

The achievement of these goals is important to NOAA's mission to improve hurricane forecasts and warnings on both the short- and long-term time scales. In the short term, this experiment seeks to provide real-time measurements of winds at the surface and at typical aircraft flight levels. In the long term, improved understanding of the behavior of the hurricane PBL over the ocean and near landfall will lead to improvements in dynamical model predictions and to improved initial data for storm surge models.

The emerging instrument capabilities for this experiment could be equally applied to a variety of atmospheric conditions over fairly large horizontal scales. The intricate coupling between the oceanic boundary layer and PBL play a very important role weekly, monthly, seasonally and annually that we are only beginning to unravel with detailed process studies, including modeling efforts, that have been learned from the hurricane research program.

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Mission Description

Airborne and satellite active and passive microwave remote sensing methods will be used to estimate surface wind and stress fields for comparison with gradient level fields and as forcing fields for coupled atmosphere-ocean response studies. A multiple-component experiment is planned consisting studies of: (1) the hurricane PBL; (2) the PBL in a decaying eastern Pacific (EPAC) TC; and (3) the calibration of surface wind estimation techniques.

The first study focuses on the determination of the vertical profile of the PBL wind and thermodynamic structure in the vicinity of a TC rainband using along-band and cross-band aircraft profiles, GPS-sondes, and airborne Doppler observations. Cross-band aircraft profiles provide the vertical thermodynamic and kinematic structure of the air entering the rainband, while the Doppler radar observations made during along-band stepped descents provide more complete mapping of the three-dimensional wind field and rainband features. Cross-wind patterns between the rainband and the eyewall will document the thermodynamic recovery of the low qe air as it flows inward from the rainband toward the eyewall. Drifting buoys and airborne expendable bathythermographs (AXBT) will help to resolve the SST distribution. Active (C band Scatterometer [C-SCAT]) and passive (Stepped-frequency Microwave Radiometer [SFMR]) remote sensors on the aircraft, will estimate surface wind and wind stress in order to calculate surface fluxes of momentum, sensible, and latent heat using the bulk aerodynamic method. Independent estimates of surface stress, sensible, and latent heat fluxes will be made from linear extrapolation of direct eddy-correlation measurements from the aircraft radome gust probe system and high temporal resolution temperature and humidity measurements at altitudes within the PBL.

The purpose of the second study is to determine the PBL structure in a decaying EPAC TC after it has crossed the San Lucas front into colder water. The goal will be to ascertain the time rate of decay of the PBL wind distribution as the TC decays, and to determine the degree to which the subcloud layer is decoupled from the surface friction layer over the cooler water. The observation strategy will employ the techniques of the first study.

The third study seeks to verify the accuracy of remote-sensing methods for the estimation of surface wind speed and direction. This option involves overflying a series of buoys in the Gulf of Mexico and the east coast of the United States during stable and unstable atmospheric conditions and during offshore and onshore flow for strong and weak wind situations.

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There are THREE options associated with this experiment:

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