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  • Author or Editor: S. K. Avery x
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W. B. Maguire II
and
S. K. Avery

Abstract

The behavior of precipitation is of great importance in obtaining a better understanding of heat transport estimates and global processes in the atmosphere. This paper discusses improvements in an earlier raindrop size distribution model that utilizes two Doppler wind profilers to obtain accurate measurements of rainfall. A UHF Doppler wind profiler provides a precipitation return while a VHF Doppler wind profiler provides vertical clear-air velocities. The model is tested using simulated data. Sensitivity tests examine the model's ability to estimate rainfall characteristics given typical (noisy) Doppler spectra, and the model's sensitivity to atmospheric parameters. Because it is necessary to estimate pressure, temperature, raindrop temperature, and water vapor pressure at the location of the radar volume, the sensitivity tests examine the effects of errors in the estimates of these parameters. Sensitivity testing of the raindrop size distribution model with simulated data indicates model accuracy and reliability and is necessary in the interpretation of results using real data. Testing shows that the model is very capable at estimating even multiple-peaked drop size distributions from noisy UHF spectra. Sensitivity analyses of atmospheric parameters reveal that model results are quite robust, yielding good accuracy for a wide range of atmospheric parameter estimation errors.

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Wayne M. Angevine
,
S. K. Avery
, and
G. L. Kok

Abstract

Measurements of the turbulent virtual heat flux in the convective atmospheric boundary layer made with a 915-MHz boundary-layer wind profiler-radio acoustic sounding system (RASS) are compared to flux measurements from a King Air aircraft. The profiler-RASS flux was calculated by a refined eddy correlation technique. The measurements were made during the Rural Oxidants in the Southern Environment II experiment in June 1992. The area over which the measurements were made is primarily pine forest, and the dominant weather conditions were hot with light winds. The profiler-RASS measurements and the aircraft measurements agree well. Even under these light wind conditions, a 2-h-average profiler-RASS measurement may be sufficiently accurate to be useful. The instrumental error is estimated to be less than the uncertainty due to sampling of the turbulence.

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Robert Schafer
,
Susan K. Avery
, and
Kenneth S. Gage

Abstract

VHF wind profiler measurements of zonal and meridional winds are compared with the NCEP–NCAR reanalysis at sites in the tropical Pacific. By December 1999 the profilers at Darwin, Australia, and Biak, Indonesia, in the western Pacific; Christmas Island, Kiribati, in the central Pacific; and Piura Peru, in the eastern Pacific had collected between 8 and 13 yr of nearly continuous data. While these profilers routinely observe winds up to about 20 km, only winds at Christmas Island are assimilated into the reanalysis. The long period of profiler operation provides an opportunity to study differences between the profiler and reanalysis winds in the equatorial Pacific, a region with geographically sparse observations. Mean and seasonal mean zonal and meridional winds are used to identify differences in the profiler and reanalysis winds. Two potential causes for the discrepancy between profiler and reanalysis winds are identified. The first of these is related to different spatial and temporal characteristics of the reanalysis and profiler data. The second cause is the geographical sparseness of rawinsonde data, and not assimilating wind profiler observations. The closest agreement between the mean and seasonal mean zonal winds was found at Christmas Island, a site at which profiler winds are assimilated. A good agreement between reanalysis and profiler meridional and zonal winds is also shown at Darwin, where nearby rawinsonde observations are available. The poorest agreement was found at Piura (where profiler winds are not assimilated), the closest rawinsonde is almost 2000 km from the profiler site, and topography is not adequately resolved in the reanalysis.

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Wayne M. Angevine
,
S. K. Avery
,
W. L. Ecklund
, and
D. A. Carter

Abstract

A 915-MHz boundary-layer wind profiler radar with radio acoustic sounding system (RASS) capability has been used to measure the turbulent fluxes of heat and momentum in the convective boundary layer by eddy correlation. The diurnal variation of the heat flux at several heights between 160 and 500 m above ground level and values of the momentum flux for 2-h periods in midday from 160 to 1000 m are presented, as well as wind and temperature data. The momentum flux is calculated both from the clear-air velocities and from the RASS velocities, and the two results are compared.

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Robert Cifelli
,
Christopher R. Williams
,
Deepak K. Rajopadhyaya
,
Susan K. Avery
,
Kenneth S. Gage
, and
P. T. May

Abstract

Drop-size distribution characteristics were retrieved in eight tropical mesoscale convective systems (MCS) using a dual-frequency (UHF and VHF) wind profiler technique. The MCSs occurred near Darwin, Australia, during the 1993/94 wet season and were representative of the monsoon (oceanic) regime. The retrieved drop-size parameters were compared with corresponding rain gauge and disdrometer data, and it was found that there was good agreement between the measurements, lending credence to the profiler retrievals of drop-size distribution parameters. The profiler data for each MCS were partitioned into a three-tier classification scheme (i.e., convective, mixed convective–stratiform, and stratiform) based on a modified version of Williams et al to isolate the salient microphysical characteristics in different precipitation types. The resulting analysis allowed for an examination of the drop-size distribution parameters in each category for a height range of about 2.1 km in each MCS.

In general, the distributions of all of the retrieved parameters showed the most variability in convection and the least in stratiform, with the mixed convective–stratiform category usually displaying intermediate characteristics. Although there was significant overlap in the range of many of the parameter distributions, the mean profiles were distinct. In the stratiform region, there was minimal vertical structure for all of the drop-size distribution parameters. This result suggests an equilibrium between depletion (e.g., evaporation) and growth (e.g., coalescence) over the height range examined. In contrast, the convective parameter distributions showed a more complicated structure, probably as a consequence of the complex microphysical processes occurring in the convective precipitation category.

Reflectivity–rainfall (Z–R) relations of the form Z = AR B were developed for each precipitation category as a function of height using linear regressions to the profiler retrievals of R and Z in log space. Similar to findings from previous studies, the rainfall decreased for a given reflectivity as the precipitation type changed from convective to stratiform. This result primarily was due to the fact that the coefficient A in the best-fit stratiform Z–R was approximately a factor of 2 greater than the convective A at all heights. The coefficient A generally increased downward with height in each category; the exponent B showed a small decrease (stratiform), almost no change (convective), or a slight increase (mixed convective–stratiform). Consequently, the amount by which convective rain rate exceeded stratiform (for a given reflectivity) varied significantly as a function of height, ranging from about 15% to over 80%.

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