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  • Author or Editor: Edward A. Brandes x
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Edward A. Brandes

Abstract

Doppler radar observations are utilized to describe the evolution of the severe thunderstorm updraft mesocyclone and its associated gust front. Tornadoes form within the elliptical mesocyclonic circulation, apparently along the major axis, and may denote a critical development stage.

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Edward A. Brandes

Abstract

Estimates of precipitation are improved when raingage observations are used to calibrate quantitative radar data as well as to estimate precipitation in areas without radar data.

Estimated areal precipitation depth errors for nine rainfalls over a 3000 km2 watershed averaged 13 and 14% (1.5 and 1.8 mm) when the radar was calibrated by networks of raingages having densities of one gage per 900 and 1600 km2. Areal precipitation estimates derived from rainfalls observed at the gages alone produced errors of 21 and 24% (2.5 and 3.0 mm). Adjusting the radar data by a single calibration factor (the simple average ratio of gage-observed and radar-inferred rainfall at all input gages without regard to the spatial variation among ratios) resulted in error reduction to 18% (2.1 mm). Radar data added to gage observations also increased the explained variance in point rainfall estimates above that from gages alone, from 53 to 77% and 46 to 72% for the above gage densities.

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Edward A. Brandes
and
Jerome Spar

Abstract

An investigation into the synoptic conditions necessary for the occurrence of heavy snow on the east coastal plain of the United States has revealed no obvious characteristic antecedent patterns either 12 or 24 hr prior to the onset of snow. The contemporaneous association of heavy snow with east coastal secondary cyclogenesis and with certain synoptic features at the 850-mb level is found to be of limited predictive value.

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Edward A. Brandes
and
Kyoko Ikeda

Abstract

A simple empirical procedure for determining freezing levels with polarimetric radar measurements is described. The algorithm takes advantage of the strong melting-layer signatures and the redundancy provided by the suite of polarimetric radar measurements—in particular, radar reflectivity, linear depolarization ratio, and cross-correlation coefficient. Freezing-level designations can be made with all volumetric scanning strategies. Application to uniform (stratiform) precipitation within 60 km of the radar and with brightband reflectivity maxima of greater than 25 dBZ suggests an accuracy of 100–200 m.

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Edward A. Brandes
,
Guifu Zhang
, and
J. Vivekanandan

Abstract

A unique dataset consisting of high-resolution polarimetric radar measurements and dense rain gauge and disdrometer observations collected in east-central Florida during the summer of 1998 was examined. Comparison of the radar measurements and radar parameters computed from the disdrometer observations supported previous studies, which indicate that oscillating drops in the free atmosphere have more spherical apparent shapes in the mean than equilibrium shapes. Radar–disdrometer comparisons improved markedly when using an empirical axis ratio relation developed from observational studies and representing more spherical drop shapes. Fixed-form power-law rainfall estimators for radar reflectivity (Z H), specific differential phase (K DP), specific differential phase–differential reflectivity (K DP, Z DR), and radar reflectivity–differential reflectivity (Z H, Z DR) were then determined using the disdrometer observations. Relations were produced for both equilibrium shapes and the empirical axis ratios. Polarimetric rainfall estimators based on more spherical shapes gave significantly improved performance. However, the improvement was largely in bias mitigation. Rainfall estimates with the Z HZ DR measurement pair had the highest correlation with rain gauge observations, the smallest range in bias factors from storm to storm, and the smallest root-mean-square error.

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Edward A. Brandes
,
Guifu Zhang
, and
J. Vivekanandan

Abstract

A method for estimating the governing parameters of gamma drop size distributions (DSDs) and associated rainfall rates from polarimetric radar measurements at the S band is examined. The technique uses radar reflectivity at horizontal polarization, differential reflectivity, and an empirical constraining relationship between the DSD shape factor and slope parameter. Retrieved DSD parameters show good agreement with disdrometer observations. Retrieved rainfall estimates are insensitive to drop climatological regime. Comparison with fixed-form power-law estimators reveals that the constrained-gamma method outperforms reflectivity estimators and is roughly equivalent to radar reflectivity–differential reflectivity estimators optimized for local DSDs.

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Edward A. Brandes
,
Guifu Zhang
, and
J. Vivekanandan
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Edward A. Brandes
,
Guifu Zhang
, and
J. Vivekanandan

Abstract

Polarimetric radar measurements are used to retrieve properties of raindrop distributions. The procedure assumes that drops are represented by a gamma distribution and retrieves the governing parameters from an empirical relation between the distribution shape and slope parameters and measurements of radar reflectivity and differential reflectivity. Retrieved physical characteristics of the drop size distribution (DSD) were generally well matched with disdrometer observations. The method is applied to select storms to demonstrate utility. Broad DSDs were determined for the core (high reflectivity) regions of thunderstorms. Largest drop median volume diameters were at the leading edge of the storm core and were displaced slightly downwind from updrafts. Rainy downdrafts exhibited what are believed to be equilibrium DSDs in which breakup and accretion are roughly in balance. DSDs for stratiform precipitation were dominated by relatively large drops. Median volume diameters at the ground were closely related to the intensity of an overlying bright band. The radar measurements suggest that, although DSDs in stratiform rain were also broad and nearly constant in the rain layer, they were not at equilibrium but were merely steady. DSD invariance is attributed to small total drop numbers, which result in few collisions.

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Edward A. Brandes
,
Guifu Zhang
, and
Juanzhen Sun

Abstract

Polarimetric radar measurements are used to retrieve drop size distributions (DSD) in subtropical thunderstorms. Retrievals are made with the single-moment exponential drop size model of Marshall and Palmer driven by radar reflectivity measurements and with a two-parameter constrained-gamma drop size model that utilizes reflectivity and differential reflectivity. Results are compared with disdrometer observations. Retrievals with the constrained-gamma DSD model gave better representation of total drop concentration, liquid water content, and drop median volume diameter and better described their natural variability. The Marshall–Palmer DSD model, with a fixed intercept parameter, tended to underestimate the total drop concentration in storm cores and to overestimate significantly the concentration in stratiform regions. Rainwater contents in strong convection were underestimated by a factor of 2–3, and drop median volume diameters in stratiform rain were underestimated by 0.5 mm. To determine possible DSD model impacts on numerical forecasts, evaporation and accretion rates were computed using Kessler-type parameterizations. Rates based on the Marshall–Palmer DSD model were lower by a factor of 2–3 in strong convection and were higher by about a factor of 2 in stratiform rain than those based on the constrained-gamma model. The study demonstrates the potential of polarimetric radar measurements for improving the understanding of precipitation processes and microphysics parameterization in numerical forecast models.

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Edward A. Brandes
,
Kyoko Ikeda
,
Gregory Thompson
, and
Michael Schönhuber

Abstract

Terminal velocities of snow aggregates in storms along the Front Range in eastern Colorado are examined with a ground-based two-dimensional video disdrometer. Power-law relationships for particles having equivalent volume diameters of 0.5–20 mm are computed for the temperatures −1°, −5°, and −10°C. Fall speeds increase with temperature. Comparison with relationships found in the literature suggests that temperature-dependent relations may be surrogates for relations based on aggregate composition (e.g., plates, columns, or dendrites) and the degree of riming.

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