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  • Author or Editor: Paul H. Herzegh x
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Paul H. Herzegh
and
Peter V. Hobbs

Abstract

The vertical air motions and microphysical structures of the clouds associated with two mesoscale precipitation systems in the Pacific Northwest are examined using rawinsonde, aircraft and vertically pointing Doppler radar data.

A rainband associated with a prefrontal surge of cold air aloft was found to consist of deep (3–4 km) convective cells. Natural seeding by cirrostratus cloud spread ice crystals throughout the rainband. All of the precipitation growth observed took place through ice-phase processes. Much of the moisture necessary for precipitation growth entered the rainband at low levels in the form of vapor and condensate associated with widespread stratiform cloud. A weakly organized cold-frontal precipitation area was found to consist of snow trails originating from shallow (1–2 km) convective cells in a “seeder” zone above 5.5 km altitude. Below this level the trails swept through a “feeder” zone which consisted of stratiform cloud. Downdrafts observed in one region of the feeder zone, and weak updrafts observed in another, resulted in precipitation growth in this zone being much more limited than that commonly observed in the feeder zones of cold-frontal precipitation systems.

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Paul H. Herzegh
and
Peter V. Hobbs

Abstract

The air motions and growth of precipitation in warm-frontal clouds containing mesoscale rainbands have been studied through Doppler radar, aircraft, rawinsonde and surface measurements.

Precipitation growth in deep warm-frontal ice clouds occurred through a “seeder-feeder” process. About 20% of the total mass of precipitation formed in a “seeder” zone (above 5 km) and 80% of the precipitation formed in a “feeder” zone (below 5 km).

Wavelike rainbands originated from ice particles which fell from linear arrays of convective generating cells in a seeder zone. Riming growth was important in these cells where updrafts reached 60 cm s−1, but deposition and aggregation dominated at lower levels where the updraft velocities were ≤15 cm s−1. Some form of ice multiplication process appeared to be active in the feeder zone.

The growth of precipitation in a warm-frontal rainband embedded in shallow liquid water clouds took place through the coalescence of water drops. The updraft velocities in this rainband reached ∼20 cm s−1 and were due to the low-level convergence of air in a stable region beneath the warm-frontal surface.

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Paul H. Herzegh
and
Arthur R. Jameson

Dual-polarization radar measurements of precipitation are primarily influenced by the size, shape, orientation, and phase of scattering hydrometeors. As a result, these measurements can serve as a tool for remote identification of hydrometeor characteristics.

This paper presents an overview of the definitions, observed values, and applications of differential reflectivity (ZDR) and linear depolarization ratio (LDR) measurements. Brief examples of these measurements are given for widespread stratiform precipitation, a rapidly developing convective cell, and a severe hailstorm. The results outline the role that ZDR can play in the differentiation of rain and solid precipitation, identification of supercooled raindrops above the 0°C level, and identification of hail at the surface. LDR measurements are seen to reveal contrasts in ice-particle shape, orientation, and particle phase. These contrasts are of particular benefit toward delineation of hail regions aloft and identification of mixed-phase particle growth environments.

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Robert A. Houze Jr.
,
Peter V. Hobbs
,
David B. Parsons
, and
Paul H. Herzegh

Abstract

No Abstract.

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Robert A. Houze Jr.
,
Peter V. Hobbs
,
Paul H. Herzegh
, and
David B. Parsons

Abstract

Measurements of the size spectra of precipitation particles have been made with Particle Measuring Systems probes aboard an aircraft flying through frontal clouds as part of the CYCLES (Cyclonic Extra-tropical Storms) PROJECT. These measurements were obtained while the aircraft flew through the clouds associated with mesoscale rainbands at temperatures ranging from −42 to +6°C. Particles ≳1.5 mm in diameter closely follow an exponential size distribution. Above the melting level precipitation occurs mainly in the form of ice particles. In this region the mean particle size of the exponential distribution increases with increasing temperature, indicating that the ice particles grow as they drift downward. The variance of the exponential distribution also increases with increasing temperature above the melting level, indicating that the particles grow particularly well by collection as they fall at various speeds. Passage of the failing particles through the melting level is accompanied by a sudden decrease in the mean and variance of the exponential size distribution.

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Michael F. Donovan
,
Earle R. Williams
,
Cathy Kessinger
,
Gary Blackburn
,
Paul H. Herzegh
,
Richard L. Bankert
,
Steve Miller
, and
Frederick R. Mosher

Abstract

Three algorithms based on geostationary visible and infrared (IR) observations are used to identify convective cells that do (or may) present a hazard to aviation over the oceans. The performance of these algorithms in detecting potentially hazardous cells is determined through verification with Tropical Rainfall Measuring Mission (TRMM) satellite observations of lightning and radar reflectivity, which provide internal information about the convective cells. The probability of detection of hazardous cells using the satellite algorithms can exceed 90% when lightning is used as a criterion for hazard, but the false-alarm ratio with all three algorithms is consistently large (∼40%), thereby exaggerating the presence of hazardous conditions. This shortcoming results in part from the algorithms’ dependence upon visible and IR observations, and can be traced to the widespread prevalence of deep cumulonimbi with weak updrafts but without lightning over tropical oceans, whose origin is attributed to significant entrainment during ascent.

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Peter V. Hobbs
,
Thomas J. Matejka
,
Paul H. Herzegh
,
John D. Locatelli
, and
Robert A. Houze Jr.

Abstract

Detailed information is deduced on the mesoscale organization of precipitation, the structures of the clouds, the air flows associated with mesoscale rainbands, and the precipitation efficiencies and the mechanisms producing precipitation in the rainbands associated with a cold front. Measurements were obtained with quantitative reflectivity and Doppler radars, two instrumented aircraft, serial rawinsondes and a network of ground stations.

The regions of heaviest precipitation were organized into a complex mesoscale rainband in the warm-sector air ahead of the front, a narrow band of precipitation at the surface cold front, and four wide cold-frontal rainbands. The wide cold-frontal rainbands and the smaller mesoscale areas of precipitation within them moved with the velocities of the winds between ∼3—6 km. The narrow rainband, which was produced by strong convergence and convection in the boundary layer, moved with the speed of the cold front at the surface. A coupled updraft and downdraft was probably responsible for the heavy precipitation on the cold front being organized, on the small mesoscale, into ellipsoidal areas with similar orientations.

The precipitation efficiencies in the warm-sector and narrow cold-frontal rainbands were ∼40–50% and ∼30–50%, respectively. One of the wide cold-frontal rainbands, in which there was a steady production of ice panicles in the main updraft, had a precipitation efficiency of at least 80%, whereas another wide cold-frontal band, in which some precipitation evaporated before reaching the surface, had a precipitation efficiency of ∼20%.

Ice particles from shallow convective cells aloft played important roles in the production of precipitation in the wide cold-frontal rainbands and in some regions of the warm-sector rainband. These “seed” ice particles grew by aggregation and by the deposition of vapor as they fell through lower level “feeder” clouds. About 20% of the mass of the precipitation reaching the ground in the wide cold-frontal rainbands originated in the upper level “seeder” zones and ∼80% in the “feeder” zones.

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