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Paul J. Neiman
,
M. A. Shapiro
, and
L. S. Fedor

Abstract

This is the second of two articles describing the evolving structure and selected physical processes within an intense extratropical marine cyclone observed during the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA) field program. Part I describes the 24-h frontal-cyclone evolution through 6-h horizontal analyses of observations taken by specially deployed observing systems from air, land, and sea. Part II presents frontal-scale and precipitation structures and physical processes from analyses based primarily on research aircraft observations taken during three phases of the cyclone's life cycle. Horizontal analyses at 350 m above ground level describe the cyclone's mesoscale frontal baroclinic structure and associated flow patterns. The vertical structure and evolution of the cyclone's cold front, warm front, and bent-back front are illustrated in cross-sectional analyses of potential temperature, wind velocity, potential vorticity (PV), front-relative transverse flow vectors, diabatic heating, and PV tendencies. Of particular interest are the lower-tropospheric positive PV anomalies within the warm front and within its bent-back extension westward into the polar airstream. Airborne radar reflectivities and Doppler velocities provide a detailed account of the precipitation elements and associated wind flow patterns in the vicinity of the fronts, including mesoconvective radar reflectivities of greater than 50 dBZ and cross-frontal convergent flow exceeding −20 × 10−4 s−1. Time series traces of the 1-s aircraft observations show large and rapid changes in meteorological variables as the aircraft crossed the narrow frontal zones.

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Michael W. Douglas
,
L. S. Fedor
, and
M. A. Shapiro

Abstract

During the 1987 Alaska Storms Program, a polar low that developed over the northern Gulf of Alaska was investigated during two flights by a WP-3D research aircraft on successive days. Analyse based on data obtained from omega dropwindsondes during the first flight showed the small (∼300 km) horizontal scale of the vortex The vortex center was characterized by suppressed cloudiness on both days and was decidedly warmer at low levels than its surroundings. The vorticity associated with the polar low was largest new the surface and decreased rapidly with height on the first day, with only a very weak circulation evident by 700 mb. Coldest 500-mb temperatures and lowest static stabilities were found directly above the surface low. On the second day the vortex was more intense at midtropospheric levels than on the previous day, and convective clouds extended to higher levels.

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V. E. Derr
,
R. S. Stone
,
H. P. Hanson
, and
L. S. Fedor

Abstract

Surface measurements of solar flux and total integrated liquid-water content, radiosonde data, and infrared satellite images are analyzed in conjunction with radiative transfer calculations to derive an empirical parameterization for the shortwave transmissivity of continental stratiform water clouds. The data were collected near Denver, Colorado, over a period of six years. Seventeen days on which uniform stratiform clouds persisted over the observing site were selected for detailed analysis, and form the basis for deriving the parameterization. A mulitiple reflection radiative transfer model is employed to estimate stratus cloud transmissivity in terms of the measurable liquid-water path (LWP). A nonlinear fit of estimated transmissivities to the corresponding observations of LWP yields close agreement with a previous, more complicated parameterization. The derived expression for cloud transmissivity is used to predict mean daily surface fluxes for 61 days during which periods of stratiform clouds were observed over the Denver area. A comparison between predicted and measured fluxes shows agreement to within ±4%, with best agreement for clouds of moderate optical thickness. Potential sources of error are identified with sensitivity studies.

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Michael W. Douglas
,
M. A. Shapiro
,
L. S. Fedor
, and
Lea Saukkonen

Abstract

The structure of a subsynoptic-scale cyclone (polar low) that formed along the cast Greenland ice edge during the 1989 Coordinated Eastern Arctic Research Experiment (CEAREX) is described using NOAA WP-3D research aircraft and satellite observations. Satellite imagery showed a well-defined 400-km-wide comma cloud pattern during the time of the aircraft observations. Frontal zones with marked wind shifts and thermal gradients near the surface were associated with the polar low. Although the polar low's vorticity decreased rapidly with height between 950 and 800 mb, a secondary vorticity maximum was found in the upper troposphere associated with a short-wave trough. Doppler radar and aircraft observations showed the structure of the main precipitation band to be similar to that of other polar lows observed by research aircraft. In general, the structure of the polar low resembled, except for horizontal scale, the structure of midlatitude cyclones at a similar stage of cloud field evolution.

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