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Richard S. Penc

Abstract

Moisture data from radar and rawinsonde observations during three lake-effect snow events are analyzed to determine entrainment rates. Type I convective boundary layers, which are those driven largely by surface heating, typically accompany these storms. Gathered during the winter of 1990, the data are a subset from the Lake Ontario Winter Storms (LOWS) Project, which deployed a mesoscale network of sensors.

Doppler wind profiler signal-to-noise ratio (SNR) data are used to derive humidity structure function parameter (C2q) time–height series analysis, which are then compared to rawinsonde specific humidity (q) plots. Visual comparison of log(C2q) and q analysis indicated a strongly positive correlation. Radar-derived humidity analysis is used to estimate the depth of the Type I (driven by surface heating), cloud-topped boundary layer (CTBL), which corresponded well with results from LOWS rawinsonde data. Calculations of the contribution of (C2q) to the refractive index structure parameter (C2n) showed the humidity correction factor (α2r) to range from 1.02 to 1.04 within the CTBL, consistent with previous findings for Type II CTBLs. A comparison of entrainment rates, computed via two different methods, were in agreement.

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Bruce A. Albrecht, Richard S. Penc, and Wayne H. Schubert

Abstract

The turbulence and mean structure of oceanic stratocumulus was studied using aircraft data collected during the summer of 1976 off the coast of California. Three cloud-topped mixed layers were studied in detail. They consisted of 1) a thin cloud capped by an inversion at a height of ∼1000 m, 2) a relatively thick but broken cloud layer capped by a weak inversion at ∼600 m and 3) a solid cloud capped by a strong inversion at ∼600 m. The mean temperature, moisture, liquid water and radiative characteristics obtained for these three cases were compared. Heat and moisture fluxes were also calculated and compared.

Although there was considerable variation in the characteristics of the three cloud-topped mixed layers studied, all indicated the validity of the general approach used in simple mixed-layer models of stratocumulus. But for these models to be useful, they should be generalized to allow clouds other than solid clouds to be modeled. The measurements indicate that cloud microphysics may be important in regulating the structure of stratocumulus clouds and that the cloud structure is important in regulating the distribution of radiative cooling. The thin cloud case satisfied conditions for cloud-top entrainment instability, but no evidence of enhanced entrainment was observed. In the analysis it is shown that this cloud may have had insufficient liquid water available to drive the instability. Restrictions on cloud-top entrainment instability criteria are discussed, and it is shown that the instability may be neither a necessary nor a sufficient condition for the breakup of stratocumulus.

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Joanne Simpson, Bruce R. Morton, Michael C. McCumber, and Richard S. Penc

Abstract

The GATE data base for days 261 and 186 is used for a combined observational and numerical investigation of interacting cumulus processes that may be important in the generation of waterspouts. The results suggest that the existence of cumulus-scale parent vortices is a necessary condition for the production of waterspouts, but not in itself sufficient. For generation of a visible funnel, the vortices must undergo intensification below cloud base to sea level during the short time span in which the convective updraft is active.

A high-resolution version of Schlesinger's three-dimensional cumulus model with a Kessler-type precipitation scheme is used to analyze the organization of cumulus-scale vorticity on these two days, which had contrasting thermal stratification and cloud features. On day 261, the soundings near but outside the cloud cluster had a relatively deeper cloud layer with weaker conditional instability and vertical wind shear at low levels. In the numerical simulation of the congestus, downdraft under-runs updraft early, so that the strong vortex pair at midcloud levels do not extend to the surface, where the vorticity remains weak even though a wide range of wind profiles was tried in the model. On day 186, the soundings near the waterspouts showed a more unstable subcloud and lower cloud layer capped by a stable dry layer restricting cumulus growth below 4 km. Four wind profiles were used in this case to initiate the numerical model; two with strong low-level shear resulted in strong parent vortices with their maximum intensity at the surface. These vortices strengthened in the convergence between side-by-side updraft and downdraft, which both extended to the surface, a rare configuration for GATE but characteristic of many midwestern tornadic developments.

The observations from both days suggest that the waterspouts formed ahead of the wind shift, resulting from passage of a gust front, both in zones where it may be surmised that two gust fronts were approaching one another. A brief summary is given of results on tropical gust fronts to provide a basis for discussing their role in the generation of tropical waterspouts, and it is shown that 1) they can produce a favorable environment for the parent vortices and 2) they can cause additional vortex intensification. Order-of-magnitude vorticity calculations suggest that small-scale, low-level convergence may have been sufficient, particularly on day 186 when the waterspout signatures were observed at intersecting convergent features to intensify the parent vortex in as little as 5–10 min.

On day 261, additional model experiments simulating pregust front conditions, namely low-level destabilization and increased shear, show stronger parent vortices at low levels. Reasons for the rarity of GATE waterspouts are suggested, and a renewed observational program is proposed for the Florida Keys relating waterspouts to cloud interactions and boundary layer features.

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Roger F. Reinking, Roger Caiazza, Robert A. Kropfli, Brad W. Orr, Brooks E. Martner, Thomas A. Niziol, Gregory P. Byrd, Richard S. Penc, Robert J. Zamora, Jack B. Snider, Robert J. Ballentine, Alfred J. Stamm, Christopher D. Bedford, Paul Joe, and Albert J. Koscielny

Snowstorms generated over the Great Lakes bring localized heavy precipitation, blizzard conditions, and whiteouts to downwind shores. Hazardous freezing rain often affects the same region in winter. Conventional observations and numerical models generally are resolved too coarsely to allow detection or accurate prediction of these mesoscale severe weather phenomena. The Lake Ontario Winter Storms (LOWS) project was conducted to demonstrate and evaluate the potential for real-time mesoscale monitoring and location-specific prediction of lake-effect storms and freezing rain, using the newest of available technologies. LOWS employed an array of specialized atmospheric remote sensors (a dual-polarization short wavelength radar, microwave radiometer, radio acoustic sounding system, and three wind profilers) with supporting observing systems and mesoscale numerical models. An overview of LOWS and its initial accomplishments is presented.

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