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Yeong-jer Lin
and
John A. Coover

Abstract

Dual-Doppler data collected from 1646 to 1648 MDT 14 July 1982 in Colorado are used to study the kinetic energy budget of a microburst-producing thunderstorm during its mature stage. Values of each term in the kinetic energy budget equation are assessed from the Doppler derived winds and retrieved thermodynamic fields using a fourth-order finite differencing with 0.5 km grid spacing. Results indicate that vertical totals of the horizontal generation and horizontal flux divergence terms act as a source of kinetic energy, while a vertical total of dissipation is a sink. The horizontal flux divergence term is nearly in balance with the vertical flux divergence term. Similarly, the vertical generation and total buoyancy production terms have the same order of magnitude but opposite signs at most levels. In the lower layer, where the microburst dominates, the kinetic energy is transported downward. In the middle and upper layers, the kinetic energy is transported upward due to the storm's strong convective updrafts.

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Yeong-Jer Lin
and
Robert Hughes

Abstract

Some dynamic and thermodynamic structures of a microburst-producing storm, which occurred on 14 July 1982 in Colorado, were studied in detail during the storm's quasi-steady mature stage. Dual-Doppler data from 1646 to 1648 MDT, collected during the project of Joint Airport Weather Studies (JAWS) at Denver's Stapleton International Airport, were objectively analyzed to produce a three-dimensional wind field. The domain of interest had a horizontal dimension of 10 km by 10 km centered on the microburst. There were 19 analysis levels in the vertical, ranging from 0.25 to 8.5 km AGL. The horizontal grid spacing was 0.5 km, while thevertical grid spacing varied from 0.25 km near the surface to 0.5 km at levels above I kin. Vertical velocities were computed by integrating the anelastic continuity equation downward from the storm's top with variational adjustment. Subsequently, fields of deviation-perturbation pressure and virtual temperature were recovered from a detailed wind field using the three momentum equations. These fields were then subjected to internal consistency checks to determine the level of confidence before interpretation.

Findings demonsUate that the thermodynamic retrieval method is feasible for investigating the structure and internal dynamics of the storm. Variational adjustment substantially reduces errors in vertical velocity fields. Results show that the microburst being investigated is embedded within the high-refiectivity region with heavy precipitation. A strong downfiow impinges upon the surface, producing a stagnation mesohigh inside the microburst. This high is accompanied by low pressure in the strongest outflow regions, forming a pronounced horizontal perturbation pressure gradient outward from the high-pressure center. Such pressure patterns are in good agreement with the surface observations in similar cases for two different storms. The outflow regions extend from the surface to approximately I km height with maximum divergence in excess of lO-: s-L The outflow air is negatively buoyant due to evaporation in the outsldn of the microburst. In the middle troposphere, hish pressure forms on the upshear side of the main ulxlraft with low pressure on the downshear side due to dynamical interactions between the updraft and the sheared environmental wind. The retrieved buoyancy field agrees well with the updraft-downdraft structure with warming in the updraft and cooling in the downdraft. The combined effects of perturbation-pressure gradients, buoyancy and precipitation loading are responsible for maintaining vigorous convection of the downdrafis which produced the strong diverging outflow at low levels.

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Tai-Chi Chen Wang
,
Yeong-Jer Lin
,
Hsi Shen
, and
Robert W. Pasken

Abstract

In this study, structural features of a subtropical squall line that occurred on 17 May 1987 over the Taiwan Straits, were investigated using dual-Doppler data collected during the Taiwan Area Mesoscale Experiment (TAMEX). Fields of the storm-relative wind and reflectivity were derived in a horizontal domain of 45 km × 25 km using an objective analysis scheme with 1 km grid spacing in all three directions. There were ten analysis levels in the vertical ranging from 0.3 to 8.8 km. Vertical velocities were computed from the anelastic continuity equation by integrating downward with variational adjustment.

Results show that many structural features of a subtropical squall line are similar to those for a fast-moving tropical squall line. A low-level jet (LLJ) associated with the frontal system provides the necessary strong shear at lower levels. On the front side of the squall line front-to-rear flow prevails at all levels and is accompanied by shallow rear-to-front flow on the back of the line. There are many individual cells embedded within the squall line. Relatively weak convective downdrafts occur between the cells and behind the main cells. Convective downdrafts on the rear of the main convective updrafts are essential to transport cooler midtropospheric air into the lower layer. Part of the negatively buoyant air from the rear continues to move forward colliding with the advancing high θ e air in the boundary layer. As a result, new convective cells form ahead of the old cells, thereby prolonging the life time of the squall line. In the convective region the low-level front-to-rear inflow is lifted at the leading edge to form the main updrafts. The lifted air continues to flow west in the middle and upper levels heading toward the trailing stratiform region. The interaction between the convective updraft and downdraft plays an important role in maintaining the three-dimensional circulation within the squall line.

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Yeong-Jer Lin
,
Hsi Shen
,
Tai-Chi Chen Wang
,
Zen-Sing Deng
, and
Robert W. Pasken

Abstract

A thermodynamic retrieval method was used to study the dynamical and thermodynamical structure of a subtropical squall line, which occurred on 17 May 1987 over the Taiwan Straits. Three-dimensional wind fields were derived from the dual-Doppler data based on the methodology presented in Part I of this paper. Subsequently, fields of perturbation pressure and temperature were retrieved from the detailed wind field using the three momentum equations.

Results show that the overall structural features of this subtropical squall line are similar to those of a tropical squall line. The orientation of the squall line is almost in a north–south direction. In the lowest layer, the gust front is located to the immediate east of the main convective updrafts. High pressure occurs behind the gust front with low pressure to its east. A buoyancy-induced low pressure area lies beneath the convective updraft corresponding to the ascending warm environmental air. In the middle and upper layers, high pressure forms on the upshear side with low pressure on the downshear at the leading edge. The orientation of horizontal pressure gradients is approximately in the direction of the average shear vector in the domain. The retrieved temperature field agrees well with the updraft–downdraft structure. The convective updrafts are warmed by the release of latent heat by condensation. Conversely, cooling prevails in the convective downdrafts due, in part, to evaporation. Precipitation loading further decreases buoyancy of the downdraft air in the high reflectivity areas. To the rear of the main (old) cells the rear-to-front air is negatively buoyant resulting in a sloping downdraft. As the cool descending midtropospheric air approaches the surface, it spreads out to form a cold outflow behind the gust front. Part of the descending air moves forward, colliding with the advancing environmental warm air at the leading edge to form new cells ahead of the old cells. The interplay between a cell's cold surface outflow and the low-level shear within the system contributes to the maintenance of the subtropical squall line. The momentum budget calculation shows that the horizontal and vertical flux convergences/divergences of horizontal momentum by the mean and eddy motions are the major contributor to maintain the mean momentum.

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