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.

Abstract

Fluctuations of the planetary-scale waves, represented by spherical harmonics of the 500-mb. geopotential field, are statistically analyzed. A study is made of the prediction of these fluctuations from previous changes and using the non-divergent spherical vorticity equation.

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.

Abstract

Meso β-scale radiosonde data at 75 km spacings and 3 or 1.5 h intervals from the fifth day of AVE-SESAME 1979 (20–21 May) are employed to investigate moisture budgets in thunderstorm environments. Budget values are computed at nine times prior to, during, and after a convective outbreak over Oklahoma. The domain under investigation includes both convective and nonconvective areas, thereby allowing budget comparisons between the two regions.

Findings show that the convective region is characterized by strong horizontal moisture flux convergence in the low levels and weak divergence aloft. Vertical motion carries moisture into the middle and upper troposphere. Magnitudes of the moisture fluxes are directly proportional to storm intensity. The vertically integrated source/sink term also is closely related to the presence and intensity of convective activity. When converted into equivalent precipitation amounts, values correspond closely with those from a rain gage network.

Moisture budgets also are obtained from routine National Weather Service rawinsonde soundings. A comparison of results for similar locations, but derived from the two different resolutions, reveals several common processes. However, magnitudes from the mesoscale data are sometimes an order of magnitude greater than those at the synoptic scale, especially in the convective areas.

Abstract

Dual-Doppler data collected during the Taiwan Area Mesoscale Experiment (TAMEX) are used to study the kinetic energy balance of a subtropical squall line over the Taiwan Straits. Values of each term in the kinetic energy budget equation are assessed from the Doppler-derived winds and retrieved thermodynamic variables using 1-km grid spacing in all three directions. The budget domain has horizontal dimensions of 23 × 40 km^2,/, covering the convective region of the squall line. Results show that vertical totals of the horizontal generation and total buoyancy production terms act as a source of kinetic energy, while vertical totals of dissipation and vertical generation provide the main sinks. The horizontal flux divergence (convergence) of kinetic energy is nearly balanced by the vertical flux convergence (divergence) at most levels. The computed tendencies agree well with the observed tendencies in the lower and upper layers. The vertical total of kinetic energy change is negative for both the observed and computed values, showing the decrease of total kinetic energy of the squall system as it approached the west coast of Taiwan.

Abstract

A thermodynamic retrieval method was employed to investigate the dynamic and thermodynamic structures of a subtropical prefrontal convective rainband associated with the Mei-Yu front on 25 June 1987 over north-western Taiwan. Three-dimensional wind fields were derived from the dual-Doppler data based on the methodology developed at Saint Louis University. Subsequently, fields of perturbation pressure and temperature were retrieved from the detailed wind field using the three momentum equations.

Results show that the maintenance of this long-lived rainband at the times of dual-Doppler analysis is caused by the combined effects of a gust front arising from the convective downdrafts ahead of the front and developments of new convection along the gust front. In the lowest layer, high pressure occurs behind the cold front with low pressure to its southeast. A buoyancy-induced low pressure area lies beneath the frontal updraft corresponding to the rising warm environmental air. The precipitation core associated with the frontal updraft is elongated toward the southeast side with the environmental shear in the mid-and upper troposphere forming the convective downdraft on the warm side of the surface front. This precipitation-induced downdraft transports cooler air downward producing a high pressure area underneath the convective downdraft. This high is accompanied by a temperature deficit resulting in cold horizontal outflows in the boundary layer. Part of these cold outflows interacts with the high-θ_e, southwest monsoonal flow to form a gust front ahead of the surface front. New convection develops along a gust front and then merges with the old convection, thereby prolonging the lifetime of the rainband. The vertical flux convergences and divergences of horizontal momentum by organized convection are largely responsible for forming a midlevel jet and weakening a low-level jet. The momentum budget calculation shows that the horizontal and vertical flux convergences and divergences of horizontal momentum by the mean and eddy motions are the major contributor to maintain the mean momentum.

Abstract

In Part I of this study, structural features of a convective rainband associated with the Mei-Yu front on 25 June 1987 over northwestern Taiwan are investigated. The dynamic and thermodynamic structures and momentum budgets of the convective rainband will be reported in Part II. Dual-Doppler data were collected from CP-4 and TOGA Doppler radars positioned along the northwest coast of Taiwan during the Taiwan Area Mesoscale Experiment (TAMEX). Fields of the system-relative wind and reflectivity were derived in a horizontal domain of 40 km × 36 km using the objective analysis scheme with a 1-km grid spacing in all three directions. There were ten analysis levels in the vertical ranging from 0.8 to 9.8 km. Vertical velocities were computed from the anelastic continuity equation by integrating downward with variational adjustment.

Results show that the low-level jet (LLJ) in the boundary layer provides large vertical shear ahead of the cold front. The orientation of the jet is in the direction almost parallel to the cold front. The depth of the cold air associated with the Mei-Yu front is rather shallow (1–2 km), in agreement with other TAMEX case studies reported in the literature. To the south of the front, winds are from the southwest in the lower troposphere and veer with height in the middle and upper troposphere. The southwest monsoon flow at low levels transports high-θ _e , environmental air toward the wind-shift line. Behind the front, the northwesterly flow dominates, carrying much cooler air from northern China. The low-level convergence enhances lifting, resulting in a narrow band of convection on the warm side of the front. The length of the rainband analyzed is 5–10 km wide and 50 km long and is composed of many cells. Each cell is accompanied by the moderate convective updraft (6–8 m s⁻¹) and weak downdraft (2–4 m s⁻¹). The precipitation is deep but not intense on the warm side of the front. The maximum reflectivity within the rainband is less than 42 dBZ. The precipitation cores are elongated southeastward in the direction parallel to the environmental shear vector between the lower and middle layers. By contrast, the precipitation behind the front is weak and shallow, showing stratiform precipitation. Because the system traveled very slowly within the domain of interest, up to 200 mm of rainfall in 24 h occurred on the west coast of central Taiwan.

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.

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.

Abstract

A long-lived highly organized squall line moved rapidly across the middle Mississippi and Ohio Valleys on 15 April 1994 within a moderately unstable, strongly sheared environment. Over Kentucky and southern Indiana, the line contained several bowing segments (bow echoes) that resulted in widespread wind damage, numerous shear vortices/rotational circulations, and several tornadoes that produced F0–F2 damage. In this study, the Louisville–Fort Knox WSR-88D is used to present a thorough discussion of a particularly long-tracked bowing line segment over central Kentucky that exhibited a very complex and detailed evolution, more so than any other segment throughout the life span of the squall line. Specifically, this segment produced abundant straight-line wind damage; cyclic, multiple core cyclonic circulations, some of which met mesocyclone criteria; several tornadoes; and embedded high precipitation supercell-like structure that evolved into a rotating comma head–comma tail pattern. The bowing segment also is examined for the presence of bookend vortices aloft and midaltitude radial convergence. In addition, the structure of other bowing segments and their attendant circulations within the squall line are discussed and compared with existing documentation. Radar sampling issues and ramifications of the squall line’s complicated structure on the warning process of future similar severe weather events are touched upon as well.