Abstract

Measurements with Doppler radar, and instrumented aircraft and towers, have revealed that surface cold fronts often have cross-frontal circulations organized on a scale of a kilometer or less. These circulations include intense updrafts (1 to 20 m s⁻¹) that result in a narrow band of heavy rainfall. We used a nonhydrostatic model to investigate the mechanism for these updrafts and to isolate those characteristics of the prefrontal environment that result in intense updrafts and narrow bands of heavy rainfall. Our simulations were initialized with a cold reservoir in a manner analogous to that used to produce a gravity current. The similarity between the observations and our simulated frontal flows supports the hypothesis that the flow at the leading edge of surface cold fronts can sometimes be represented by gravity-current dynamics. We also found that the differences between frontal circulations and classical dry gravity currents can be explained by the effects of precipitation and vertical shear. In our simulations, the intense updrafts at the leading edge of the cold air mass were associated with a strong upward-directed pressure force and were not associated with significant parcel buoyancy. The conditions for intense updrafts and heavy rainfall in our simulations were 1) strong deep cold pools, 2) a prefrontal environment that contains deep layers of air that are nearly saturated with a lapse rate that is nearly neutral to moist ascent, and 3) intense low-level vertical shear in the cross-frontal direction of the horizontal wind. These conditions are typical of maritime surface cold fronts that often have strong updrafts and bands of heavy rainfall. In our simulations, the vertical shear in the cross-frontal direction exerted a strong influence on the strength and character of the frontal updraft. For a given magnitude and depth of the cold air an optimal vertical shear existed where the updraft tended vertical, intense, and result in a narrow band of heavy precipitation. With decreasing vertical shear, the updraft tended to weaken, tilt back over the cold air, and result in a broad band of lighter prcipitation. An unsteady system resulted at shears higher than optimal. The dependence of the updraft character on vertical shear is similar to that predicted by recent theoretical work on squall lines.

Abstract

Comparisons are made between the characteristics of several types of rainbands observed in an extratropical cyclone and dynamical mechanisms relevant on the mesoscale.

The warm-sector flow ahead of the cold front and above the cold-frontal zone aloft was unstable to conditional symmetric instability, and theoretical predictions for this mechanism am consistent with several aspects of the warm-sector and wide cold-frontal rainbands. In the case of the warm-sector rainbands, other mechanisms (e.g., wave-CISK and mixed dynamic/convective instabilities) may have also played a role.

The core structure of the narrow cold-frontal rainbands appeared to be affected by an instability that derived its energy from the horizontal shear across the surface front. Also, many aspects of the narrow cold-frontal rainband were similar to a density current. Shear-induced gravity waves appeared to be responsible for the wavelike rainbands observed in the vicinity of the cold-frontal zone aloft.

The orientation of the postfrontal rainbands suggests that energy from the mean flow was responsible for their organization. Convection, in the presence of horizontal temperature gradients and vertical shear, could explain the existence of the postfrontal rainbands through either wave-CISK or a mixed dynamic/convective instability. Since the postfrontal rainbands are often aligned along the thermal gradient, the symmetric instabilities may also play a role in their formation. Buoyant vertical motions under relatively uniform conditions can explain the hexagonally-shaped convective cells observed well behind the cold front.

Abstract

The effects of orography on the mesoscale structures and precipitation processes in warm-frontal, warm-sector, wide cold-frontal, narrow cold-frontal and post-frontal rainbands in four Pacific cyclones are described. The rainbands were tracked with a Doppler radar as they approached the Washington coast and then for ∼150 km inland as they passed over topographic features ranging from modest hills to mountain ranges. The rainbands were affected in a variety of ways by orography, ranging from dissipation to formation, and precipitation from the bands was enhanced and reduced in different situations. These effects are discussed with respect to the large-scale flow, mesoscale air motions and precipitation growth mechanisms.

Abstract

Mesoscale measurements on five Pacific cyclones are used to investigate the formation, movement, development, interaction and dissipation of warm-sector, prefrontal cold-surge, narrow cold-frontal, wide cold-frontal, wavelike and postfrontal rainbands.

Warm-sector rainbands formed near the leading edge of the cold front and often moved away from the front. Prefrontal cold-surge and wide cold-frontal rainbands formed aloft and behind the surface cold front and they also advanced relative to the front. The clearest interactions between rainbands occurred when wide cold-frontal rainbands overtook narrow cold-frontal rainbands; in this case the narrow cold-frontal rainband may be either temporarily or permanently dissipated, or the wide cold-frontal rainband may be dissipated, depending on the relative strengths of the rainbands. Wavelike rainbands, with very uniform properties, were initiated primarily in the vicinity of the cold front aloft; despite their small scale, these rainbands were relatively long-lived. Postfrontal rainbands, some of which contained oriented precipitation cores, moved with the winds within the postfrontal airmass, and exhibited a variety of lifecycles.

Abstract

Previous studies have shown that a surface cold front often coincides with a heavy band of precipitation commonly designated as a narrow cold-frontal rainband. The maximum rainfall rate within this band can exceed 100–200 mm h⁻¹. This study uses a nonhydrostatic two-dimensional cloud model with ice microphysics to investigate the precipitation processes within this type of rainband. Despite the relatively simple initialization and two-dimensionality, many aspects of these storms were well simulated. In these simulations, the intense but shallow updrafts produced large amounts of cloud water that were transformed primarily into rain and graupel within the zone of heavy precipitation and, to a lesser extent, into snow. The graupel and snow produced a zone of trailing stratiform precipitation. While the heavy rainfall could be represented in a warm rain model of the storm, an ice phase was needed in order to replicate the stratiform precipitation. Feedbacks of microphysical processes upon the dynamics of the flow were investigated. Sublimation and melting of frozen hydrometeors produced a pronounced cooling within the cold air mass, which slowly increased the depth and intensity of the cold air mass. This diabatic cooling within the cold air could potentially play a role in maintaining or even intensifying the circulations that lead to these rainbands. Previous studies of these types of fronts have instead concentrated on the role of melting in maintaining these structures through producing a stable layer across the cold air interface that could inhibit mixing.

Abstract

Previous studies have revealed that convective storms often contain intense small-scale downdrafts, termed “downbursts,” that are a significant hazard to aviation. These downbursts sometimes possess strong rotation about their vertical axis in the lower and middle levels of the storm, but studies of how this rotation is produced and how it impacts downdraft strength are lacking. In this study a three-dimensional cloud model was used to simulate a rotating downburst based on conditions observed on a day that produced rotating downbursts. It was found that rotating downbursts may occur when the direction of the wind shear vector in the middle levels of the troposphere varies with height. In the early stages of the convective system, vertical vorticity is generated from tilting of the ambient vertical shear by the updraft, resulting in a vertical vorticity couplet on the flanks of the updraft. Later, the negative buoyancy associated with precipitation loading causes the updraft to collapse and to be eventually replaced by a downdraft downshear of the midlevel updraft. When the direction of the vertical shear vector varies with height, a correlation may develop between the location of the vertical vorticity previously produced by the updraft at midlevels and the location of the developing downdraft. This mechanism causes downbursts to rotate cyclonically when the vertical shear vector veers with height and to rotate anticyclonically when the vertical shear vector backs with height. The rotation associated with the downburst, however, does not significantly enhance the peak downdraft magnitude. The mechanism for the generation of vorticity in a downburst is different from that found for supercell downdrafts, and, for a given vertical shear vector, downbursts and supercell downdrafts will rotate in the opposite sense.

Abstract

Details of the structure of a moderate reflectivity microburst were provided by dual-Doppler radar measurements during the Phoenix II convective boundary layer experiment. The dated allowed high resolution of the descending microburst in both time and space. Thermodynamic fields of virtual potential temperature and buoyancy retrieved from the radar measurements indicated that the downdraft was associated with a minimum in virtual potential temperature, rather than coinciding with a maximum in precipitation loading. The physical separation of the downdraft from the reflectivity maximum was especially pronounced during the later stages of the microburst and was partly due to the tilled reflectivity core descending more rapidly than the downdraft. The downdraft corms also descended at a rate slower than the magnitude of the maximum downdraft so that air was continually converging and entraining into the downdraft above the level of its peak value and was detraining and diverging below it. The retrieved pressure fields and simple analytical calculations showed that this slower descent and internal circulation coincided with an upward-directed pressure form. Simple calculations also suggest that this influence of the pressure force on the vertical accelerations depends strongly on the aspect ratio of the negatively buoyant parce1; horizontally narrow and vertically deep negatively buoyant parcels result in stronger downdraft than wider and shallower parcels. Our study suggests the internal circulation and the relatively slow descent of the peak downdraft should be inherent characteristics of microbursts driven by corms of low virtual potential temperature air, while microbursts driven primarily by water loading could be expected to have a different structure. In the case of the microbursts driven by corms of cool air, observation and recognition of the convergence and divergence associated with the internal circulation provides important precursors to microburst activity. In this study, the Doppler measurements showed that the microburst descending into a stable layer may have enhanced the divergence pattern below the peak downdraft.

Abstract

Recent studies using data from the Tropical Ocean and Global Atmosphere program’s Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) have shown that synoptic-scale areas of extremely dry air can occur in the troposphere over the equatorial western Pacific. These layers of extremely dry air modify convective activity and the vertical profile of radiation in clear air. At the present time there is some disagreement as to the dynamic mechanism responsible for these events and a number of their characteristics are relatively unknown. In this study, the origin and characteristics of the dry air events were investigated through analysis of TOGA COARE rawinsonde data and examination of global analyses from two different forecast centers. These drying events were found to be very common and evidence was presented that their intensity was underestimated in the global analyses. These dry events were shown to most often originate in the Northern (winter) Hemisphere as troughs associated with baroclinic waves intensified and expanded equatorward, leading to a process analogous to Rossby wave breaking. In these cases, the dry air at the edge of the westerlies at upper levels was incorporated into the equatorward extension of thin NE–SW tropospheric troughs, where it subsided and was subsequently advected equatorward. If sufficient subsidence took place, the dry air continued flowing equatorward on the eastern edge of well-defined anticyclones in the lower troposphere. The dry air in one case originated in a Southern (summer) Hemisphere trough that was associated with midlatitude baroclinic waves that propagated equatorward and developed into a series of distinct disturbances along a subtropical jet. In both the Northern and Southern Hemisphere events, the subsiding dry air in the midtroposphere was injected into the fringes of the Tropics, where it was able to reach equatorial regions if it interacted with favorable meridional flow in the Tropics. Past studies have proposed that these intrusions of dry air could induce droughts in the Tropics through decreasing deep convective activity. The implication of this study is that these droughts are actually induced by midlatitude processes.

Abstract

This study investigates a nocturnal mesoscale convective system (MCS) observed during the Plains Elevated Convection At Night (PECAN) field campaign. A series of wavelike features were observed ahead of this MCS with extensive convective initiation (CI) taking place in the wake of one of these disturbances. Simulations with the WRF-ARW Model were utilized to understand the dynamics of these disturbances and their impact on the MCS. In these simulations, an “elevated bore” formed within an inversion layer aloft in response to the layer being lifted by air flowing up and over the cold pool. As the bore propagated ahead of the MCS, the lifting created an environment more conducive to deep convection allowing the MCS to discretely propagate due to CI in the bore’s wake. The Scorer parameter was somewhat favorable for trapping of this wave energy, although aspects of the environment evolved to be consistent with the expectations for an n = 2 mode deep tropospheric gravity wave. A bore within an inversion layer aloft is reminiscent of disturbances predicted by two-layer hydraulic theory, contrasting with recent studies that suggest bores are frequently initiated by the interaction between the flow within stable nocturnal boundary layer and convectively generated cold pools. Idealized simulations that expand upon this two-layer approach with orography and a well-mixed layer below the inversion suggest that elevated bores provide a possible mechanism for daytime squall lines to remove the capping inversion often found over the Great Plains, particularly in synoptically disturbed environments where vertical shear could create a favorable trapping of wave energy.

Abstract

The cloud and precipitation structure and the airflow associated with wavelike rainbands in a cold-frontal zone have been investigated with Doppler radar, instrumented aircraft, rawinsondes and a network of ground stations. The rainbands were oriented perpendicular to the cold front and embedded within wide cold-frontal rainbands. The wavelike rainbands were 20–40 km long, 3–6 km wide, spaced 9–13 km apart and their tops ranged from 3-5 km in height. The radar reflectivities, convergence/divergence and airflow show regular patterns associated with the rainbands.

There is evidence that wavelike rainbands were associated with generating cells aloft. These rainbands may have been initiated by shear instability in the frontal zone, since the resonant mode for such an instability had a similar orientation, movement and spacing to those observed for the rainbands.