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

This study investigates the evolution of a mesoscale convective system that formed during the Taiwan Area Mesoscale Experiment (TAMEX) on the 19 June 1987. With respect to the upstream flow, the convective system formed in the lee with rainfall totals from this system exceeding 100 mm. Any event that produces over 100 mm of rain in 24 h is thought to be capable of producing, flooding over the steep orography of Taiwan. Analysis of Doppler radar data showed that the convective cells during this heavy rainfall event repeatedly formed near a fixed location over the foothills and moved slowly (˜4 m s⁻¹) northward. Although the radar reflectivities within these cells were relatively modest (35–40 dBZ), the repeated passage of slowly moving cells partly supplemented by relatively steady, stratiform rainfall during the later stages of the event resulted in the high precipitation totals. The heavy rain in this event resulted from a number of factors including 1) a moist, convectively unstable southerly flow of tropical origin, 2) a shallow convergence zone on the western side of the island dividing flow with a northerly component from that with a southerly component, 3) a quasi-stationary area of storm formation, and 4) a mesoscale environment that produced convective systems with a favorable storm structure and movement. Due to the fact that the large-scale forcing—as evidenced by vertical ascent calculated using rawinsonde data—was small, the authors believe that the conceptual model of low-Froude number flow around the island of Taiwan in the presence of heating can be used to explain the local convergent region that initiated this convective event. A numerical simulation of flow around the island of Taiwan in the presence of surface heating predicted a persistent quasi-stationary area of convergence in the foothills near the location of the observed convection. In this study the hypothesis is discussed that this persistent, quasi-stationary area of convergence may have also played a role in maintaining this convective system. These results and the application of this conceptual model will he discussed within the more general context of forecasting flash floods in Taiwan and the United States.

Abstract

Time continuous data assimilation or four-dimensional data assimilation (FDDA) is a collection of techniques where observations are ingested into a numerical model during the simulation in order to produce a physically balanced estimate of the true state of the atmosphere. Application of FDDA to the mesoalpha and subalpha scales is relatively new. One of many strategies for undertaking FDDA on the mesoscale is to employ Newtonian relaxation on increasingly finer horizontal grids. Encouraging results were found using this technique by Kuo et al. on a 40-km grid and by Stauffer and Seaman in a nested model with a 10-km inner grid. In these studies, the model is nudged toward the observations through adding an extra term(s) based on the difference between observations and the model predictions to the model’s prognostic equation(s). Since the model must retain a balance, this adjustment is spread over relatively large spatial and long temporal scales, and the nudging term is also multiplied by a coefficient that keeps the adjustment relatively small. Despite the positive findings of past studies, a number of questions arise in the application of this technique to fine grids. One area yet to be tested is how nudging will behave on fine grids under conditions with sharp horizontal and temporal gradients. Little improvement or even degradation of the model by the nudging might be expected as the timescale of nudging is relatively slow compared to the rapid evolution of the atmosphere, and spreading the observations out in time and space may not be representative of the actual atmospheric conditions. Other questions include 1) how the behavior of nudging at these scales and in active convection depends on boundary conditions, network density, and areal extent; 2) how the results depend on variations in the nudging coefficients; and 3) how nudging compares to simple objective analysis of the observations. In this study, Newtonian relaxation is used in a moist, full physics, nonhydrostatic mesoscale model to conduct simulations with horizontal resolutions as fine as 5 km in environments with deep convection and in mountainous terrain. Observing system simulation experiments were designed to address the previously mentioned questions. The authors show that nudging on these scales and in these conditions tends not to produce any large degradations but instead leads to improvements in the simulations even with a small number of observing sites. In applying nudging to a limited mesoscale area, the authors found that the results were more favorable if the nudging was undertaken over larger regions, which supports the nested approach used by Stauffer and Seaman. Some negative aspects of nudging were also uncovered with locally high rms errors due to data representativity problems and predictability issues. The accuracy of objective analysis was also explored and discussed in the context of the Atmospheric Radiation Measurement (ARM) Program. In agreement with Mace and Ackerman, the errors associated with objective analysis can be too large for the goals of ARM. However, the authors also found that a method proposed by Mace and Ackerman to detect time periods where significant errors exist in the objective analysis was not valid for this case. Based on this work, the authors propose that for a modest network of observing sites FDDA has a number of advantages over objective analysis.

Abstract

A large nocturnal mesoscale convective complex (MCC) developed on 4 June 1985 during the Oklaboma-Kansas Preliminary Regional Experiment for STORM-Central (PRE-STORM) field phase. It occurred near the climatological center of the nocturnal maximum in warm-season precipitation situated over the central United States. In this study special rawinsonde and surface mesonet data have been used to examine how the environmental conditions, which supported MCC development, evolved at night over this region. The MCC of interest was the fourth in a series of MCCS, three of which propagated east-northeastward, 100–300 km north of a quasi-stationary surface front. The region where the MCC experienced its most intensive growth was initially characterized by dry and hydrostatically stable conditions (associated with the passage of the previous MCC) above the shallow, wedge-shaped cold air mass. In less than 3 h, interaction between the diurnally varying low-level jet and the frontal boundary led to a local increase in convective available potential energy (CAPE) of over 2000 J kg⁻¹ for air parcels averaged through a 50-mb-deep layer immediately above the Frontal surface.

Our analysis shows that the region north of the quasi-stationary surface front became a favored zone for nocturnal MCC development when 1) particularly high CAPE arose due to the transport of moist, high-θ _E air northward above the frontal surface by the diurnally modulated low-level jet into a region of significantly colder midtropospheric conditions, and 2) adiabatic mesoscale ascent, which was particularly strong near the northern terminus of the low-level jet, resulted in significant cooling above the jet axis. The cooling acted together with the strong moisture advection to eliminate convective inhibition [negative CAPE below the level of free convection (LFC)], thus enabling air parcels over a mesoscale region to more easily attain their LFC. Strong and deep mesoscale ascent was absent south of the front. In this region the surface-based deep convection that was supported during the evening hours weakened overnight as the low-level jet veered to a southwesterly direction, resulting in less favorable vertical shear for the sustenance of convective updrafts, while diurnal cooling increased the convective inhibition and raised the height of the LFC.

Abstract

In this study a three-dimensional numerical cloud model is used to examine the early evolution of deep convective rainbands that occur in an environment of weak to moderate buoyancy and directionally varying lower-tropospheric vertical wind shear. A simulation based on a case observed on 8 June 1987 during the Taiwan Area Mesoscale Experiment produced a narrow bow-shaped rainband that comprised 1) short-lived updrafts along the downshear portion of the weak rain-induced cold pool and 2) more persistent updrafts along its southern flank, which were highly correlated with vertical vorticity. Trajectory calculations and an analysis of the dynamic portion of the perturbation pressure field are presented to illustrate the hybrid dynamical character of the simulated rainband. The shorter-lived updrafts were associated with weak upward-directed pressure gradient forces at the leading edge of the surface-based cold pool. The more persistent updrafts exhibited much stronger upward-directed pressure gradient forces, which have previously been noted to play an important role in the longevity and propagation of updrafts in midlatitude supercell storms.

While this work was motivated by the desire to better understand mechanisms important to the. formation of heavy rainfall that occurs in association with prefrontal low-level jets over Taiwan, direct verification of the control simulation was hindered by the lack of available Doppler radar observations and difficulties in unambiguously determining initial conditions. Therefore, the simulation results were viewed as idealized and interpreted within the context of a series of sensitivity experiments. These experiments revealed that updraft dynamics and convective organization were strongly dependent on the magnitude of the ambient vertical shear. At weaker vertical shears, low-level updrafts were generally weaker and not associated with strong vertical vorticity. Maximum rainwater mixing ratios were also significantly weaker for less ambient vertical shears despite the specification of identical initial profiles of temperature and moisture for all simulations. This suggests that the strong vertical shear associated with the low-level jet provides a mechanism for producing greater local rainfall rates by allowing enhanced forcing for low-level updrafts in the nearly saturated ambient environment.

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.