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David B. Parsons

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.

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David B. Parsons and Jimy Dudhia

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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.

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Kunio Yoneyama and David B. Parsons

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.

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Stanley B. Trier and David B. Parsons

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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.

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Shushi Zhang, David B. Parsons, and Yuan Wang

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.

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David B. Parsons and Pete V. Hobbs

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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.

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David B. Parsons and Morris L. Weisman

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.

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Kenji Akaeda, Jon Reisner, and David Parsons

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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.

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David B. Parsons and Robert A. Kropfli

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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.

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Tammy M. Weckwerth and David B. Parsons

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The International H2O Project (IHOP_2002) included four complementary research components: quantitative precipitation forecasting, convection initiation, atmospheric boundary layer processes, and instrumentation. This special issue introductory paper will review the current state of knowledge on surface-forced convection initiation and then describe some of the outstanding issues in convection initiation that partially motivated IHOP_2002. Subsequent papers in this special issue will illustrate the value of combining varied and complementary datasets to study convection initiation in order to address the outstanding issues discussed in this paper and new questions that arose from IHOP_2002 observations.

The review will focus on convection initiation by boundaries that are prevalent in the U.S. southern Great Plains. Boundary layer circulations, which are sometimes precursors to deep convective development, are clearly observed by radar as reflectivity fine lines and/or convergence in Doppler velocity. The corresponding thermodynamic distribution, particularly the moisture field, is not as readily measured. During IHOP_2002, a variety of sensors capable of measuring atmospheric water vapor were brought together in an effort to sample the three-dimensional time-varying moisture field and determine its impact on forecasting convection initiation. The strategy included examining convection initiation with targeted observations aimed at sampling regions forecast to be ripe for initiation, primarily along frontal zones, drylines, and their mergers.

A key aspect of these investigations was the combination of varied moisture measurements with the detailed observations of the wind field, as presented in many of the subsequent papers in this issue. For example, the high-resolution measurements are being used to better understand the role of misocyclones on convection initiation. The analyses are starting to elucidate the value of new datasets, including satellite products and radar refractivity retrievals. Data assimilation studies using some of the state-of-the-art datasets from IHOP_2002 are already proving to be quite promising.

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