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Christopher A. Davis
and
David A. Ahijevych

Abstract

Conditional composites of dropsondes deployed into eight tropical Atlantic weather systems during 2010 are analyzed. The samples are conditioned based on cloud-top temperature within 10 km of the dropsonde, the radius from the cyclonic circulation center of the disturbance, and the stage of system development toward tropical cyclogenesis. Statistical tests are performed to identify significant differences between composite profiles. Cold-cloud-region-composite profiles of virtual temperature deviations from a large-scale instantaneous average indicate enhanced static stability prior to genesis within 200 km of the center of circulation, with negative anomalies below 700 hPa and larger warm anomalies above 600 hPa. Moist static energy is enhanced in the middle troposphere in this composite mainly because of an increase in water vapor content. Prior to genesis the buoyancy of lifted parcels within 200 km of the circulation center is sharply reduced compared to the buoyancy of parcels farther from the center. These thermodynamic characteristics support the conceptual model of an altered mass flux profile prior to genesis that strongly favors convergence in the lower troposphere and rapid increase of circulation near the surface. It is also noted that the air–sea temperature difference is greatest in the inner core of the pregenesis composite, which suggests a means to preferentially initiate new convection in the inner core where the rotation is greatest.

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Christopher A. Davis
and
David A. Ahijevych

Abstract

Three well-observed Atlantic tropical weather systems that occurred during the 2010 hurricane season are analyzed. One case was former Tropical Storm Gaston that failed to redevelop into a tropical cyclone; the other two cases were developing storms Karl and Matthew. Geostationary satellite, multisensor-derived precipitation, and dropsondes from the National Science Foundation (NSF)–NCAR Gulfstream V (GV), NASA DC-8, and the NOAA Gulfstream IV (G-IV) and WP-3D Orion (P-3) aircraft are analyzed in a system-following frame to quantify the mesoscale dynamics of these systems.

Gaston featured extensive dry air surrounding an initially moist core. Vertical shear forced a misalignment of midtropospheric and lower-tropospheric circulation centers. This misalignment allowed dry air to intrude above the lower-tropospheric center and severely limited the area influenced by deep moist convection, thus providing little chance of maintaining or rebuilding the vortex in sheared flow. By contrast, Karl and Matthew developed in a moister environment overall, with moisture increasing with time in the middle and upper troposphere. Deep moist convection was quasi-diurnal prior to genesis. For Karl, deep convection was initially organized away from the lower-tropospheric circulation center, creating a misalignment of the vortex. The vortex gradually realigned over several days and genesis followed this realignment within roughly one day. Matthew experienced weaker shear, was vertically aligned through most of its early evolution, and developed more rapidly than Karl. The evolutions of the three cases are interpreted in the context of recent theories of tropical cyclone formation.

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Matthew D. Parker
and
David A. Ahijevych

Abstract

Nine years of composited radar data are investigated to assess the presence of organized convective episodes in the east-central United States. In the eastern United States, the afternoon maximum in thunderstorms is ubiquitous over land. However, after removing this principal diurnal peak from the radar data, the presence and motion of organized convective systems becomes apparent in both temporally averaged fields and in the statistics of convective episodes identified by an objective algorithm. Convective echoes are diurnally maximized over the Appalachian chain, and are repeatedly observed to move toward the east. Partly as a result of this, the daily maximum in storms is delayed over the Piedmont and coastal plain relative to the Appalachian Mountains and the Atlantic coast. During the 9 yr studied, the objective algorithm identified 2128 total convective episodes (236 yr−1), with several recurring behaviors. Many systems developed over the elevated terrain during the afternoon and moved eastward, often to the coastline and even offshore. In addition, numerous systems formed to the west of the Appalachian Mountains and moved into and across the eastern U.S. study domain. In particular, many nocturnal convective systems from the central United States entered the western side of the study domain, frequently arriving at the eastern mountains around the next day’s afternoon maximum in storm frequency. A fraction of such well-timed systems succeeded in crossing the Appalachians and continuing across the Piedmont and coastal plain. Convective episodes were most frequent during the high-instability, low-shear months of summer, which dominate the year-round statistics. Even so, an important result is that the episodes still occurred almost exclusively in above-average vertical wind shear. Despite the overall dominance of the diurnal cycle, the data show that adequate shear in the region frequently leads to long-lived convective episodes with mesoscale organization.

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Christopher A. Davis
,
David A. Ahijevych
, and
Stanley B. Trier

Abstract

The authors perform a statistical and dynamical analysis of midtropospheric mesoscale vortices captured by analyses from the Rapid Update Cycle, version 2 (RUC-2), during the period 1 May to 31 August 1999. A total of 203 vortices meeting conditions of an automated algorithm were found. Of these, 86 were observed to form within organized convection and were termed mesoscale convective vortices (MCVs). MCVs were favored over a broad area from eastern Colorado and western Nebraska to the Mississippi River valley, essentially collocated with the loci of organized convection. The remaining 117 vortices (termed dry vortices) clustered in the immediate lee of the Rocky Mountains and over the southeastern United States.

Vortices arising from convection had considerably greater intensity and longevity than dry vortices. They were roughly five times more likely to be involved with the triggering of new convection. A relationship was found between intensity and longevity such that there appears to be a maximum vortex lifetime that can be estimated from its maximum intensity. Vortices arising from convection had markedly greater humidity and water vapor mixing ratio underneath their centers compared to dry vortices, consistent with many dry vortices having a topographic origin and MCVs arising from organized convection. Parameters such as horizontal scale, background vertical wind shear, and horizontal deformation were not systematically related to intensity or longevity.

Prediction of mesoscale vortices by the RUC-2 was examined for a subsample of all cases. In general, the RUC-2 was able to predict the evolution of vortices once analyzed, but had virtually no skill at predicting (12 h in advance) the formation of the vortices. However, forecasts of organized convection should still benefit from accurate predictions of long-lived vortex tracks.

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Jason C. Knievel
,
David A. Ahijevych
, and
Kevin W. Manning

Abstract

The authors demonstrate that much can be learned about the performance of a numerical weather prediction (NWP) model by examining the temporal modes of its simulated rainfall. Observations from the Weather Surveillance Radar-1988 Doppler (WSR-88D) network are used to evaluate the rainfall frequency, and its diurnal and semidiurnal modes, in simulations made by a preliminary version of the Weather Research and Forecasting (WRF) model for the conterminous United States during the summer of 2003.

Simulations and observations were broadly similar in the normalized amplitudes of their diurnal and semidiurnal modes, but not in the modes' phases, and not in overall frequency of rain. Simulated rain fell too early, and light rain was too frequent. The model also did not produce the distinct, nocturnal maximum in rainfall frequency that is integral to the hydrologic cycle of the Great Plains. The authors provide evidence that there were regional and phenomenological dependencies to the WRF model's performance.

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Christopher A. Davis
,
David A. Ahijevych
,
Julie A. Haggerty
, and
Michael J. Mahoney

Abstract

Microwave temperature profiler (MTP) data are analyzed to document temperature signatures in the upper troposphere and lower stratosphere that accompany Atlantic tropical weather disturbances. The MTP was deployed on the National Science Foundation–National Center for Atmospheric Research Gulfstream V (GV) aircraft during the Pre-Depression Investigation of Cloud-Systems in the Tropics (PREDICT) in August and September 2010.

Temporal variations in cold-point temperature compared with infrared cloud-top temperature reveal that organized deep convection penetrated to near or beyond the cold point for each of the four disturbances that developed into a tropical cyclone. Relative to the lower-tropospheric circulation center, MTP and dropsonde data confirmed a stronger negative radial gradient of temperature in the upper troposphere (10–13 km) of developing disturbances prior to genesis compared with nondeveloping disturbances. The MTP data revealed a somewhat higher and shallower area of relative warmth near the center when compared with dropsonde data. MTP profiles through anvil cloud depicted cooling near 15 km and warming in the lower stratosphere near the time of maximum coverage of anvil clouds shortly after sunrise. Warming occurred through a deep layer of the upper troposphere toward local noon, presumably associated with radiative heating in cloud. The temperature signatures of anvil cloud above 10-km altitude contributed to the radial gradient of temperature because of the clustering of deep convection near the center of circulation. However, it is concluded that these signatures may be more a result of properties of convection than a direct distinguishing factor of genesis.

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Lawrence D. Carey
,
Steven A. Rutledge
,
David A. Ahijevych
, and
Tom D. Keenan

Abstract

A propagation correction algorithm utilizing the differential propagation phase (ϕ dp) was developed and tested on C-band polarimetric radar observations of tropical convection obtained during the Maritime Continent Thunderstorm Experiment. An empirical procedure was refined to estimate the mean coefficient of proportionality a (b) in the linear relationship between ϕ dp and the horizontal (differential) attenuation throughout each radar volume. The empirical estimates of these coefficients were a factor of 1.5–2 times larger than predicted by prior scattering simulations. This discrepancy was attributed to the routine presence of large drops [e.g., differential reflectivity Z dr ≥ 3 dB] within the tropical convection that were not included in prior theoretical studies.

Scattering simulations demonstrated that the coefficients a and b are nearly constant for small to moderate sized drops (e.g., 0.5 ≤ Z dr ≤ 2 dB; 1 ≤ diameter D 0 < 2.5 mm) but actually increase with the differential reflectivity for drop size distributions characterized by Z dr > 2 dB. As a result, large drops 1) bias the mean coefficients upward and 2) increase the standard error associated with the mean empirical coefficients down range of convective cores that contain large drops. To reduce this error, the authors implemented a “large drop correction” that utilizes enhanced coefficients a* and b* in large drop cores.

Validation of the propagation correction algorithm was accomplished with cumulative rain gauge data and internal consistency among the polarimetric variables. The bias and standard error of the cumulative radar rainfall estimator R(Z h ) [R(K dp,Z dr)], where Z h is horizontal reflectivity and K dp is specific differential phase, were substantially reduced after the application of the attenuation (differential attenuation) correction procedure utilizing ϕ dp. Similarly, scatterplots of uncorrected Z h (Z dr) versus K dp substantially underestimated theoretical expectations. After application of the propagation correction algorithm, the bias present in observations of both Z h (K dp) and Z dr(K dp) was removed and the standard errors relative to scattering simulation results were significantly reduced.

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Stanley B. Trier
,
John H. Marsham
,
Christopher A. Davis
, and
David A. Ahijevych

Abstract

A convection-permitting numerical model is used to simulate the postsunrise reorganization of a nocturnal mesoscale convective system (MCS) observed over western and central Oklahoma on 13 June 2002 during the International H2O (IHOP_2002) Field Experiment. The MCS reorganization consists of a transition from northwest–southeast-oriented convective rainbands near sunrise to a single northeast–southwest (NE–SW)-oriented convective rainband with trailing stratiform precipitation later in the morning.

Results indicate the importance of environmental preconditioning on MCS reorganization. In particular, the development of the NE–SW rainband that redefines the MCS organization is facilitated by a similarly oriented zone of antecedent mesoscale upward motion, which increases the depth of large water vapor mixing ratios. This allows convective updrafts to be fed primarily by moist and conditionally unstable air from 1 to 2.5 km AGL in the NE–SW-oriented rainband, which lacks a surface cold pool during its incipient postsunrise stage.

The MCS develops a strong surface cold pool from latent cooling–induced downdrafts by midmorning and evolves into an upshear-tilted squall-type system. These downdrafts and the resulting cold pool are not necessary for the overall reorganization and maintenance of the MCS in this environment where earlier mesoscale ascent has occurred. However, the latent cooling from downdrafts does influence the MCS strength, vertical structure, and horizontal motion by early in the postsunrise stage. In contrast, surface heating of the preconvective environment has little effect on the strength and structural characteristics of the MCS until midday, by which time the convection has become primarily surface based.

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Stanley B. Trier
,
Glen S. Romine
,
David A. Ahijevych
,
Ryan A. Sobash
, and
Manda B. Chasteen

Abstract

A 50-member convection-allowing ensemble was used to examine environmental factors influencing afternoon convection initiation (CI) and subsequent severe weather on 5 April 2017 during intensive observing period (IOP) 3b of the Verification of the Origins of Rotation in Tornadoes Experiment in the Southeast (VORTEX-SE). This case produced several weak tornadoes (rated EF1 or less), and numerous reports of significant hail (diameter ≥ 2 in.; ≥~5 cm), ahead of an eastward-moving surface cold front over eastern Alabama and southern Tennessee. Both observed and simulated CI was facilitated by mesoscale lower-tropospheric ascent maximized several tens of kilometers ahead of the cold-frontal position, and the simulated mesoscale ascent was linked to surface frontogenesis in the ensemble mean. Simulated maximum 2–5 km AGL updraft helicity (UHmax) was used as a proxy for severe-weather-producing mesocyclones, and considerable variability in UHmax occurred among the ensemble members. Ensemble members with UHmax > 100 m2 s−2 had stronger mesoscale ascent than in members with UHmax < 75 m2 s−2, which facilitated timelier CI by producing greater adiabatic cooling and moisture increases above the PBL. After CI, storms in the larger UHmax members moved northeastward toward a mesoscale region with larger convective available potential energy (CAPE) than in smaller UHmax members. The CAPE differences among members were influenced by differences in the location of an antecedent mesoscale convective system, which had a thermodynamically stabilizing influence on the environment toward which storms were moving. Despite providing good overall guidance, the model ensemble overpredicted severe weather likelihoods in northeastern Alabama, where comparisons with VORTEX-SE soundings revealed a positive CAPE bias.

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Stanley B. Trier
,
Christopher A. Davis
,
David A. Ahijevych
, and
Kevin W. Manning

Abstract

A method based on parcel theory is developed to quantify mesoscale physical processes responsible for the removal of inhibition energy for convection initiation (CI). Convection-permitting simulations of three mesoscale convective systems (MCSs) initiating in differing environments are then used to demonstrate the method and gain insights on different ways that mesoscale thermodynamic destabilization can occur.

Central to the method is a thermodynamic quantity B min, which is the buoyancy minimum experienced by an air parcel lifted from a specified height. For the cases studied, vertical profiles of B min using air parcels originating at different heights are qualitatively similar to corresponding profiles of convective inhibition (CIN). Though it provides less complete information than CIN, an advantage of using B min is that it does not require vertical integration, which simplifies budget calculations that enable attribution of the thermodynamic destabilization to specific physical processes. For a specified air parcel, B min budgets require knowledge of atmospheric forcing at only the parcel origination level and some approximate level where B min occurs.

In a case of simulated daytime surface-based CI, destabilization in the planetary boundary layer (PBL) results from a combination of surface fluxes and upward motion above the PBL. Upward motion effects dominate the destabilizing effects of horizontal advections in two different simulated elevated CI cases, where the destabilizing layer occurs from 1 to 2.5 km AGL. In an elevated case with strong warm advection, changes to the parcel at its origination level dominate the reduction of negative buoyancy, whereas for a case lacking warm advection, adiabatic temperature changes to the environment near the location of B min dominate.

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