VORTEX2 Observations of a Low-Level Mesocyclone with Multiple Internal Rear-Flank Downdraft Momentum Surges in the 18 May 2010 Dumas, Texas, Supercell*

Abstract Observations collected in the second Verification of the Origins of Rotation in Tornadoes Experiment during a 15-min period of a supercell occurring on 18 May 2010 near Dumas, Texas, are presented. The primary data collection platforms include two Ka-band mobile Doppler radars, which collected a near-surface, short-baseline dual-Doppler dataset within the rear-flank outflow of the Dumas supercell; an X-band, phased-array mobile Doppler radar, which collected volumetric single-Doppler data with high temporal resolution; and in situ thermodynamic and wind observations of a six-probe mobile mesonet. Rapid evolution of the Dumas supercell was observed, including the development and decay of a low-level mesocyclone and four internal rear-flank downdraft (RFD) momentum surges. Intensification and upward growth of the low-level mesocyclone were observed during periods when the midlevel mesocyclone was minimally displaced from the low-level circulation, suggesting an upward-directed perturbation pressure gradient force aided in the intensification of low-level rotation. The final three internal RFD momentum surges evolved in a manner consistent with the expected behavior of a dynamically forced occlusion downdraft, developing at the periphery of the low-level mesocyclone during periods when values of low-level cyclonic azimuthal wind shear exceeded values higher aloft. Failure of the low-level mesocyclone to acquire significant vertical depth suggests that dynamic forcing above internal RFD momentum surge gust fronts was insufficient to lift the negatively buoyant air parcels comprising the RFD surges to significant heights. As a result, vertical acceleration and the stretching of vertical vorticity in surge parcels were limited, which likely contributed to tornadogenesis failure.

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On the fine vertical structure of the low troposphere over the coastal margins of East Antarctica

10.5194/acp-2018-1197 ◽

2019 ◽

Author(s):

Étienne Vignon ◽

Olivier Traullé ◽

Alexis Berne

Keyword(s):

Pressure Gradient ◽

Vertical Structure ◽

East Antarctica ◽

Radiosonde Data ◽

Gradient Force ◽

Pressure Gradient Force ◽

Near Surface ◽

Ice Shelves ◽

Low Level ◽

Humidity Profiles

Abstract. Eight years of high-resolution radiosonde data at nine Antarctic stations are analysed to provide the first large scale characterization of the fine scale vertical structure of the low troposphere up to 3 km of altitude over the coastal margins of East Antarctica. Radiosonde data show a large spatial variability of wind, temperature and humidity profiles, with different features between stations in katabatic regions (e.g., Dumont d'Urville and Mawson stations), stations over two ice shelves (Neumayer and Halley stations) and regions with complex orography (e.g., Mc Murdo). At Dumont d'Urville, Mawson and Davis stations, the yearly median wind speed profiles exhibit a clear low-level katabatic jet. During precipitation events, the low-level flow generally remains of continental origin and its speed is even reinforced due to the increase in the continent- ocean pressure gradient. Meanwhile, the relative humidity profiles show a dry low troposphere, suggesting the occurence of low-level sublimation of precipitation in katabatic regions but such a phenomenon does not appreciably occur over the ice-shelves near Halley and Neumayer. Although ERA-Interim and ERA5 reanalyses assimilate radiosoundings at most stations considered here, substantial – and sometimes large – low-level wind and humidity biases are revealed but ERA5 shows overall better performances. A free simulation with the regional model Polar WRF (at a 35-km resolution) over the entire continent shows too strong and too shallow near-surface jets in katabatic regions especially in winter. This may be a consequence of an understimated coastal cold air bump and associated sea-continent pressure gradient force due to the coarse 35 km resolution of the Polar WRF simulation. Beyond documenting the vertical structure of the low troposphere over coastal East-Antarctica, this study gives insights into the reliability and accuracy of two major reanalysis products in this region on the Earth and it raises the difficulty of modeling the low-level flow over the margins of the ice sheet with a state-of-the-art climate model.

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Nocturnal Destabilization Associated with the Summertime Great Plains Low-Level Jet

Monthly Weather Review ◽

10.1175/mwr-d-19-0394.1 ◽

2020 ◽

Vol 148 (11) ◽

pp. 4641-4656

Author(s):

Thomas R. Parish ◽

Richard D. Clark ◽

Todd D. Sikora

Keyword(s):

Great Plains ◽

The United States ◽

Temperature Gradients ◽

Gradient Force ◽

Wind Component ◽

Pressure Gradient Force ◽

Near Surface ◽

Low Level ◽

Temperature Advection ◽

Low Level Jet

AbstractThe Great Plains low-level jet (LLJ) has long been associated with summertime nocturnal convection over the central Great Plains of the United States. Destabilization effects of the LLJ are examined using composite fields assembled from the North American Mesoscale Forecast System for June and July 2008–12. Of critical importance are the large isobaric temperature gradients that become established throughout the lowest 3 km of the atmosphere in response to the seasonal heating of the sloping Great Plains. Such temperature gradients provide thermal wind forcing throughout the lower atmosphere, resulting in the establishment of a background horizontal pressure gradient force at the level of the LLJ. The attendant background geostrophic wind is an essential ingredient for the development of a pronounced summertime LLJ. Inertial turning of the ageostrophic wind associated with LLJ provides a westerly wind component directed normal to the terrain-induced orientation of the isotherms. Hence, significant nocturnal low-level warm-air advection occurs, which promotes differential temperature advection within a vertical column of atmosphere between the level just above the LLJ and 500 hPa. Such differential temperature advection destabilizes the nighttime troposphere above the radiatively cooled near-surface layer on a recurring basis during warm weather months over much of the Great Plains and adjacent states to the east. This destabilization process reduces the convective inhibition of air parcels near the level of the LLJ and may be of significance in the development of elevated nocturnal convection. The 5 July 2015 case from the Plains Elevated Convection at Night field program is used to demonstrate this destabilization process.

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On the fine vertical structure of the low troposphere over the coastal margins of East Antarctica

Atmospheric Chemistry and Physics ◽

10.5194/acp-19-4659-2019 ◽

2019 ◽

Vol 19 (7) ◽

pp. 4659-4683 ◽

Cited By ~ 11

Author(s):

Étienne Vignon ◽

Olivier Traullé ◽

Alexis Berne

Keyword(s):

Pressure Gradient ◽

Vertical Structure ◽

East Antarctica ◽

Radiosonde Data ◽

Gradient Force ◽

Pressure Gradient Force ◽

Near Surface ◽

Ice Shelves ◽

Low Level ◽

Humidity Profiles

Abstract. In this study, 8 years of high-resolution radiosonde data at nine Antarctic stations are analysed to provide the first large-scale characterization of the fine vertical structure of the low troposphere up to 3 km altitude over the coastal margins of East Antarctica. Radiosonde data show a large spatial variability of wind, temperature and humidity profiles, with different features between stations in katabatic regions (e.g., Dumont d'Urville and Mawson stations), stations over two ice shelves (Neumayer and Halley stations) and regions with complex orography (e.g., McMurdo). At the Dumont d'Urville, Mawson and Davis stations, the yearly median wind speed profiles exhibit a clear low-level katabatic jet. During precipitation events, the low-level flow generally remains of continental origin and its speed is even reinforced due to the increase in the continent–ocean pressure gradient. Meanwhile, the relative humidity profiles show a dry low troposphere, suggesting the occurrence of low-level sublimation of precipitation in katabatic regions but such a phenomenon does not appreciably occur over the ice shelves near Halley and Neumayer. Although ERA-Interim and ERA5 reanalyses assimilate radiosoundings at most stations considered here, substantial – and sometimes large – low-level wind and humidity biases are revealed but ERA5 shows overall better performance. A free simulation with the regional polar version of the Weather Research and Forecasting model (Polar WRF) (at a 35 km resolution) over the entire continent shows too-strong and too-shallow near-surface jets in katabatic regions especially in winter. This may be a consequence of an underestimated coastal cold air bump and associated sea–continent pressure gradient force due to the coarse 35 km resolution of the Polar WRF simulation. Beyond documenting the vertical structure of the low troposphere over coastal East Antarctica, this study gives insights into the reliability and accuracy of two major reanalysis products in this region on the Earth. The paper further underlines the difficulty of modeling the low-level flow over the margins of the ice sheet with a state-of-the-art atmospheric model.

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Radar Refractivity Retrieval: Validation and Application to Short-Term Forecasting

Journal of Applied Meteorology ◽

10.1175/jam-2204.1 ◽

2005 ◽

Vol 44 (3) ◽

pp. 285-300 ◽

Cited By ~ 49

Author(s):

Tammy M. Weckwerth ◽

Crystalyne R. Pettet ◽

Frédéric Fabry ◽

Shin Ju Park ◽

Margaret A. LeMone ◽

...

Keyword(s):

Great Plains ◽

Doppler Radar ◽

Doppler Velocity ◽

Good Representation ◽

Short Term ◽

Near Surface ◽

National Network ◽

Low Level ◽

Short Term Forecasting ◽

Convection Initiation

Abstract This study will validate the S-band dual-polarization Doppler radar (S-Pol) radar refractivity retrieval using measurements from the International H2O Project conducted in the southern Great Plains in May–June 2002. The range of refractivity measurements during this project extended out to 40–60 km from the radar. Comparisons between the radar refractivity field and fixed and mobile mesonet refractivity values within the S-Pol refractivity domain show a strong correlation. Comparisons between the radar refractivity field and low-flying aircraft also show high correlations. Thus, the radar refractivity retrieval provides a good representation of low-level atmospheric refractivity. Numerous instruments that profile the temperature and moisture are also compared with the refractivity field. Radiosonde measurements, Atmospheric Emitted Radiance Interferometers, and a vertical-pointing Raman lidar show good agreement, especially at low levels. Under most daytime summertime conditions, radar refractivity measurements are representative of an ∼250-m-deep layer. Analyses are also performed on the utility of refractivity for short-term forecasting applications. It is found that the refractivity field may detect low-level boundaries prior to the more traditional radar reflectivity and Doppler velocity fields showing their existence. Data from two days on which convection initiated within S-Pol refractivity range suggest that the refractivity field may exhibit some potential utility in forecasting convection initiation. This study suggests that unprecedented advances in mapping near-surface water vapor and subsequent improvements in predicting convective storms could result from implementing the radar refractivity retrieval on the national network of operational radars.

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Evolution of Low-Level Angular Momentum in the 2 June 1995 Dimmitt, Texas, Tornado Cyclone

Journal of the Atmospheric Sciences ◽

10.1175/jas3829.1 ◽

2007 ◽

Vol 64 (4) ◽

pp. 1365-1378 ◽

Cited By ~ 17

Author(s):

Erik N. Rasmussen ◽

Jerry M. Straka

Keyword(s):

Angular Momentum ◽

Life Cycle ◽

Axisymmetric Flow ◽

Tangential Velocity ◽

Secondary Circulation ◽

Cloud Base ◽

Gradient Force ◽

Pressure Gradient Force ◽

Low Level ◽

Debris Cloud

Abstract The life cycle of the 2 June 1995 Dimmitt, Texas, tornado cyclone, observed during the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX), is described. The tornado cyclone here is defined as a significantly axisymmetric flow larger than the visible tornado and characterized by increasing angular momentum with increasing radius. Its life cycle included three phases with somewhat differing evolution of angular momentum, herein called intensifying, transition, and weakening. During the intensifying stage, the funnel and debris cloud gradually increased in size. The azimuthally averaged secondary circulation of the larger-scale tornado cyclone, as determined using high-resolution single-Doppler data obtained by a mobile radar, was primarily inward and upward, consistent with the presence of a wall cloud outside the tornado. The azimuthally averaged angular momentum increased monotonically away from the tornado, so inward advection allowed the angular momentum to increase slowly with time in part of the tornado cyclone. During the transition phase, downdrafts began to occur within the tornado cyclone. The transport of angular momentum by the secondary circulation nearly was offset by eddy flux convergence of angular momentum so that the azimuthally averaged angular momentum tendency was only weakly negative at most radii. The tornado was visually impressive during this stage, featuring a 400-m diameter debris cloud extending to cloud base, while the surrounding wall cloud shrank and eroded. During the weakening phase, the funnel and debris cloud gradually shrank, and the funnel went through a rope stage prior to disappearing. The weakening phase was characterized by extensive downdrafts at all radii outside the tornado, and large-scale near-ground outflow as observed by mobile mesonet systems in a portion of the tornado cyclone. The secondary circulation acted to transport smaller angular momentum downward from aloft, and outward along the ground. All terms of the angular momentum budget became negative throughout most of the low-level (0–800-m AGL) tornado cyclone during the weakening phase. Several hypotheses for this evolution are evaluated, including changes in water loading in the tornado cyclone, cooling of the near-ground air, and the distribution of tangential velocity with height with its concomitant influence on the nonhydrostatic vertical pressure gradient force.

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Large Sensitivity of Near-Surface Vertical Vorticity Development to Heat Sink Location in Idealized Simulations of Supercell-Like Storms

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-16-0372.1 ◽

2017 ◽

Vol 74 (4) ◽

pp. 1095-1104 ◽

Cited By ~ 9

Author(s):

Paul M. Markowski ◽

Yvette P. Richardson

Keyword(s):

Heat Sink ◽

Vertical Axis ◽

Gradient Force ◽

Pressure Gradient Force ◽

Near Surface ◽

Vertical Vorticity ◽

Supercell Storm ◽

History Of ◽

Perturbation Pressure ◽

Negative Buoyancy

Abstract In idealized numerical simulations of supercell-like “pseudostorms” generated by a heat source and sink in a vertically sheared environment, a tornado-like vortex develops if air possessing large circulation about a vertical axis at the lowest model levels can be converged. This is most likely to happen if the circulation-rich air possesses only weak negative buoyancy (the circulation-rich air has a history of descent, so typically possesses at least some negative buoyancy) and is subjected to an upward-directed vertical perturbation pressure gradient force. This paper further explores the sensitivity of the development of near-surface vertical vorticity to the horizontal position of the heat sink. Shifting the position of the heat sink by only 2–3 km can significantly influence vortex intensity by altering both the baroclinic generation of circulation and the buoyancy of circulation-rich air. Many of the changes in the pseudostorms that arise from shifting the position of the heat sink would be difficult to anticipate. The sensitivity of the pseudostorms to heat sink position probably at least partly explains the well-known sensitivity of near-surface vertical vorticity development to the microphysics parameterizations in more realistic supercell storm simulations, as well as some of the failures of actual supercells to produce tornadoes in seemingly favorable environments.

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A numerical study of back-building process in a quasistationary rainband with extreme rainfall over northern Taiwan during 11–12 June 2012

Atmospheric Chemistry and Physics ◽

10.5194/acp-16-12359-2016 ◽

2016 ◽

Vol 16 (18) ◽

pp. 12359-12382 ◽

Cited By ~ 10

Author(s):

Chung-Chieh Wang ◽

Bing-Kui Chiou ◽

George Tai-Jen Chen ◽

Hung-Chi Kuo ◽

Ching-Hwang Liu

Keyword(s):

Numerical Study ◽

Leading Edge ◽

Extreme Rainfall ◽

Cold Pool ◽

Gradient Force ◽

Pressure Gradient Force ◽

Near Surface ◽

Convective Systems ◽

Mesoscale Convective ◽

Northern Taiwan

Abstract. During 11–12 June 2012, quasistationary linear mesoscale convective systems (MCSs) developed near northern Taiwan and produced extreme rainfall up to 510 mm and severe flooding in Taipei. In the midst of background forcing of low-level convergence, the back-building (BB) process in these MCSs contributed to the extreme rainfall and thus is investigated using a cloud-resolving model in the case study here. Specifically, as the cold pool mechanism is not responsible for the triggering of new BB cells in this subtropical event during the meiyu season, we seek answers to the question why the location about 15–30 km upstream from the old cell is still often more favorable for new cell initiation than other places in the MCS. With a horizontal grid size of 1.5 km, the linear MCS and the BB process in this case are successfully reproduced, and the latter is found to be influenced more by the thermodynamic and less by dynamic effects based on a detailed analysis of convective-scale pressure perturbations. During initiation in a background with convective instability and near-surface convergence, new cells are associated with positive (negative) buoyancy below (above) due to latent heating (adiabatic cooling), which represents a gradual destabilization. At the beginning, the new development is close to the old convection, which provides stronger warming below and additional cooling at mid-levels from evaporation of condensates in the downdraft at the rear flank, thus yielding a more rapid destabilization. This enhanced upward decrease in buoyancy at low levels eventually creates an upward perturbation pressure gradient force to drive further development along with the positive buoyancy itself. After the new cell has gained sufficient strength, the old cell's rear-flank downdraft also acts to separate the new cell to about 20 km upstream. Therefore, the advantages of the location in the BB process can be explained even without the lifting at the leading edge of the cold outflow.

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On the Orographically Generated Low-Level Easterly Jet and Severe Downslope Storms of March 2006 over the Tacheng Basin of Northwest China

Monthly Weather Review ◽

10.1175/mwr-d-17-0355.1 ◽

2018 ◽

Vol 146 (6) ◽

pp. 1667-1683 ◽

Cited By ~ 1

Author(s):

Guangxing Zhang ◽

Da-Lin Zhang ◽

Shufang Sun

Keyword(s):

Large Scale ◽

Inversion Layer ◽

Dust Storms ◽

Frozen Soil ◽

Gradient Force ◽

Mountain Range ◽

Pressure Gradient Force ◽

Low Level ◽

Downslope Winds ◽

The Impact

A high-latitude low-level easterly jet (LLEJ) and downslope winds, causing severe dust storms over the Tacheng basin of northwestern China in March 2006 when the dust source regions were previously covered by snow with frozen soil, are studied in order to understand the associated meteorological conditions and the impact of complex topography on the generation of the LLEJ. Observational analyses show the development of a large-scale, geostrophically balanced, easterly flow associated with a northeastern high pressure and a southeastern low pressure system, accompanied by a westward-moving cold front with an intense inversion layer near the altitudes of mountain ridges. A high-resolution model simulation shows the generation of an LLEJ of near-typhoon strength, which peaked at about 500 m above the ground, as well as downslope windstorms with marked wave breakings and subsidence warming in the leeside surface layer, as the large-scale cold easterly flow moves through a constricting saddle pass and across a higher mountain ridge followed by a lower parallel ridge, respectively. The two different airstreams are merged to form an intense LLEJ of cold air, driven mostly by zonal pressure gradient force, and then the LLEJ moves along a zonally oriented mountain range to the north. Results indicate the importance of the lower ridge in enhancing the downslope winds associated with the higher ridge and the importance of the saddle pass in generating the LLEJ. We conclude that the intense downslope winds account for melting snow, warming and drying soils, and raising dust into the air that is then transported by the LLEJ, generated mostly through the saddle pass, into the far west of the basin.

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Origin of Strong Winds in an Explosive Mediterranean Extratropical Cyclone

Monthly Weather Review ◽

10.1175/mwr-d-19-0009.1 ◽

2019 ◽

Vol 147 (10) ◽

pp. 3649-3671 ◽

Cited By ~ 5

Author(s):

Mihaela Brâncuş ◽

David M. Schultz ◽

Bogdan Antonescu ◽

Christopher Dearden ◽

Sabina Ştefan

Keyword(s):

Boundary Layer ◽

Conveyor Belt ◽

Heat Fluxes ◽

The Black Sea ◽

Gradient Force ◽

Pressure Gradient Force ◽

Strong Wind ◽

Near Surface ◽

Cyclone Center ◽

Strong Winds

Abstract During 2–3 December 2012, the Black Sea and east coast of Romania were affected by a rapidly deepening Mediterranean cyclone. The cyclone developed a bent-back front along which short-lived (2–4 h) strong winds up to 38 m s−1 were recorded equatorward of the cyclone center. A mesoscale model simulation was used to analyze the evolution of the wind field, to investigate the physical processes that were responsible for the strong winds and their acceleration, and to investigate the relative importance of the stability of the boundary layer to those strong winds. The origin of the air in the wind maximum equatorward of the cyclone center was twofold. The first was associated with a sting jet, a descending airstream from the midlevels of the cloud head and the lower part of the cyclonic branch of the warm conveyor belt. The sting jet started to descend west of the cyclone center, ending at the frontolytic tip of the bent-back front. The second was a low-level airstream associated with the cold conveyor belt that originated northeast of the cyclone center and traveled below 900 hPa along the cold side of the bent-back front, ending behind the cold front. Both airstreams were accelerated by the along-flow pressure gradient force, with the largest accelerations acting on the sting-jet air before entering into the near-surface strong-wind area. The sensible heat fluxes destabilized the boundary layer to near-neutral conditions south of the cyclone center, facilitating downward mixing and allowing the descending air to reach the surface. Mesoscale instabilities appeared to be unimportant in the sting-jet formation.

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Processes Preventing the Development of a Significant Tornado in a Colorado Supercell on 26 May 2010

Monthly Weather Review ◽

10.1175/mwr-d-19-0288.1 ◽

2020 ◽

Vol 148 (5) ◽

pp. 1753-1778

Author(s):

Shawn S. Murdzek ◽

Paul M. Markowski ◽

Yvette P. Richardson ◽

Robin L. Tanamachi

Keyword(s):

Small Radius ◽

Future Research ◽

Gradient Force ◽

Pressure Gradient Force ◽

Far Field ◽

Streamwise Vorticity ◽

Low Level ◽

Precipitation Region ◽

Negative Buoyancy

Abstract A supercell produced a nearly tornadic vortex during an intercept by the Second Verification of the Origins of Rotation in Tornadoes Experiment on 26 May 2010. Using observations from two mobile radars performing dual-Doppler scans, a five-probe mobile mesonet, and a proximity sounding, factors that prevented this vortex from strengthening into a significant tornado are examined. Mobile mesonet observations indicate that portions of the supercell outflow possessed excessive negative buoyancy, likely owing in part to low boundary layer relative humidity, as indicated by a high environmental lifted condensation level. Comparisons to a tornadic supercell suggest that the Prospect Valley storm had enough far-field circulation to produce a significant tornado, but was unable to converge this circulation to a sufficiently small radius. Trajectories suggest that the weak convergence might be due to the low-level mesocyclone ingesting parcels with considerable crosswise vorticity from the near-storm environment, which has been found to contribute to less steady and weaker low-level updrafts in supercell simulations. Yet another factor that likely contributed to the weak low-level circulation was the inability of parcels rich in streamwise vorticity from the forward-flank precipitation region to reach the low-level mesocyclone, likely owing to an unfavorable pressure gradient force field. In light of these results, we suggest that future research should continue focusing on the role of internal, storm-scale processes in tornadogenesis, especially in marginal environments.

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