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M. I. Biggerstaff and R. A. Houze Jr.

Abstract

High-frequency (90 min) rawinsonde data from a special mesoscale network (26 sites) have been combined with wind profiler, dense automated surface network data (80 stations spaced 50 km apart), and a series of high-resolution dual-Doppler radar analyses in a common framework attached to a moving squall-line system to form a comprehensive dataset describing the mature phase of the 10–11 June 1985 squall line observed during PRE-STORM. The dual-Doppler radar analyses covered a 200 × 300 km2 area, from the leading edge of the convective line to the back edge of the trailing stratiform precipitation region, thus, providing high-resolution wind information over a very broad portion of the storm system.

The comprehensive analysis is used to resolve several aspects of the trailing stratiform region that had remained unclear from previous studies. First, a difference in the horizontal scale was found between the mesoscale updraft, which at upper levels was on the scale of the trailing stratiform cloud, and the strong mesoscale downdraft, which at mid-to-lower levels was on the scale of the trailing stratiform precipitation. Second, the region of heaviest stratiform precipitation (the secondary band) was found to be immediately downwind of the most intense portions of the convective line, and the width of the trailing stratiform precipitation region was controlled by a combination of the wind velocity and microphysical fall-speed scales. Third, the radar reflectivity minimum observed at mid-to-lower levels in the region just behind the convective line was found to coincide with deep subsidence from mid-to-upper levels, which may have reduced the mass of the hydrometeors through sublimation and evaporation. However, precipitation trajectories computed from the comprehensive analysis indicate another contributing factor; namely, the source region of hydrometeors at low levels just behind the convective line was at a lower altitude than the source region of low-level hydrometeors in the heavy stratiform precipitation farther behind the convective line. Thus, even if all other factors had been the same, the hydrometeors in the heavy stratiform rain would have had more time to grow than those found in the region of the radar reflectivity minimum just behind the convective line. Moreover, hydrometeor detrainment may have been greater near cloud top than at lower levels.

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J. Sun and R. A. Houze Jr.

Abstract

A thermodynamic retrieval technique, which uses the equation of motion in conjunction with the thermodynamic equation, is validated in a two-dimensional numerical model simulation of a squall line with a trailing stratiform region. Model wind and reflectivity output are used as input to the retrieval. The availability of model thermodynamic output allows us to examine the performance of the retrieval. The computational technique involved is found to be valid. When the retrieval is applied to the time-averaged model wind fields, reasonably accurate results are obtained for the stratiform region, but errors arise for the convective region because of the neglect of eddy correlations of the temporally fluctuating wind components. Application of the retrieval to instantaneous model wind fields demonstrates that very high lime resolution is needed in the wind data (< about 2 min) to obtain reliable results where time changes are large and nonlinear.

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K. L. Rasmussen and R. A. Houze Jr.

Abstract

Satellite radar and radiometer data indicate that subtropical South America has some of the deepest and most extreme convective storms on Earth. This study uses the full 15-yr TRMM Precipitation Radar dataset in conjunction with high-resolution simulations from the Weather Research and Forecasting Model to better understand the physical factors that control the climatology of high-impact weather in subtropical South America. The occurrence of intense storms with an extreme horizontal dimension is generally associated with lee cyclogenesis and a strengthening South American low-level jet (SALLJ) in the La Plata basin. The orography of the Andes is critical, and model sensitivity calculations removing and/or reducing various topographic features indicate the orographic control on the initiation of convection and its upscale growth into mesoscale convective systems (MCSs). Reduced Andes experiments show more widespread convective initiation, weaker average storm intensity, and more rapid propagation of the MCS to the east (reminiscent of the MCS life cycle downstream of lower mountains such as the Rockies). With reduced Andes, lee cyclogenesis and SALLJ winds are weaker, while they are stronger in increased Andes runs. The presence of the Sierras de Córdoba (secondary mountain range east of the Andes in Argentina) focuses convective initiation and results in more intense storms in experiments with higher Andes. Average CAPE and CIN values for each terrain modification simulation show that reduced Andes runs had lower CIN and CAPE, while increased Andes runs had both stronger CAPE and CIN. From this research, a conceptual model for convective storm environments leading to convective initiation has been developed for subtropical South America.

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M. I. Biggerstaff and R. A. Houze, Jr.

Abstract

The comprehensive analysis of the kinematic structure of the mature phase of the 10–11 June 1985 squall-line system was used to examine the midlevel vertical-vorticity structure of the storm to show that relative vertical vorticity in the stratiform region at midlevels was organized into bands (both cyclonic and anticyclonic) oriented parallel to the convective line, with anticyclonic vorticity between the rear of the convective line and the heaviest stratiform precipitation and cyclonic vorticity farther back. Since previous studies have not found anticyclonic vorticity over such a large portion of the stratiform region at midlevels and since the concentration of anticyclonic vorticity may have been detrimental to the longevity of the storm by limiting the development of an inertially stable cyclonic circulation, the tilting and stretching terms of the vertical-vorticity equation were computed to determine how the observed vertical-vorticity pattern was maintained. Tilting of horizontal vorticity into vertical vorticity by gradients of vertical motion was a factor of 2–10 greater than the stretching of vertical vorticity. Below the melting level and at the rear of the stratiform region, tilting was associated with gradients of mesoscale vertical motion. Above the melting level, near the convective line, tilting was associated with gradients of mean convective motions.

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H. H. Schiesser, R. A. Houze Jr., and H. Huntrieser

Abstract

The structures of severe mesoscale precipitation systems (MPS) in Switzerland have been classified by analyzing radar images obtained over a 5-yr period. Severe MPSs were defined to be those producing most of the damage on days on which at least 5 (out of 2400) communities reported water and/or at least 20 reported hail damage. Of 94 MPSs selected, 82 had radar reflectivity of 47 dBZ or greater and were referred to as mesoscale convective systems (MCS). The 12 remaining MPSs consisted of less intense, long-lasting, and widespread frontal or orographic rainfall.

Subclasses of MCSs were defined according to their internal arrangements of cell complexes (CC). A CC was defined as an echo contour of 40 dBZ surrounding echo maxima of at least 47 dBZ. Four general categories of organization were found: isolated CC, a group of CCs, and a broken or continuous line of CCs. All categories can be purely convective at the mature stage, or the CCs may be juxtaposed with a stratiform precipitation area, usually behind moving convection. The stratiform region often developed as a decaying convective area. These categories were examined in relation to sounding, surface mesonet, synoptic weather type, and severe weather information.

In 26 cases, the MCS had “leading line-trailing stratiform” structure. These MCSs were graded according to a classification scheme previously used to characterize spring rainstorms in Oklahoma. Only moderately and weakly classifiable storm systems occurred in Switzerland. The mountain barriers apparently interfered with the airflow such that MCSs were prevented from having enough time and space to develop to a higher degree of organization as is possible over the relatively flat terrain of Oklahoma. In addition, the instability and the wind shear in the Swiss storm environment was found to be weaker.

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Scott W. Powell, Robert A. Houze, Jr., and Stella R. Brodzik

Abstract

An algorithm used to classify precipitation echoes by rain type without interpolating radar data to a constant height is detailed. The method uses reflectivity data without clutter along the lowest available scan angle so that the classifications yield a more accurate representation of the rain type observed at the surface. The algorithm is based on that of Steiner et al. but is executed within a polar coordinate system. An additional procedure allows for more small, isolated, and/or weak echo objects to be appropriately identified as convective. Echoes in the immediate vicinity of convective cores are included in a new transition category, which consists mostly of echoes for which a convective or stratiform determination cannot be confidently made. The new algorithm more effectively identifies shallow convection embedded within large stratiform regions, correctly identifies isolated shallow and weak convection as such, and more often appropriately identifies periods during which no stratiform precipitation is present.

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R. A. Houze Jr., L. A. McMurdie, K. L. Rasmussen, A. Kumar, and M. M. Chaplin

Abstract

Conditions producing disastrous flooding in Uttarakhand, India, in June 2013 differed from conditions that produced other notorious floods in the Himalayan region in recent years. During the week preceding the Uttarakhand flood, deep convection moistened the mountainsides, making them vulnerable to flooding. However, the precipitation producing the flood was not associated with a deep convective event. Rather, an eastward-propagating upper-level trough in the westerlies extended abnormally far southward, with the jet reaching the Himalayas. The south end of the trough merged with a monsoon low moving westward across India. The merged system produced persistent moist low-level flow oriented normal to the Himalayas that advected large amounts of water vapor into the Uttarakhand region. The flow was moist neutral when it passed over the Himalayan barrier, and orographic lifting produced heavy continuous rain over the region for 2–3 days. The precipitation was largely stratiform in nature although embedded convection of moderate depth occurred along the foothills, where some mild instability was being released. The Uttarakhand flood had characteristics in common with major 2013 floods in the Rocky Mountains in Colorado and Alberta, Canada.

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S. E. Yuter, R. A. Houze Jr., B. F. Smull, F. D. Marks Jr., J. R. Daugherty, and S. R. Brodzik

An electronic atlas of research aircraft missions in TOGA COARE (Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment) has been prepared and is available on the Internet via World Wide Web browsers such as Mosaic. These maps are in the form of time sequences of color imagery assembled using the NCAR Zebra software. Initial versions of these maps were prepared in the field at the TOGA COARE Honiara Operations Center to aid in the evaluation of each aircraft mission immediately after it was flown. The maps prepared in the field have been updated, corrected, and remapped at standard scales and with common color schemes. They show the meteorological setting of sampling by all seven aircraft participating in TOGA COARE—the two NOAA WP-3D aircraft, the NCAR Electra, the FIAMS C-340, the UK C-130, and the NASA DC-8 and ER-2—by overlaying flight tracks, GMS satellite infrared data, and NOAA WP-3D airborne radar images. The map sequences are combined with text of scientists' notes and other background information on the research flights to form a summary of each aircraft mission. The resulting aircraft mission summaries are intended as a road map to the COARE aircraft dataset. They indicate where and when data were collected and the meteorological context for those data. As an electronic document, the atlas of aircraft mission summaries is available on demand, and it is dynamic: as further information becomes available, the mission summaries will continue to be added to and updated as appropriate, and new releases will be issued periodically.

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R. A. Houze Jr., W. Schmid, R. G. Fovell, and H-H. Schiesser

Abstract

In the central region of Switzerland, lying between the Jura Mountains to the north and the Alps to the south, severe hailstorms are a common summertime phenomenon. Eight years of data on these hailstorms show that they are nearly equally divided between left- and right-moving storms. Depending on the exact environmental conditions, the severe hailstorms consist variously of left- or right-moving ordinary-cell storms, left- or right-moving supercell storms, and left-moving storms of an intermediary type (i.e., supercellular in some but not all respects). The left movers of the intermediary type sometimes exhibit a cyclonically rotating echo appendage on the right-rear flank of the storm. This appendage to the left mover resembles a book echo associated with a classic supercell. It is dubbed a false hook, since it has a dynamical configuration substantially different from that of a classic supercell. This difference is demonstrated by the fact that the false hook appears on the wrong side of the left mover for it to be a mirror image of a classic right-moving supercell.

Sounding data show that at bulk Richardson numbers less than 30–50, the right-moving severe hailstorms in central Switzerland tend to be stronger and are more likely to be supercellular, though they are almost never tornadic. The hodograph of the wind in the environment of the storms shows that the winds are about one-half to two-thirds the strength of the winds associated with tornadic storms over the central United States. The wind-shear vector turns generally clockwise with increasing height through the lowest 5–6 km, with a maximum south-westerly wind at about the 3-km level. On days when left-moving storms occur, the shear vector in the lowest 2–3 km of the generally clockwise-turning layer tends to exhibit a slight counterclockwise turning with height.

Model calculations have been carried out for a day on which slight counterclockwise shear was present in the lowest 2–3 km and on which both a right-moving supercell and a left-moving false-hook storm occurred. In addition to rawinsonde data, observations were obtained by three radars, surface stations, and a hailpad network. The model produces splitting storms. The right-and left-moving model storms match the observed storms quite well. The left-hook mover was a false-hook storm, since the separate, cyclonically rotating updraft in the false-hook region does not separate from the left-moving storm. The false-hook appendage is found to consist of updraft and precipitation advected westward and southward in the cyclonically rotating south near flank of the storm. It bounds a cyclonically rotating downdraft on the south side of the storm. When the model simulation is repeated after modifying the environment wind hodograph by reversing the sense of the turning of the shear vector at low levels, so that the environment wind-shear vector turned in the clockwise sense with increasing height throughout the entire lowest 5–6 km, the second split of the left mover occurs much sooner. Consequently, the southern echo appendage is only a transitory feature, and a long-lived false-hook storm is not maintained.

The model simulations indicate that the basic characteristics of thunderstorms in central Switzerland can be realistically reproduced in a numerical model with a flat lower boundary. Hence, the environmental wind and thermodynamic stratification are inferred to be the primary factors determining storm structure. However, the environment supports multiple storm structures, and those storm modes selected by nature at a specific time and location may be determined by very subtle local effects, such as whether the low-level wind hodograph exhibits a slight clockwise or counterclockwise perturbation. Such local variability of the winds is likely related, directly or indirectly, to orography. Such variability is evidently random, though, resulting in the even climatological distribution between left- and right-moving storms.

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J. Sun, S. Braun, M. I. Biggerstaff, R. G. Fovell, and R. A. Houze Jr.

Abstract

Thermodynamic retrieval analysis applied to a composite of dual-Doppler radar data obtained in the 10–11 June 1985 PRE-STORM (Preliminary Regional Experiment for STORM-Central) squall line and a model simulation of a similar squall line show that the upper-level downdrafts located ahead of and behind the main convective updraft zone were generally positively buoyant. As a result, the upper-level downdrafts contributed negatively to the system heat flux.

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