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Olivier Bousquet and Bradley F. Smull

Abstract

Although airborne Doppler radar is increasingly relied upon to provide detailed descriptions of mesoscale precipitation systems in remote and complex meteorological settings, the utility of these observations has often been limited by the considerable difficulty in their manual processing to remove ground clutter and other sources of contamination, which is a prerequisite to synthesis of reliable airflow and reflectivity fields. This difficulty is further magnified over mountainous terrain, where these sources of contamination take on increased spatial extent and geometric complexity. Removal of such contamination has traditionally required tedious and time-consuming manual editing. As such, routine retrieval of near-surface airflow and precipitation characteristics over steep orography and within hydrologically critical zones, such as deep valleys cutting through mountainous regions (along which population and transportation corridors are frequently concentrated), has been impractical. A new approach is described that largely automates this data-editing procedure for airborne radar platforms, achieving reliable elimination of corrupted data with minimal loss of meteorological signal. Subjective decisions are minimized through a judicious combination of data renavigation, pattern recognition, and reliance upon high-resolution digital terrain information. This technique is applied to data obtained over the Alps by the NCAR Electra and NOAA P-3 aircraft during the recent Mesoscale Alpine Programme field campaign. Three-dimensional airflow and reflectivity fields are shown to illustrate the power and fidelity of this new approach by capitalizing on data collected near, and even beneath, the aircraft track to provide a unique and highly illuminating description of airflow deep within Alpine river valleys and their tributaries during two contrasting orographic precipitation events. The validity of these results is explored through quantitative comparison of this output with independent kinematic measures obtained from ground-based Doppler radar. The utility of airborne radar to provide comprehensive and near-simultaneous views reaching into multiple valleys hidden from the view of ground-based radars is highlighted for a notable case of “down valley” flow, more comprehensively illustrating the nature and extent of low-level upstream blocking during a widespread orographic precipitation event.

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Bradley F. Smull and Robert A. Houze Jr.

Abstract

The mesoscale structure of a squall-line system that passed over Oklahoma on 22 May 1976 is investigated by dual-Doppler radar analysis. The mature storm consisted of a leading line of deep convection, which exhibited organized multicellular structure, trailed by an extensive region of stratiform precipitation marked by a radar bright band at the melting level. These contrasting radar echo regimes were separated by a narrow band of weak reflectivity at lower levels, which has been termed the “transition zone.” While conventional and single-Doppler radar analyses documented the persistence of this precipitation structure and revealed the corresponding kinematic structure in one part of the mature storm, the dual-Doppler analysis demonstrates the pervasiveness of these features over much of the squall line's length.

The structure of a midtropospheric maximum of rearward, system-relative flow crossing the system is particularly well described by the dual-Doppler data. This mesoscale current originated ahead of the storm. gained strength while passing through the convective line, spanned the transition zone, and extended to near the back edge of the stratiform region. It strongly influenced precipitation growth and radar echo structure by promoting the transfer of ice particles from convective cells across the transition zone into the trailing stratiform region. Deep, intense updrafts occurred in association with convective cells along the leading edge of the system. Convective downdrafts were apparently active both in the lower troposphere, where thermodynamic data showed they were a source of air feeding the leading gust front, and at upper levels, where the Doppler analysis indicated they were forced by convergence of air detrained from the tops of the updrafts with slower moving ambient air. Horizontal momentum transported vertically by convective motions converged at midlevels, accelerating parcels rearward and so bolstering the front-to-rear flow.

Profiles of radar-derived mean vertical motion confirm the presence of a mesoscale updraft overlying a mesoscale downdraft in the transition and trailing stratiform regions. The mean descent in the lower troposphere was particularly deep and intense in the transition zone and may have contributed to the decreased reflectivity values observed there.

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Bradley F. Smull and Robert A. Houze Jr.

Abstract

The relative airflow and accompanying precipitation structure of squall lines trailed by mesoscale regions of stratiform rain are examined with emphasis on the occurrence of “rear inflow,” i.e. the intrusion of environmental air into these storms across the trailing precipitation boundary. Three cases from Kansas and Oklahoma provide examples of “Strong Rear Inflow,” which crosses the back edge of the stratiform precipitation area at relative speeds exceeding 10 m s−1. The vertical profiles of relative flow at the trailing precipitation boundary of these three systems were remarkably similar, with the rear inflow confined to a jet-like layer centered at about 550 mb. Doppler radar data for two of these cases showed that the rear inflow jet occupied a continuous channel extending from middle levels at the back edge of the stratiform region to lower levels of the leading convective region, where it merged with outflow from convective downdrafts to bolster the leading gust front. Strong front-to-rear flow occurred both above and below this layer. The front-to-rear flow lying above the rear inflow jet was consistently strengthened in the vicinity of convective cells and was separated from the rear inflow by an interface of strong vertical wind shear that sloped upward toward the rear of the storm. Soundings indicate this interface marked the division between cloudy air (the trailing “anvil”) associated mesoscale updraft (above) and subsaturated air in the mesoscale downdraft (below).

An inclusive review of the literature reveals that these three midlatitude squall lines had stronger rear inflow than any previously described squall lines with trailing stratiform precipitation. Five “Weak Rear Inflow” cases had peak inflows at the back edge of the stratiform rain area between 5 and 10 m s−1. The vertical profiles of relative flow at the trailing edge exhibited a variety of structures, with the rear inflow maximum sometimes at higher altitude and sometimes at lower altitude than in the Strong Rear Inflow cases. The literature further reveals ten studies in which the relative flow at the back edge of the precipitation showed little if any rear inflow (<5 m s−1), suggestive of a stagnation of the midlevel system-relative flow as air at the back edge of the stratiform region moved at or near the speed of the system. These “Stagnation Zone” squall systems, like both the Strong and Weak Rear Inflow cases, exhibited maxima of front-to-rear flow at both upper and lower levels; however, the front-to-rear flow was not as strong as in the Strong Rear Inflow cases. The stagnation layer was located between 650 and 750 mb, considerably below the height of the inflow jet in the Strong Rear Inflow systems. While no appreciable rear inflow occurred at the back edge of Stagnation Zone cases, rear-to-front flow has been observed to develop at midlevels in the interior of their stratiform regions, suggesting that physical processes internal to the mesoscale system are capable of generating rear-to-front flow behind the convective line without the aid of ambient flow entering the storm.

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Bradley F. Smull and John A. Augustine

Abstract

A multiscale analysis reveals diverse atmospheric structure and processes within a mesoscale convective complex (MCC) observed during the Oklahoma-Kansas Preliminary Regional Experiment for STORM-Central (PRE-STORM) experiment. This midlatitude system was the second in a series of four MCCs that developed and traveled along a quasi-stationary frontal zone over the central United States on 3–4 June 1985. Objectively analyzed mesoscale upper-air soundings encompassing the MCC are interpreted in tandem with more detailed dual-Doppler radar measurements that disclose the storm's internal airflow and precipitation structure. The mature MCC is found to include a variety of local environments and associated weather, ranging from tornadic thunderstorms to more linear convective bands and widespread chilling rains. A corresponding spectrum of mesoscale ver6cW-motion profiles is documented. These findings are related to previous composite-based portrayals of MCCs, as well as detailed case studies of simpler squall-type convective systems.

A hallmark of this storm was its “open-wave” precipitation pattern, in which two convective bands intersected so as to resemble a miniature developing frontal cyclone. This resemblance proves superficial, however, since 1) anticyclonic lower-tropospheric flow was observed in place of the expected cyclonic circulation near the convective apex, and 2) the accompanying wavelike lower-tropospheric temperature pattern was strongly influenced by moist processes intrinsic to the MCC (e.g., evaporative cooling), as opposed to horizontal advection about a developing vortex. The storm's intriguing organization is instead postulated to have resulted from the superposition of two preferred convective modes: one aligned with the mean vertical wind-shear vector, accompanied by marked cross-band thermal contrast and deformation through a deep layer, and another oriented perpendicular to the low-level shear, which exhibited a shallow gust front and mesoscale cold pool as found in squall-line systems. Highly three-dimensional airflow within the mature MCC and a pronounced modulation of convective instability across an embedded frontal-like zone further promoted the storm's asymmetric precipitation pattern.

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David P. Jorgensen and Bradley F. Smull

During the spring of 1991, scientists from the National Severe Storms Laboratory conducted a field observational program to obtain a better understanding of the processes responsible for organizing and maintaining the dynamical and electrical structure of mesoscale convective systems (MCSs), as well as mechanisms acting to organize and propagate the dryline. Extensive use was made of a relatively new observing tool, the airborne Doppler radar installed on one of the NOAA P-3 research aircraft, to map the precipitation and kinematic structure of large mesoscale convective systems. The radar was operated in an innovative scanning mode in order to collect pseudo-dual-Doppler wind data from a straight-line flight path. This scanning method, termed the fore/aft scanning technique (FAST), effectively maps out the three-dimensional wind field over mesoscale domains (e.g., 80 km × 100 km) in ~15 min with horizontal data spacing of 1–2 km. Several MCSs were observed over central Oklahoma during May and June of 1991, and one such system exhibiting a “bow-echo” structure is described. Many observed features of this MCS correspond to structures seen in nonhydrostatic numerical simulations. These features include a pronounced bulge or “bow” in the convective line (convex toward the storm's direction of propagation), a strong descending rear inflow jet whose axis is aligned with the apex of the bow, and a cyclonic vortex (most pronounced at heights of 2–3 km) situated in the trailing stratiform region lateral to the axis of strongest rear inflow. Dopplerderived wind analyses reveal the likely role played by the mesoscale circulation in twisting environmental vertical shear and converging ambient vertical vorticity in maintaining and amplifying the vortex. The relatively detailed yet horizontally extensive airflow analyses also reveal the utility and advantages of airborne Doppler radar in the study of large convective systems.

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Bradley F. Smull and Robert A. Houze Jr.

Abstract

A squall line exhibiting an extensive trailing region of stratiform precipitation passed over the observational network of the National Severe Storms Laboratory on 22 May 1976. Satellite imagery and conventional radar observations document its evolution from a broken line of thunderstorms to a system of mesoscale proportions, and single-Doppler radar observations describe aspects of its mature structure. Satellite measurements of cloud-top temperature showed the system to be a mesoscale convective complex (MCC). The life cycle of the system exhibited the stages of development seen in tropical cloud clusters.

At maturity, two prominent mesoscale flow regimes were identified at midlevels: one marked by inflow into the system's front and continuing toward its rear, and another associated with inflow entering the extreme rear of the system.

The rear inflow was associated with a cyclonic midlevel vortex in the stratiform precipitation region. It produced a concavity, or “notch”, in the back edge of the precipitation echo. Shortly after the appearance of the notch, a downwind segment of the leading convective line accelerated forward. The notch persisted through the dissipating stage, at which time secondary notches also formed. The last remnant of the stratiform precipitation area took the form of a chain of three comma-shaped vortices, whose origin could be traced in time back to the primary and secondary notches.

The inflow at the front of the system spanned both the leading convective and trailing stratiform regions. Convective-scale velocity maxima were superimposed on this front-to-rear flow in the convective region, while a broad maximum of the rearward current occurred in the stratiform region, just above the melting layer. This rearward system-relative flow apparently promoted the broad structure of the precipitation area. Slowly falling ice particles originating at convective cell tops were evidently advected rearward and dispersed over a 50–100 km wide region, whereupon their melting produced a prominent radar bright band.

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Matthew F. Garvert, Bradley Smull, and Cliff Mass

Abstract

This study combines high-resolution mesoscale model simulations and comprehensive airborne Doppler radar observations to identify kinematic structures influencing the production and mesoscale distribution of precipitation and microphysical processes during a period of heavy prefrontal orographic rainfall over the Cascade Mountains of Oregon on 13–14 December 2001 during the second phase of the Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE-2) field program. Airborne-based radar detection of precipitation from well upstream of the Cascades to the lee allows a depiction of terrain-induced wave motions in unprecedented detail.

Two distinct scales of mesoscale wave–like air motions are identified: 1) a vertically propagating mountain wave anchored to the Cascade crest associated with strong midlevel zonal (i.e., cross barrier) flow, and 2) smaller-scale (<20-km horizontal wavelength) undulations over the windward foothills triggered by interaction of the low-level along-barrier flow with multiple ridge–valley corrugations oriented perpendicular to the Cascade crest. These undulations modulate cloud liquid water (CLW) and snow mixing ratios in the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5), with modeled structures comparing favorably to radar-documented zones of enhanced reflectivity and CLW measured by the NOAA P3 aircraft.

Errors in the model representation of a low-level shear layer and the vertically propagating mountain waves are analyzed through a variety of sensitivity tests, which indicated that the mountain wave’s amplitude and placement are extremely sensitive to the planetary boundary layer (PBL) parameterization being employed. The effects of 1) using unsmoothed versus smoothed terrain and 2) the removal of upstream coastal terrain on the flow and precipitation over the Cascades are evaluated through a series of sensitivity experiments. Inclusion of unsmoothed terrain resulted in net surface precipitation increases of ∼4%–14% over the windward slopes relative to the smoothed-terrain simulation. Small-scale waves (<20-km horizontal wavelength) over the windward slopes significantly impact the horizontal pattern of precipitation and hence quantitative precipitation forecast (QPF) accuracy.

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Cheng-Ku Yu and Bradley F. Smull

Abstract

This study uses airborne Doppler radar observations to describe the mesoscale structure and evolution of a cold frontal system as it made landfall on the mountainous coast of Oregon and northern California on 1 December 1995 during the Coastal Observations and Simulations with Topography experiment. This section of coastline constitutes a steep, approximately two-dimensional north–south-oriented orographic barrier. The front exhibited a northeast–southwest orientation and thus intersected the axis of high terrain at an acute angle. The along-barrier pressure gradient and low-level winds increased with time along the coastal zone and reached a maximum as the front made landfall. Stably stratified prefrontal flow was strongly blocked by the orography, resulting in a confluent transition from pervasive southwesterly winds offshore to a narrow zone of accelerated south-southwesterly flow near the coast, where wind speeds approached 30 m s−1 at a height of 750 m above mean sea level. Postfrontal flow was much less affected by the topography, probably because of its weaker static stability. Upstream blocking by the steep coastal terrain also evidently led to modifications of precipitation in the vicinity of the front, including the rapid genesis of a narrow cold-frontal rainband (NCFR) and nearshore enhancement of two prefrontal precipitation bands. This evolution of the NCFR is interpreted in conjunction with changes in prefrontal vertical wind shear, which favored more upright convective ascent as the front neared shore and encountered accelerated along-barrier flow adjacent to the steep terrain. In addition, a statistical examination of observed radar reflectivity patterns shows that the intensity of frontal precipitation systematically decreased with upstream distance away from the orographic barrier.

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Brian A. Colle, Clifford F. Mass, and Bradley F. Smull

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This paper documents the three-dimensional flow and precipitation structures associated with a weak cold front interacting with the Olympic Mountains and the subsequent development of a Puget Sound convergence zone. This study utilizes data collected during COAST IOP5 (the fifth intensive observing period of the Coastal Observation and Simulation with Topography field experiment) that took place on 11–12 December 1993. One of the most important data sources was a NOAA P-3 aircraft, which provided flight-level data, radar reflectivity, and Doppler winds as it circumnavigated the Olympics. Initially, frontal passage along the western foothills of the Olympics was accompanied by a 2°–3°C temperature drop, a rapid wind shift to northwesterlies, and an intense line of precipitation (35–45 dBZ); however, the wind shift and associated precipitation structures attenuated when the front began to ascend the windward slopes of the Olympics. Surface and P-3 observations document the deformation of the front around the Olympics. The front accelerated and became more intense and shallow to the northeast of the Olympics where it encountered prefrontal downslope warming and strong southerlies. This portion of the front and the associated precipitation band moved southward toward central Puget Sound and eventually became stationary to the east of the Olympics, where a Puget Sound convergence zone developed and subsequently dissipated after a few hours.

This case was simulated down to 3-km horizontal resolution using the Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5). Frontogenesis calculations using the model data suggest that prefrontal flow splitting around the Olympics intensified the front by enhanced stretching deformation as it approached the barrier. A simulation with half-height Olympics showed that reduced flow splitting resulted in a weaker front approaching the windward foothills. As observed by P-3 observations and in accordance with previous theoretical studies, frontolysis occurred as the modeled front ascended the windward slope. Sensitivity experiments showed that diabatic effects were important in maintaining the front as it rounded the north side of the Olympics and pushed southward toward central Puget Sound.

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Scott A. Braun, Robert A. Houze Jr., and Bradley F. Smull

Abstract

Airborne Doppler radar data, collected off the Pacific Northwest coast by a NOAA WP-3D Orion aircraft over an 8-h period on 8 December 1993 during the Coastal Observations and Simulations with Topography experiment, reveal the mesoscale structure of an intense frontal system while it was well offshore and as it approached within 20 km of the Oregon coastline. During the offshore stage, a portion of the narrow cold-frontal rainband was characterized by deep convective cores. Pseudo-dual-Doppler analyses characterize the kinematic and precipitation structure of the deep convection.

Pseudo-dual-Doppler analyses describe the subsequent evolution of the narrow cold-frontal rainband as it approached to within 20 km of the Oregon coast. Deformation of the frontal zone appeared to cause the dissipation of one of three precipitation cores contained within the dual-Doppler area. The precipitation cores and the strong convergence zone associated with the front conformed to some degree to the shape of the coastline near Cape Blanco, Oregon, as the front neared the coast. Changes in the prefrontal flow that occurred as the front approached the coast were qualitatively consistent with theoretical and numerical studies of upstream orographic influence. Comparison of pseudo-dual-Doppler-derived velocity profiles with idealized numerical model calculations suggests that the nearshore evolution of the frontal rainband was significantly affected by an upstream influence of the coastal orography.

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