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Tammy M. Weckwerth

Abstract

Observations during the Convection and Precipitation/Electrification (CaPE) project illustrate that horizontal convective rolls are capable of providing sufficient forcing to initiate free moist convection. Rolls occurred on the majority of days during CaPE but on only some of those days were they able to trigger thunderstorms. This study was undertaken to ascertain the difference between the two types of roll days: the storm days and the no-storm days. All obvious sounding parameters were examined: stability parameters, midlevel moisture, and vertical wind shear. None of them showed a difference between the storm and no-storm days. This is not surprising in light of recent work showing that soundings within rolls are not representative of the environmental stability unless they happen to be launched into roll updraft branches. This is due to the upward transport of warm, moist air in the roll updraft regions atop which cloud streets and sometimes thunderstorms form. Numerous other parameters examined were also fruitless in identifying any difference between the days. These included surface station measurements, cell motion relative to roll updraft locations, surface topography, and roll circulation strength and depth. The only useful predictor was obtained by modifying the soundings using aircraft data as they were flying across the rolls and sampling moisture contained within the roll updraft branches. Using these roll updraft moisture measurements to recalculate sounding stability parameters provided an effective means of predicting thunderstorm formation.

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Tammy M. Weckwerth and Ulrike Romatschke

Abstract

The overarching goal of the Plains Elevated Convection At Night (PECAN) field campaign was to improve understanding of the processes contributing to the nocturnal precipitation maximum in the U.S. Great Plains. This study presents the precipitation pattern surrounding PECAN and addresses the origin, timing, duration, and potential causes contributing to that pattern. It is shown that the precipitation occurs most frequently at night, as expected. The maximum in the precipitation pattern occurred in the northeastern portion of the PECAN radar domain. The source of the rainfall was attributed to mountain-initiated precipitation, plains-initiated precipitation, precipitation advecting over the border of the radar domain, and episodes in which different initiation categories merged together. Through the combination of mountain-initiated, border, and merged episodes, 70% of the Great Plains precipitation was caused by episodes that formed outside of the PECAN domain and propagated into the region. The remaining 30% of the precipitation was attributed to plains-initiated storms. The mountain-initiated storms formed primarily in the afternoon and typically dissipated near the mountains. For those that survived, they propagated eastward, grew upscale, and contributed 27% of the precipitation in the plains. The plains-initiated precipitation fell mostly during the afternoon but also contributed to overnight rainfall and those locally triggered systems tended to be relatively smaller and shorter lived. For the top 10% rain-producing events, composite reanalysis fields showed that synoptic-scale features influenced the precipitation pattern and timing: an approaching trough established southwesterly moist flow throughout the region and a nocturnal low-level jet transported moisture to its terminus in the northeast corner of the PECAN domain.

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Tammy M. Weckwerth and Roger M. Wakimoto

Abstract

On 13 July 1986 a cold-air outflow from thunderstorms over Illinois and Missouri propagated through the MIST (Microburst and Severe Thunderstorm) network over northern Alabama. The study of this outflow is important since the gust front was solely responsible for the initiation of numerous convective cells. Previous studies have documented the initiation of convection due to colliding gust fronts. In addition, there was a pronounced mesoscale organization of the cells atop the outflow boundary. This was most likely due to a combination of Kelvin–Helmholtz (K–H) and internal gravity (IG) wave activity. In contrast to previous cases, the K–H wave crests were oriented nearly perpendicular to the gust front within the analysis area. The resulting intersections between the circulations associated with the K–H waves and the gust front produced favorable locations for the initiation of convection. Subsequently, the convective cells remained along the updraft side of the K–H wave circulations as they propagated back relative to the gust front. In addition, the gust front induced IG waves that were oriented parallel to the gust front. The enhanced upward motions associated with the IG waves resulted in a periodic arrangement of the convective cells along the updraft side of the K–H waves. The combined motion of the K–H and IG waves was consistent with the cell movement atop the cold-air outflow.

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Tammy M. Weckwerth and David B. Parsons

Abstract

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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David P. Jorgensen and Tammy M. Weckwerth

Abstract

From its initial deployment as a research tool following the second World War, radar has played a fundamental role in revealing the forces that initiate and organize severe storms and larger mesoscale convective systems composed of a conglomeration of convective storm cells. Early radar observations were primarily descriptive and showed the tremendous variety of precipitating moist convection types and sizes. Examples include single convective storms, longer-lived multicellular storms, fast-moving squall lines, slower-moving linear and nonlinear convective systems, and long-lived supercell storms. Certain modes or types of convective systems were shown to possess a variety of hazardous weather that includes very heavy rain, large hail, straight-line damaging winds, tornadoes, and lightning. It was soon recognized that the type of convective system was strongly dependent on the environment in which it was embedded. Researchers determined that two variables were particularly important in describing convective behavior: the vertical profile of the horizontal wind and potential instability of the air feeding the system [convective available potential energy (CAPE)]. The types of convective systems are discussed here according to their typical shear and CAPE values. In addition to the knowledge gained from observational radar studies, considerable advancement in understanding of convective system dynamics has resulted from high-resolution numerical simulations.

In addition to being a critical factor in determining the particular structure and organization that convective systems assume once convection is initiated, radar (particularly in clear air mode) has been a leading tool in identifying forcing mechanisms for convective initiation. In particular, the role of “boundary layer forcing” in initiating convection has received much attention in recent years. Boundary layer circulations, which are sometimes precursors to deep convective development, are clearly observed by radar as reflectivity fine lines and/or discontinuities in Doppler velocity. Some of these mesoscale boundary layer mechanisms for producing upward motion include horizontal convective roles, sea-breeze circulations, drylines, gust fronts, orographic circulations (e.g., mountain–valley), and circulations resulting from horizontal inhomogeneities in surface character. Convection initiation sometimes does not occur continuously along boundaries but only at preferred along-boundary locations. Location preferences can sometimes be identified with boundary intersections, such as colliding gust fronts, sea-breeze fronts and rolls, and drylines and rolls. It is not always clear, however, why convection forms at certain locations along boundaries and not others. It is possible that low-level waves, bores, and other features, which may not always be apparent in radar data, may also play an important role in convection initiation processes.

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Tammy M. Weckwerth, James W. Wilson, and Roger M. Wakimoto

Abstract

Data from the Convection and Precipitation/Electrification (CaPE) Experiment conducted during the summer of 1991 are used to examine and quantify the horizontal variability of temperature and moisture within the convective boundary layer (CBL). Potential temperature variations were only about 0.5 K, while variations in water vapor mixing ratio values of 1.5–2.5 g kg−1 were observed throughout the CBL. Using radar, aircraft, and sounding data, it is shown that horizontal convective rolls are the likely cause of these variabilities. The enhanced moisture occurred within the roll updraft regions, thus rolls were transporting moist air from the surface upward. The observed cloud-base heights, obtained from cloud photogrammetry, were produced from the highest moisture values within the roll updraft regions. Since the roll ascending branches contained moisture values that were most representative of the observed cloud-base heights, it is likely that measurements from within the roll updrafts would provide the best estimate of the potential for deep, moist convection.

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Nolan T. Atkins, Roger M. Wakimoto, and Tammy M. Weckwerth

Abstract

The three-dimensional kinematic structures of offshore and onshore flow sea-breeze fronts observed during the CaPE experiment are shown using high resolution dual-Doppler and aircraft data. The fronts interact with horizontal convective rolls (HCRs) that develop within the convective boundary layer. Nearly perpendicular intersections between the HCRs and sea-breeze front were observed during the offshore flow case. Close to the front, the HCR axes were tilted upward and lifted by the frontal updrafts. Consequently, a deeper updraft was created at the intersection points, providing additional impetus for cloud development. Furthermore, clouds forming at periodic intervals along the NCRs intensified as they propagated over the front.

During the onshore flow case, the HCR orientation was nearly parallel to the front. Extended sections of the front “merged” with the HCRs. This process strengthened the front and is explained as the merger of like-sign vortices associated with both the front and HCRs. Clouds formed along the intensified portions of the front and at the locations of periodic enhancements on the HCR, which were present prior to the merger.

Documentation of two distinct frontal boundaries is presented for the onshore flow case. The first is a kinematic sea-breeze front delineating the region of maximum near-surface convergence between the sea-breeze air and the warmer, drier environmental air. The second is a thermodynamic sea-breeze front, which delineates the location where the mean thermodynamic properties differ from the ambient air mass. It is generated by the interaction of the HCRs with the sea breeze and extends a few kilometers ahead of the kinematic frontal position.

The kinematic differences between the two cases are quantitatively illustrated. The offshore flow case exhibited stronger low-level convergence, larger vertical velocities, and larger radar reflectivity values. The source air for the clouds developing along the front originated from the ambient and moist sea-breeze air masses for the offshore and onshore now cases, respectively.

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Tammy M. Weckwerth, Thomas W. Horst, and James W. Wilson

Abstract

A comprehensive observational dataset encompassing the entire temporal evolution of horizontal convective rolls was obtained for the first time. Florida, Illinois, and Kansas measurements from preroll conditions through the development of well-defined rolls to their dissipation were utilized to determine the factors influencing roll evolution. When the buoyancy flux reached a critical value of 35–50 W m−2, the first form of boundary layer convection resolved by radar was rolls. It was noted that two-dimensional convective rolls can evolve in a convective boundary layer in the absence of significant wind speed and shear. In fact, the value of wind speed or shear in itself did not seem to determine when or if rolls would form, although it did influence roll evolution. Well-defined, two-dimensional rolls only occurred while −z i/L, where z i is the convective boundary layer depth and L is the Monin–Obukhov length, was less than ∼25, which is consistent with previous studies. As −z i/L increased throughout the day, either open cellular convection or unorganized boundary layer convection was the dominant clear-air convective mode. If the wind speed was low (mean boundary layer winds <3 m s−1 or 10-m winds <2 m s−1) during roll occurrences, rolls evolved into open cells. Alternatively, if the wind speed throughout the day was relatively high, rolls broke apart into random, unorganized convective elements. These are unprecedented observations of two-dimensional convection evolving into three-dimensional convection over land, which is analogous to laboratory convection where increased thermal forcing can produce a transition from two-dimensional to three-dimensional structures. Finally, the roll orientation was governed primarily by the mean convective boundary layer wind direction.

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Ya-Chien Feng, Frédéric Fabry, and Tammy M. Weckwerth

Abstract

Accurate radar refractivity retrievals are critical for quantitative applications, such as assimilating refractivity into numerical models or studying boundary layer and convection processes. However, the technique as originally developed makes some simplistic assumptions about the heights of ground targets () and the vertical gradient of refractivity (). In reality, the field of target phases used for refractivity retrieval is noisy because of varying terrain and introduces estimation biases. To obtain a refractivity map at a constant height above terrain, a 2D horizontal refractivity field at the radar height must be computed and corrected for altitude using an average . This is achieved by theoretically clarifying the interpretation of the measured phase considering the varying and the temporal change of . Evolving causes systematic refractivity biases, as it affects the beam trajectory, the associated target range, and the refractivity field sampled between selected targets of different heights. To determine and changes, a twofold approach is proposed: first, can be reasonably inferred based on terrain height; then, a new method of estimation is devised by using the property of the returned powers of a pointlike target at successive antenna elevations. The obtained shows skill based on in situ tower observation. As a result, the data quality of the retrieved refractivity may be improved with the newly added information of and .

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Tammy M. Weckwerth, James W. Wilson, Roger M. Wakimoto, and N. Andrew Crook

Abstract

Data from the Convection and Precipitation/Electrification (CaPE) project, as well as results from numerical simulations, are used to study horizontal convective rolls. The environmental conditions necessary for sustaining rolls and for influencing the aspect ratio, ratio of roll wavelength to convective boundary layer (CBL) depth, and orientation are examined. Observations and numerical model simulations both suggest that a moderate surface sensible heat flux and some vertical wind shear are necessary for roll existence. Unlike some previous studies, however, it is shown that rolls occurred within very low CBL shear conditions (∼2 × 10−3 s−1). In addition, the low-level (i.e., ∼200 m) shear seems to be more important than the shear through the depth of the CBL in roll sustenance. The aspect ratio is shown to be proportional to the CBL instability, measured in terms of the Monin–Obukhov length. The roll orientation is similar to the wind direction at 10 m AGL, the CBL wind direction, the inversion-level wind direction, and the CBL shear direction. This is not surprising since there was very little directional shear observed within the CBL during CaPE.

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