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Roger M. Wakimoto
and
Hanne V. Murphey

Abstract

An analysis of a dryline that did not initiate convection during the observational period is presented. The dryline was the weakest kinematic boundary observed during the International H2O Project (IHOP), but was associated with a large moisture gradient. Detailed dual-Doppler wind syntheses from an airborne Doppler radar were combined with radar refractivity measurements providing a rare opportunity to examine both the kinematic and moisture characteristics of this boundary. The radar thin line denotes the approximate kinematic position of the dryline and was quasi-linear on this day. In contrast, the moisture pattern across the dryline was more complex than was suggested by the characteristics of the thin line. Prominent in the horizontal plots was the presence of narrow (few kilometers wide) channels of moisture extending 15–20 km into the dry air mass. Past studies have suggested that echo thin lines observed in the clear air can be used as a proxy for delineating the moisture contrast across the dryline. In contrast, the “moisture extrusions” were present even though the thin line was quasi-linear and were located in weak-echo regions along the thin line. It is hypothesized that transverse rolls developed at an angle to the boundary layer winds and intersected the dryline. The kinematic airflow associated with these rolls could have protected the moist tongues from the eroding effect of the dry flow west of the dryline. The moisture extrusions appear to diminish with time as they mix with the surrounding dry air.

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Roger M. Wakimoto
and
Hanne V. Murphey

Abstract

A detailed analysis of a dryline that formed on 22 May 2002 during the International H2O Project (IHOP) is presented. The dryline was classified as a null case since air parcels lifted over the convergence boundary were unable to reach the level of free convection preventing thunderstorms from forming. A secondary dryline associated with a distinct moisture discontinuity developed to the west of the primary dryline. The primary dryline exhibited substantial along-frontal variability owing to the presence of misocyclones. This nonlinear pattern resembled the precipitation core/gap structure associated with cold fronts during one of the analysis times although the misocyclones were positioned within the gap regions. Radar refractivity has been recently shown to accurately retrieve the low-level moisture fields within the convective boundary layer; however, its use in forecasting the initiation of convection has been restricted to qualitative interpretations. This study introduces the total derivative of radar refractivity as a quantitative parameter that may improve nowcasts of convection. Although no storms developed on this day, there was a tendency for maxima of the total derivative to be near regions where cumulus clouds were developing near a convergence boundary.

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Roger M. Wakimoto
and
Hanne V. Murphey

Abstract

An analysis of a cold front over the eastern Atlantic Ocean based on airborne Doppler wind syntheses and dropsonde data is presented. The focus and unique aspect of this study is a segment of the front that was near the center of the cyclone. The dual-Doppler wind synthesis of the frontal zone combined with an average dropsonde spacing of ∼30 km covers a total distance of >450 km in the cross-frontal direction. The finescale resolution and areal coverage of the dataset are believed to be unprecedented. The cold front was characterized by a distinct wind shift and a strong horizontal temperature gradient. The latter was most intense aloft and not at the surface, in contrast to the classical paradigm of surface cold fronts. The shear of the alongfront component of the wind was relatively uniform as a function of height within the frontal zone. This observation is contrary to studies suggesting that frontal zones decrease in intensity above the surface. The surface convergence within the frontal zone was weak. This may have been related to the closeness of the analysis region to the surface low pressure. The prefrontal low-level jet and the upper-level polar jet were both shown to be supergeostrophic based on the analysis of the geopotential height field. It is believed that a major contributing factor to the former was the isallobaric wind from the large pressure tendencies associated with the moving cyclone. A dry pocket accompanied by descending air was noted out ahead of the low-level jet. This pocket produced a region of potential instability that could have supported deep convection, although none was observed on this day. The vertical structure of the front revealed couplets of potential vorticity that appeared to be the result of diabatic heat sources from condensation. The diabatic effect in the frontogenesis equation was the dominant term, exceeding the combined effects of the confluence and tilting terms. As a result, an alternating pattern of frontogenesis–frontolysis developed along the flanks of the maxima of diabatic heating. This study highlights the importance of taking diabatic heating into account even in the absence of deep convection.

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Roger M. Wakimoto
and
Hanne V. Murphey

Abstract

An analysis of six convergence boundaries observed during the International H2O Project (IHOP_2002) is presented. The detailed kinematic and thermodynamic structure of these boundaries was examined using data collected by an airborne Doppler radar and a series of dropsondes released by a jet flying at ∼500 mb. The former and latter platforms were able to resolve the meso-γ- and meso-β-scale circulations, respectively. Convection initiated on three of the days while no storms developed in the regions targeted by the mobile platforms on the other days (referred to as null cases). The airborne radar resolved the finescale structure of four drylines, a cold front, and an outflow boundary on the six days. Horizontal profiles through radar-detected thin lines revealed “bell-shaped distributions” and there appeared to be a seasonal dependence of the peak values of radar reflectivity. The echo profiles through the fine line in May were, in general, greater than those plotted for the June cases. There was no apparent relationship between the intensity of the low-level updraft and convection initiation. The strongest updraft resolved in the dual-Doppler wind synthesis was associated with a null case. There was also no relationship between the strength of the moisture discontinuity across the boundaries and convection initiation.

The three days during which the storms developed were all associated with two convergence boundaries that were adjacent to each other. The two boundaries collided on one of the days; however, the boundaries on the other two days were approximately parallel and remained separated by a distance of 5–15 km. The total derivative of the horizontal vorticity rotating along an axis parallel to the boundary was calculated using dropsonde data. The horizontal gradient of buoyancy was the largest contributor to the change in vorticity and revealed maximum and minimum values that would support the generation of counterrotating circulations, thus promoting vertically rising air parcels. These updrafts would be more conducive to convection initiation. The null cases were characterized by a low-level vorticity generation of only one sign. This pattern would support tilted updrafts. The results presented in this study suggest that it is not necessary for two boundaries to collide in order for thunderstorms to develop. Solenoidally generated horizontal circulations can produce conditions favorable for convection initiation even if the boundaries remain separate.

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Roger M. Wakimoto
,
Huaqing Cai
, and
Hanne V. Murphey

Two remarkable supercell storms developed on 22 June 2003 in eastern Nebraska. One of the thunderstorms, located near the town of Aurora, Nebraska, produced the largest known hailstone on record. Receiving far less attention was an adjacent supercell that was equally impressive and is referred to as the Superior, Nebraska, supercell. The two supercells formed during the Bow Echo and Mesoscale Convective Vortex (MCV) Experiment (BAMEX), operated in the spring and summer of 2003. One of the main platforms used during BAMEX was the airborne Electra Doppler Radar (ELDORA). ELDORA was deployed on the Superior supercell several hours after it initiated. Striking in one of the flybys past the storm was the characteristics of the parent circulation. The Superior supercell was associated with a mesocyclone that was the largest (~9 km in diameter) and the most intense (118ms−1 velocity differential) ever documented. Ground-based observations from a nearby Weather Surveillance Radar-1988 Doppler (WSR-88D) located in Hastings, Nebraska (UEX), could not resolve the Doppler velocities correctly owing to the intensity of the mesocyclone. The environmental conditions, satellite imagery, and Doppler radar observations of this supercell are presented.

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Roger M. Wakimoto
,
Hanne V. Murphey
,
Edward V. Browell
, and
Syed Ismail

Abstract

An analysis of the initiation of deep convection near the triple point between a cold front and dryline is presented. High-spatial-resolution Doppler wind syntheses combined with vertical cross sections of mixing ratio (q) and aerosol scattering ratio retrieved from a lidar flying over the triple point provide an unprecedented view of the initiation process. The Doppler wind synthesis revealed variability along the dryline similar to the precipitation core/gap structure documented for oceanic cold fronts. Vertical cross sections through the dryline suggest a density current–like structure with the hot and dry air being forced up and over the moist air. Double thin lines associated with moisture gradients were also resolved. The vertical profile of retrieved q, approximately perpendicular to the dryline, showed a pronounced jump in the depth of the moisture layer across the triple point. Analyses of dropsonde data show the existence of virtual potential temperature (θV ) gradients across the cold front and the dryline. Although the vertical velocity was strong at the triple point, deep convection initiated ∼50 km to the east. The location where convection first developed was characterized by a prominent aerosol and moisture plume, reduced static stability, and the largest potential instability. An internal gravity wave may have provided the lift to initiate convection.

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Katja Friedrich
,
David E. Kingsmill
,
Cyrille Flamant
,
Hanne V. Murphey
, and
Roger M. Wakimoto

Abstract

Kinematic and thermodynamic structures of a nonprecipitating cold front observed in west-central Kansas on 10 June 2002 during the International H2O Project (IHOP) are examined with dropsondes and airborne instrumentation that includes Doppler radars, a differential absorption lidar, and in situ sensors. Intensive observations were collected along a 125-km segment of the front, with coverage of both the cold front leading edge and the post- and prefrontal areas. Whereas the first part of this two-part series of papers focused on across-front kinematic and moisture characteristics, the study herein investigates alongfront structures relevant for convection initiation. A northeast–southwest-oriented cold front moved into the observational domain from the northwest, but its motion slowed to less than 1 m s−1 in the early afternoon. In the late afternoon it was intersected by a north-northeast–south-southwest-oriented reflectivity thin line that was advected from the southwest, and another boundary that is an extension of a large-scale dryline paralleling the thin line but located farther to the east. Doppler wind synthesis suggests an increase in low-level horizontal wind shear across the cold front leading edge with the approach and intersection of the boundaries causing an increase in low-level convergence (up to ∼1 × 10−3 s−1), positive vertical vorticity (up to ∼0.5 × 10−3 s−1), and upward motion (up to ∼1 m s−1). An organized pattern of misocyclones (vertical vorticity maxima <4 km) and enhanced updrafts with a spacing of ∼5–8 km were observed at the cold front leading edge. At the same time vortex lines manifested as horizontal vorticity maxima were observed within the cold air oriented perpendicular to the cold front leading edge and on top of the vertical wind shear layer. The analysis suggests that inflection point instability was the dominant mechanism for their development. Low Richardson number (0.3–0.4), short lifetime (<2 h), horizontal wavelength of 3–6 km, and collocation with strong horizontal and vertical wind shear are characteristics that support the hypothesis that these instabilities were Kelvin–Helmholtz waves. Towering cumulus developed along the cold front forming a convective cell close to the intersection of the cold front, dryline, and reflectivity thin line.

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Hanne V. Murphey
,
Roger M. Wakimoto
,
Cyrille Flamant
, and
David E. Kingsmill

Abstract

The evolution and finescale structure of a dryline that initiated a line of thunderstorms is presented. Both the along-line variability and mean vertical structure were examined using data collected by an airborne Doppler radar and a water vapor differential absorption lidar (DIAL). The initiation of convection appeared to result from the diurnally induced easterly flow in the maritime air east of the dryline that typically develops late in the day. This flow increased the low-level convergence and allowed rising parcels of air to reach the level of free convection. The along-line variability was largely attributed to numerous misocyclones distorting the thin line of radar reflectivity by advecting dry (moist) air across the dryline south (north) of the misocyclone. The misocyclones also influenced the location of the updrafts, with most of the peak values positioned north of the circulations. As a result, these updrafts were fortuitously positioned in regions of high mixing ratio where the first convective cells initiated.

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Roger M. Wakimoto
,
Hanne V. Murphey
,
David C. Dowell
, and
Howard B. Bluestein

Abstract

A detailed aerial and ground survey of a long-track (∼50 km) F5 tornado is presented. The survey revealed that the tornado exhibited unusual nonlinear movements at two different locations. One portion of the track was associated with a pronounced sinusoidal pattern while another location was characterized by a cusplike pattern. For the first time, high-resolution dual-Doppler wind measurements can be used to evaluate mechanisms for such deviations from a linear tornado path. The analyses of data collected with Electra Doppler Radar (ELDORA) suggest that these departures are trochoidal marks produced as the tornado was revolving within the larger-scale mesocyclone. Retrieved perturbation pressures indicate that the mesocyclone departed significantly from a cyclostrophically balanced state during these deviations. The maximum vorticity associated with the mesocyclone at low levels was shown to be an unreliable indicator of the tornado's intensity.

Vertical cross sections of wind, vertical vorticity, radar reflectivity, and perturbation pressure were photogrammetrically superimposed onto two pictures of the tornado. This merger of data provides a unique view of the structural relationship between the hook echo and the mesocyclone. One of the important conclusions was the lack of a definitive relationship between the widths and strengths of the mesocyclone and the tornado.

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Roger M. Wakimoto
,
Hanne V. Murphey
,
Robert G. Fovell
, and
Wen-Chau Lee

Abstract

Finescale radar observations of intense thermals/starting plumes, during the early stages of precipitation formation, were collected by an airborne Doppler radar on two separate days. The radar data were recorded as the aircraft flew underneath the developing echoes. Mantle echoes (echoes that often appear as an inverted U shape) were observed on both days. Striking in one of the scans was the resemblance of the echo to a mushroom cloud resulting from a nuclear explosion. Numerical simulations using a two-dimensional cloud-resolving model were run to augment the interpretation of the observations. One of the important conclusions was the proposed modification to the default bulk microphysical scheme used in the model. The default scheme yields “a rush to precipitation” leading to the early establishment of large precipitation contents, which is not supported by the observations. Suggested modifications to the scheme are presented.

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