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.

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.

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.

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.

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.

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 (θ_V ) 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.

Abstract

The detailed analysis of the three-dimensional structure of a dryline observed over the Oklahoma panhandle during the International H₂O Project (IHOP_2002) on 11 June 2002 is presented. High-resolution observations obtained from the National Center for Atmospheric Research Electra Doppler Radar (ELDORA), S-band dual-polarization Doppler radar (S-Pol), water vapor differential absorption lidar (DIAL) Lidar pour l'Etude des Interactions Aérosols Nuages Dynamique Rayonnement et du Cycle de l'Eau (LEANDRE II; translated as Lidar for the Study of Aerosol–Cloud–Dynamics–Radiation Interactions and of the Water Cycle) as well as Learjet dropsondes are used to reveal the evolution of the dryline structure during late afternoon hours when the dryline was retreating to the northwest. The dryline reflectivity shows significant variability in the along-line direction. Dry air was observed to overrun the moist air in vertical cross sections similar to a density current. The updrafts associated with the dryline were 2–3 m s⁻¹ and were able to initiate boundary-layer-based clouds along the dryline. The formation of this dryline was caused by high equivalent potential temperature air pushing northwestward toward a stationary front in the warm sector.

Middle-level clouds with radar reflectivity greater than 18 dBZ _e near the dryline were detected by ELDORA. A roll boundary, which was associated with larger convergence and moisture content, was evident in the S-Pol data. It is found that the instability parameters most favorable for convection initiation were actually associated with the roll boundary, not the dryline. A storm was initiated near the roll boundary probably as a result of the combination of the favorable instability parameters and stronger upward forcing. It is noted that both the 11 June 2002 dryline and the roll boundary presented in this paper would not be identified if the special datasets from IHOP_2002 were not available.

Although all model runs [fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5), Meso Eta, and Rapid Update Cycle (RUC)] suggested deep convection over the Oklahoma panhandle and several cloud lines were observed near the dryline, the dryline itself did not initiate any storms. The reasons why the dryline failed to produce any storm inside the IHOP_2002 intensive observation region are discussed. Both synoptic-scale and mesoscale conditions that were detrimental to convection initiation in this case are investigated in great detail.

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.

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.

Abstract

A wide array of ground-based and airborne instrumentation is used to examine the kinematic and moisture characteristics of a nonprecipitating cold front observed in west-central Kansas on 10 June 2002 during the International H₂O Project (IHOP). This study, the first of two parts, is focused on describing structures in the across-front dimension. Coarsely resolved observations from the operational network and dropsondes deployed over a 200-km distance centered on the front are combined with higher-resolution observations from in situ sensors, Doppler radars, a microwave radiometer, and a differential absorption lidar that were collected across a ∼40-km swath that straddled a ∼100-km segment of the front.

The northeast–southwest-oriented cold front moved toward the southeast at ∼8–10 m s⁻¹ during the morning hours, but its motion slowed to less than 1 m s⁻¹ in the afternoon. In the early afternoon, the cold front separated cool air with a northerly component flow of 2–4 m s⁻¹ from a 10-km-wide band of hot, dry air with 5 m s⁻¹ winds out of the south-southwest. The average updraft at the frontal interface was ∼0.5 m s⁻¹ and slightly tilted back toward the cool air. A dryline was located to the southeast of the front, separating the hot, dry air mass from a warm, moist air mass composed of 10 m s⁻¹ southerly winds. Later in the afternoon, the warm, moister air moved farther to the northwest, approaching the cold front. The dryline was still well observed in the southwestern part of the observational domain while it vanished almost completely in the northeastern part. Low-level convergence (∼1 × 10⁻³ s⁻¹), vertical vorticity (∼0.5 × 10⁻³ s⁻¹), and vertical velocity (∼1 m s⁻¹) increased. The strong stable layer located at ∼2.0–2.5 km MSL weakened in the course of the afternoon, providing a basis for the development of isolated thunderstorms. The applicability of gravity current theory to the cold front was studied. There was evidence of certain gravity current characteristics, such as Froude numbers between 0.7 and 1.4, a pronounced feeder flow toward the leading edge, and a rotor circulation. Other characteristics, such as a sharp change in pressure and lobe and cleft structures, remain uncertain due to the temporally and spatially variable nature of the phenomenon and the coarse resolution of the measurements.