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Robert S. Gaza

Abstract

This study examines the relationship between mesoscale meteorological conditions and high-ozone days in the northeastern United States. It is proposed that the leeside trough and the sea-breeze front are two mesoscale features that can be important on these high-ozone days. The 19 June and 4 July 1995 case studies are presented to illustrate the role of each feature, respectively.

The 19 June case revealed that the leeside trough initially formed to the lee of the high terrain from southeast Pennsylvania to the Maine coast but shifted southeastward during the course of the day. As the trough advanced toward the coast, the surface winds ahead of the trough remained southwesterly, paralleling the source regions of the major metropolitan areas. It appears that the surface flow near the trough acted to elevate and concentrate ozone and its precursors in a narrow band along and just south of the trough, with much lower values north and west of the trough, where cleaner trajectories were encountered. In the 4 July case, the sea-breeze front initially formed parallel to the Atlantic coast and advanced northward through southeast New York and southern New England as the larger-scale flow became more southerly. High ozone concentrations developed in a narrow, east–west band along, and just south of, the northward-moving boundary. The combination of the boundary’s initial location near a major source region, confluent airflow along the front, and a capping inversion on the front’s cool side is presented as the likely cause for the observed pattern of high ozone. The high ozone values occurred despite relatively low peak values up and down the coast the previous day.

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Robert S. Gaza and Lance F. Bosart

Abstract

Split flow, defined by the presence or two separate westerly airstreams in the mid- and upper troposphere, is common in middle latitudes. Occasionally, individual troughs in the separate branches of the westerlies will merge into one trough with a single vorticity maximum. Illustrative examples are used to demonstrate the significance of the trough merger process to the cyclogenesis process.

The results of a subjective climatological study revealed the existence of 21 merger events across North America based upon the amalgamation of separate 500-mb vorticity centers. Eighteen of these occurred during the 1978–85 sampling period and three were added from other years. The trough merger cases were found to occur only in central and eastern North America east of 100°W and north of 35°N. Trough merger locations varied seasonally with the position of the principal tropospheric baroclinic zone. A composite of 17 trough-merger events demonstrated that the orientation of the 500-mb trough axis changed from northern-southwest to northwest-southeast (negative tilt) while the trough amplitude nearly doubled in the 24 hours preceding trough merger. Explosive cyclogenesis occurred in 14 out of 21 events in response to the development of the negatively tilted 500-mb trough associated with the merger process. The trough merger process was not well simulated by the then operational limited fine mesh (LFM) prediction model with 12 (2) of the 21 events correctly predicted 24 h (48 h) in advance. Finally, the physical implications of the climatological findings are discussed.

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Robert S. Gaza and Lance F. Bosart

Abstract

The severe weather and localized flooding which hit Kansas City on the afternoon of 4 June 1979 are investigated through a detailed synoptic scale and mesoscale analysis of the events leading up to the formation of the storms. The results indicate that despite a middle and upper troposphere characterized by weak synoptic scale forcing and an absence of many of the “classic” severe weather parameters, a line of deep convection developed from north central Kansas through northwest Missouri and into southern lowa. The key feature was to be found in the lower troposphere in the form of a trough line which had formed along the lee side of the Rocky Mountains and moved eastward into the Great Plains area with the approach of an upper level shortwave from the northwest. Convergence and southwesterly flow ahead of the low-level trough helped create a thermodynamically favorable environment, while the associated ascent provided the necessary lift to ignite the convection. The most intense cells were confined to a small area in extreme northwest Missouri and formed in response to the intersection of the trough line with air behind an old outflow boundary which contained high values of equivalent potential temperature. In several respects the storm was not well forecast which partially can be attributed to the failure of the operational Limited Area Fine Mesh quantitative precipitation forecasts which generated no precipitation in an area where flash flooding later occurred. The storm was an example of a northwest flow severe weather event. Our findings are compared and contrasted with the published northwest flow severe weather climatology by Johns.

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Ronald W. Kessler, Lance F. Bosart, and Robert S. Gaza

An analysis has been conducted of a suspected daily maximum temperature (DMT) bias introduced by the replacement of the National Weather Service (NWS) HO-63 hygrothermograph with a modernized HO-83 instrument at Albany, New York, on 6 February 1985. The analysis involves an assessment of the bias before and after the changeover and has four components: 1) a bias comparison with seven surrounding NWS Cooperative Observer Network (COOP) stations; 2) a comparison of the DMT with the highest reported hourly temperature during the summer; 3) a comparison of 1800 and 2100 UTC model output statistics (MOS) operational temperature forecasts from the 0000 UTC forecast cycle during the summer with the observed temperatures; and 4) a comparison of the reported maximum surface temperature with the observed 850-mb temperature at the time of the surface maximum temperature in July for days with at least 75% of possible sunshine.

The results show that the reported DMT at Albany has increased by ~0.5°C relative to the surrounding COOP locations since the introduction of the HO-83 sensor. The warm bias is largest on sunny, light wind days. Roughly two-thirds (one-third) of the warm bias can be attributed to stratification of the data by wind speed (percent of possible sunshine), suggestive of an aspiration problem in the sensor housing. Subsequent to the installation of the HO-83 sensor, 1) the percentage of observed DMTs that exceeded the highest reported hourly value increased from 39.8% to 64.1% (8.7% and 16.1%) across the 0.56°C (1.12°C) threshold, amounting to an overall 0.18°C warm bias, 2) the MOS temperature analysis revealed the existence of a net warming of 0.56°C (0.35°C) at 1800 UTC (2100 UTC), and 3) the observed maximum surface temperature warmed 1.17°C relative to the given 850-mb temperature on sunny July days.

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