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A polar–subtropical jet superposition is preceded by the development of a polar cyclonic potential vorticity (PV) anomaly at high latitudes and a tropical anticyclonic PV anomaly at subtropical latitudes. A confluent large-scale flow pattern can lead to the juxtaposition of these respective PV anomalies at middle latitudes, resulting in the addition of the nondivergent circulations induced by each PV anomaly and an increase in upper-tropospheric wind speeds at the location of jet superposition. Once these PV anomalies become juxtaposed, vertical motion within the near-jet environment facilitates the advection and diabatic redistribution of tropopause-level PV, and the subsequent formation of the steep, single-step tropopause structure that characterizes a jet superposition. Given the importance of vertical motion during the formation of jet superpositions, this study adopts a quasigeostrophic (QG) diagnostic approach to quantify the production of vertical motion during three types of jet superposition events: polar dominant, eastern subtropical dominant, and western subtropical dominant. The diagnosis reveals that the geostrophic wind induced by polar cyclonic QGPV anomalies is predominantly responsible for QG vertical motion in the vicinity of jet superpositions. The QG vertical motion diagnosed from the along-isotherm component of the Q vector, which represents the vertical motion associated with synoptic-scale waves, is dominant within the near-jet environment. The QG vertical motion diagnosed from the across-isotherm component of the Q vector, which represents the vertical motion associated with frontal circulations in the vicinity of the jet, is subordinate within the near-jet environment, but is relatively more important during eastern subtropical dominant events compared to polar dominant and western subtropical dominant events. [ABSTRACT FROM AUTHOR]

Recent case-study analyses produced a conceptual model suggesting that lower-stratospheric fronts, the lower-stratospheric half of a tropopause jet-front system, develop preferentially in southwesterly flow in the presence of lower-stratospheric geostrophic warm air advection and ascent within the jet core. This conceptual understanding is examined by performing climatological and case analyses of 185 objectively identified lower-stratospheric fronts that occurred over North America during the winter seasons (December, January and February) of 2004-2012 in the 1.0°×1.0°NCEP/NCAR Global Forecast System analysis. The climatological analysis shows a preference for strong southwesterly flow lower-stratospheric fronts to occur over the Intermountain West region and relatively weaker southwesterly flow lower-stratospheric fronts to occur over eastern North America. The southwesterly flow lower-stratospheric fronts were composited according to their geographical location and the analysis shows that the composite lower-stratospheric and tropospheric structures in these two locations have statistical differences. The case-study analyses of southwesterly flow lower-stratospheric fronts in these two regions highlight different, geographically dependent, frontal evolutions. The strong lower-stratospheric front identified in the Intermountain West region interacted with an orographic gravity wave and associated lower-stratospheric ascent that rearranged the local thermal field. The lower-stratospheric front in eastern North America developed parallel to a surface front and was associated with diabatically reduced static stability in the upper troposphere, geostrophic warm air advection, and enhanced ascent. [ABSTRACT FROM AUTHOR]

Characteristics of flow in the planetary boundary layer (PBL) strongly affect the design of tall structures. PBL modelling in building codes, based as it is on empirical data from the 1960s and 1970s, differs significantly from contemporary PBL models, which account for both 'neutral' flows, and 'conventionally neutral' flows. PBL heights estimated in these relatively sophisticated models are typically approximately half as large as those obtained using the classical asymptotic similarity approach, and are one order of magnitude larger than those specified in North American and Japanese building codes. A simple method is proposed for estimating the friction velocity and PBL height as functions of specified surface roughness and geostrophic wind speed. Based on published results, it is tentatively determined that, even at elevations as high as 800 m above the surface, the contribution to the resultant mean flow velocity of the component V normal to the surface stress is negligible and the veering angle is of the order of only 5 $$^{\circ }$$ . This note aims to encourage dialogue between boundary-layer meteorologists and structural engineers. [ABSTRACT FROM AUTHOR]

On 12-13 November 2003, a mid-latitude cyclone caused a widespread non-convective high wind event across the Great Lakes region. In this paper, we attempt to explain the dynamical cause for these winds using ageostrophic wind theory. First, the theory of ageostrophic winds is explored and related to some conventional rules of thumb for wind forecasting. Next, ageostrophic wind terms relating o the isallobaric wind, horizontal advective processes, vertical advective processes, and friction are calculated from the North American Regional Reanalysis. The aggregate total wind results for each region are compared to observed winds at three different pressure levels: 925, 850, and 700 hPa. The aggregate results are in good agreement with observations, particularly at lower altitudes in the Lake Erie region, lending credence to our approach. The dominant ageostrophic contributor to the high winds for this storm was the isallobaric wind, but all terms played non-negligible roles at one location, level, or time. Analyses that focus only on one term or do not vectorially combine all geostrophic and ageostrophic contributions are therefore likely to be misleading. [ABSTRACT FROM AUTHOR]

This study documents the convective storm structures and ambient conditions associated with severe storms (wind, hail, and tornado) over the northeastern United States for two warm seasons (May-August), including 2007 and a warm season comprising randomly selected days from 2002 to 2006. The storms were classified into three main convective organizational structures (cellular, linear, and nonlinear) as well as several subcategories. The same procedure was applied to the highly populated coastal zone of the northeastern United States, including New Jersey, Connecticut, Rhode Island, and New York. The coastal analysis included six warm seasons from 2002 to 2007. Over the Northeast, severe wind events are evenly distributed among the cellular, linear, and nonlinear structures. Cellular structures are the primary hail producers, while tornadoes develop mainly from cellular and linear structures. Over the coastal zone, primarily cellular and linear systems produce severe wind and hail, while tornadoes are equally likely from all three convective structures. Composites were generated for severe weather days over the coastal region for the three main convective structures. On average, severe cellular events develop during moderate instability [most unstable CAPE (MUCAPE) ~1200 J kg−1], with low-level warm-air advection and frontogenesis at the leading edge of a thermal ridge collocated with an Appalachian lee trough. Severe linear events develop in a similar mean environment as the cellular events, except that most linear events occur with a surface trough upstream over the Ohio River valley and half of the linear events develop just ahead of progressive midlevel troughs. Nonlinear severe events develop with relatively weak mean convective instability (MUCAPE ~460 J kg−1), but they are supported by midlevel quasigeostrophic (QG) forcing for ascent. [ABSTRACT FROM AUTHOR]

Both low- and high-resolution studies of wind and upwelling were conducted off the west coasts of North and South America between August 1999 and December 2001. For the low-resolution study, done at 25 regional sites spanning both coasts, weekly mean winds were calculated from satellite measurements, from geostrophic estimates, and from an operational 100-km-resolution global atmospheric model. The satellite-measured winds, used as the reference, showed that the coastal regions of both North and South America were divided into fairly uniform climatic domains located at higher, mid, and lower latitudes. All three weekly-mean wind data sets were compared at each of the 25 sites, and then used to estimate upwelling based on the method employed by NOAA's Pacific Fisheries Environmental Laboratory. Within each of the wind domains, model-derived wind and upwelling estimates agreed with satellite-derived values better than those from geostrophic-derived estimates. To investigate variability between the regional sites, a 9-km-resolution atmospheric model was run for an area off California which spanned four of the regional sites. This high-resolution model, verified with satellite measurements, revealed jets of wind curving and intensifying around coastal promontories. These near-shore wind jets, undetected in the global model, resulted in strong upwelling bands on the order of 50 km alongshore by 20 km offshore. These bands are critical for calculating local upwelling. Our results indicate that regional upwelling estimates for fisheries research can be improved by replacing geostrophic estimates of winds with those from a global atmospheric model. For localized coastal upwelling estimates, however, models with resolution an order of magnitude higher are required. [ABSTRACT FROM AUTHOR]