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An analysis of atmospheric rivers (ARs) as defined by an automated AR detection tool based on integrated water vapor transport (IVT) and the connection to heavy precipitation in the southeast United States (SEUS) is performed. Climatological water vapor and water vapor transport fields are compared between the U.S. West Coast (WCUS) and the SEUS, highlighting stronger seasonal variation in integrated water vapor in the SEUS and stronger seasonal variation in IVT in the WCUS. The climatological analysis suggests that IVT values above ~500 kg m−1 s−1 (as incorporated into an objective identification tool such as the AR detection tool used here) may serve as a sensible threshold for defining ARs in the SEUS. Atmospheric river impacts on heavy precipitation in the SEUS are shown to vary on an annual cycle, and a connection between ARs and heavy precipitation during the nonsummer months is demonstrated. When identified ARs are matched to heavy precipitation days (>100 mm day−1), an average match rate of ~41% is found. Results suggest that some aspects of an AR identification framework in the SEUS may offer benefit in forecasting heavy precipitation, particularly at medium- to longer-range forecast lead times. However, the relatively high frequency of SEUS heavy precipitation cases in which an AR is not identified necessitates additional careful consideration and incorporation of other critical aspects of heavy precipitation environments such that significant predictive skill might eventually result.

The city of Houston, Texas, is near a complex coastline and numerous petrochemical plants, the combination of which plays a large role in Houston’s air pollution events. It has long been known that the thermally driven afternoon onshore flow (sea breeze or gulf breeze) transports ozone-rich air inland. As a way of quantifying the role of the gulf breeze in Houston’s high-ozone events, cluster analysis of hourly averaged surface winds from a regional network of meteorological sensors was performed for 27 summer days of 2000, with the dates coinciding with the Texas Air Quality Study 2000 (TexAQS 2000). Hourly averaged winds were partitioned into 16 independent clusters, or wind patterns, while simultaneously keeping track of the maximum ozone in the network for each hour. Clusters emerged that represented various wind patterns, including thermally driven flows, stagnant winds, and a thunderstorm outflow. All clusters were used to assess which wind patterns were most likely to be coincident with the maximum ozone of the day. High ozone was most likely to occur with clusters representing the gulf breeze. Clusters occurring before the ozone maximum of the day were analyzed to determine which sequences of wind patterns were most likely to precede high ozone. A transition from offshore flow to onshore flow, with at least 1 h of stagnant winds in between, routinely occurred in the 6 h preceding ozone measurements reaching ≥ 120 parts per billion by volume (ppbv). On nontransition days with high ozone, ozone maxima ≥ 120 ppbv often occurred the hour after a wind direction shift of greater than about 45°.

SUMMARYThis paper examines the three-dimensional structure and dynamics of southerly hybrid gap/mountain ow through the Wipp valley (Wipptal), Austria, observed on 30 October 1999 using high-resolution observations and model simulations. The observations were obtained during a shallow south föhn event documented in the framework of the Mesoscale Alpine Programme (MAP). Three important data sources were used: the airborne differential-absorption lidar LEANDRE 2, the ground-based Doppler lidar TEACO2 and in situ measurements from the National Oceanic and Atmospheric Administration P-3 aircraft. This event was simulated down to 2 km horizontal resolution using the non-hydrostatic mesoscale model Meso-NH. The structure and dynamics of the ow were realistically simulated. The combination of high-resolution observations and numerical simulations provided a comprehensive three-dimensional picture of the ow through the Wipptal: in the gap entrance region (Brenner Pass, Austria), the low-level jet was not solely due to the channelling of the southerly synoptic ow through the elevated gap. Part of the Wipptal ow originated as a mountain wave at the valley head wall of the Brenner Pass. Downstream of the pass, the shallow föhn ow had the characteristics of a downslope windstorm as it rushed down towards the Inn valley (Inntal) and the City of Innsbruck, Austria. Downhill of the Brenner Pass, the strongest ow was observed over a small obstacle along the western side wall (the Nösslachjoch), rather than channelled in the deeper part of the valley just to the east. Further north, the low-level jet was observed in the centre of the valley. Approximately halfway between Brenner Pass and Innsbruck, where the along-axis direction of the valley changes from north to north-north-west, the low-level jet was observed to be de ected to the eastern side wall of the Wipptal. Interaction between the Stubaier Alpen (the largest and highest topographic feature to the west of the Wipptal) and the south-westerly synoptic ow was found to be the primary mechanism responsible for the de ection. The along-and cross-valley structure and dynamics of the ow were observed to be highly variable due to the in uence of surrounding mountains, localized steep slopes within the valley and out ows from tributaries (the Gschnitztal and the Stubaital) to the west of the Wipptal.For that shallow föhn case, observations and simulations provided a large body of evidence that downslope ow created thinning/ thickening uid and accelerations/ decelerations reminiscent of mountain wave/hydraulic theory. Along the Wipptal, two hydraulic-jump-like transitions were observed and simulated, (i) on the lee slope of the Nösslachjoch and (ii) in the Gschnitztal exit region. A hydraulic solution of the ow was calculated in the framework of reduced-gravity shallow-water theory. The down-valley evolution of the Froude number computed using LEANDRE 2, P-3 ight level and TEACO2 measurements con rmed that these transitions were associated with super-to subcritical transitions.