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Isabel F. Trigo, Grant R. Bigg, and Trevor D. Davies

Abstract

A general climatology of the main mechanisms involved in Mediterranean cyclogenesis is presented. A diagnostic study of both composite means and case studies is performed to analyze processes occurring in different seasons, and in different cyclogenetic regions within the same season. It is shown that cyclones that developed over the three most active areas in winter—the Gulf of Genoa, the Aegean Sea, and the Black Sea—are essentially subsynoptic lows, triggered by the major North Atlantic synoptic systems being affected by local orography and/or low-level baroclinicity over the northern Mediterranean coast. It is also suggested that cyclones in two, or all three, of these regions often occur consecutively, linked to the same synoptic system. In spring and summer, thermally induced lows become progressively more important, despite the existence of other factors, such as the Atlas Mountains contributing to lee cyclogenesis in northern Africa, or the extension of the Asian monsoon into the eastern part of the Mediterranean. As a consequence, the behavior of Mediterranean cyclones becomes modulated by the diurnal forcing; the triggering and mature stages are mostly reached by late afternoon or early nighttime, while cyclolysis tends to occur in early morning.

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Isabel F. Trigo, Trevor D. Davies, and Grant R. Bigg

Abstract

An objective cyclone detection and tracking analysis is performed over an 18-yr period, for the Mediterranean basin. The high-resolution (1.125° × 1.125° grid) European Centre for Medium-Range Weather Forecasts data used in this study proved to be particularly suitable for the detection and tracking techniques and to identify subsynoptic-scale Mediterranean lows, which have often been underestimated in previous studies.

The major characteristics of Mediterranean cyclones are examined and compared with other Northern Hemisphere depressions. Both cyclogenesis and cyclolysis regions are identified in the domain of study. In addition, characteristics of Mediterranean depressions, such as cyclone duration and intensity, as well as their persistence throughout the year, are shown to be quite variable for different formation areas. Overall, the regions where cyclogenesis is mainly controlled by topography, like the Gulf of Genoa and south of the Atlas Mountains, seem to generally account for the most intense events.

Finally, a statistical analysis based on a k-means clustering procedure summarizes trajectory information obtained from the 18-yr climatology. The method proved to be efficient in grouping cyclone paths from similar cyclogenesis regions and with similar characteristics of movement, showing generally more clusters in the western Mediterranean than in the eastern part of the basin.

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Sarah F. Kew, Michael Sprenger, and Huw C. Davies

Abstract

Inspection of the potential vorticity (PV) distribution on isentropic surfaces in the lowermost stratosphere reveals the ubiquitous presence of numerous subsynoptic positive PV anomalies. To examine the space–time characteristics of these anomalies, a combined “identification and tracking” tool is developed that can catalog each individual anomaly’s effective amplitude, location, overall spatial structure, and movement from genesis to lysis. A 10-yr winter climatology of such anomalies in the Northern Hemisphere is derived for the period 1991–2001 based upon the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40). The climatology indicates that the anomalies are frequently evident above high topography and in a quasi-annular band at about 70°N, are long lived (days to weeks), and that their effective amplitude is typically 2 PV units (PVU) higher than that of the ambient environment. In addition, the derived climatologies and associated composites pose questions regarding the origin of the anomalies, detail their life cycle, and shed light on their dynamics and role as long-lived precursors of surface cyclogenesis.

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F. Davies, C. G. Collier, G. N. Pearson, and K. E. Bozier

Abstract

Analysis of radial wind velocity data from the Salford pulsed Doppler infrared lidar is used to calculate turbulent spectral statistics over the city of Salford in the United Kingdom. The results presented here, first, outline the error estimation procedure used to correct the radial wind velocity measurements from the Salford lidar system; second, they correct the data for the spatial averaging effects of the Salford lidar pulse; and finally, they use the corrected data to calculate turbulent spectral statistics. Using lidar data collected from the Salford Urban Meteorological Experiment (SALFEX), carried out in May 2002, kinetic energy dissipation rates, radial velocity variance, and integral length scales are calculated for the boundary layer above an urban canopy. The estimates of the kinetic energy dissipation rate from this method are compared to calculations using more traditional spectral methods. The estimates of the kinetic energy dissipation rate for the two methods are correlated and both show an increase in dissipation rate through the day. The procedure followed for the correction of the spatial averaging effects of the lidar pulse shape actually uses the Salford lidar pulse shape profile.

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Jonathan M. Davies, Charles A. Doswell III, Donald W. Burgess, and John F. Weaver

This paper considers a tornadic storm that struck south-central and eastern Kansas on 13 March 1990. Most of the devastation was associated with the first tornado from the storm as it passed through Hesston, Kansas. From the synoptic-scale and mesoscale viewpoints, the event was part of an outbreak of tornadoes on a day when the tornado threat was synoptically evident. Satellite imagery, combined with conventional data, suggest that the Hesston storm was affected by a preexisting, mesoscale outflow boundary laid down by morning storms. Radar and satellite data give clear indication of the supercellular character of the storm, despite limited radar data coverage.

Because of the considerable photographic coverage, several interesting features of the storm were recorded and are analyzed here. These include the following: 1) the movement and dissipation of a cloud band associated with an apparent rear-flank downdraft; 2) a transition from a rather large funnel through an apparent dissipation to the formation of a narrow funnel, during which the damage on the ground was continuous; and 3) a period of interaction between the first and second tornadoes.

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Norman G. Loeb, Bruce A. Wielicki, Wenying Su, Konstantin Loukachine, Wenbo Sun, Takmeng Wong, Kory J. Priestley, Grant Matthews, Walter F. Miller, and R. Davies

Abstract

Observations from the Clouds and the Earth’s Radiant Energy System (CERES), Moderate Resolution Imaging Spectroradiometer (MODIS), Multiangle Imaging Spectroradiometer (MISR), and Sea-Viewing Wide-Field-of-View Sensor (SeaWiFS) between 2000 and 2005 are analyzed in order to determine if these data are meeting climate accuracy goals recently established by the climate community. The focus is primarily on top-of-atmosphere (TOA) reflected solar radiances and radiative fluxes. Direct comparisons of nadir radiances from CERES, MODIS, and MISR aboard the Terra satellite reveal that the measurements from these instruments exhibit a year-to-year relative stability of better than 1%, with no systematic change with time. By comparison, the climate requirement for the stability of visible radiometer measurements is 1% decade−1. When tropical ocean monthly anomalies in shortwave (SW) TOA radiative fluxes from CERES on Terra are compared with anomalies in Photosynthetically Active Radiation (PAR) from SeaWiFS—an instrument whose radiance stability is better than 0.07% during its first six years in orbit—the two are strongly anticorrelated. After scaling the SeaWiFS anomalies by a constant factor given by the slope of the regression line fit between CERES and SeaWiFS anomalies, the standard deviation in the difference between monthly anomalies from the two records is only 0.2 W m−2, and the difference in their trend lines is only 0.02 ± 0.3 W m−2 decade−1, approximately within the 0.3 W m−2 decade−1 stability requirement for climate accuracy. For both the Tropics and globe, CERES Terra SW TOA fluxes show no trend between March 2000 and June 2005. Significant differences are found between SW TOA flux trends from CERES Terra and CERES Aqua between August 2002 and March 2005. This discrepancy is due to uncertainties in the adjustment factors used to account for degradation of the CERES Aqua optics during hemispheric scan mode operations. Comparisons of SW TOA flux between CERES Terra and the International Satellite Cloud Climatology Project (ISCCP) radiative flux profile dataset (FD) RadFlux product show good agreement in monthly anomalies between January 2002 and December 2004, and poor agreement prior to this period. Commonly used statistical tools applied to the CERES Terra data reveal that in order to detect a statistically significant trend of magnitude 0.3 W m−2 decade−1 in global SW TOA flux, approximately 10 to 15 yr of data are needed. This assumes that CERES Terra instrument calibration remains highly stable, long-term climate variability remains constant, and the Terra spacecraft has enough fuel to last 15 yr.

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J. D. Price, S. Vosper, A. Brown, A. Ross, P. Clark, F. Davies, V. Horlacher, B. Claxton, J. R. McGregor, J. S. Hoare, B. Jemmett-Smith, and P. Sheridan

During stable nighttime periods, large variations in temperature and visibility often occur over short distances in regions of only moderate topography. These are of great practical significance and yet pose major forecasting challenges because of a lack of detailed understanding of the processes involved and because crucial topographic variations are often not resolved in current forecast models. This paper describes a field and numerical modeling campaign, Cold-Air Pooling Experiment (COLPEX), which addresses many of the issues.

The observational campaign was run for 15 months in Shropshire, United Kingdom, in a region of small hills and valleys with typical ridge–valley heights of 75–150 m and valley widths of 1–3 km. The instrumentation consisted of three sites with instrumented flux towers, a Doppler lidar, and a network of 30 simpler meteorological stations. Further instrumentation was deployed during intensive observation periods including radiosonde launches from two sites, a cloud droplet probe, aerosol monitoring equipment, and an instrumented car. Some initial results from the observations are presented illustrating the range of conditions encountered.

The modeling phase of COLPEX includes use of the Met Office Unified Model at 100-m resolution, and some brief results for a simulation of an intensive observation period are presented showing the model capturing a cold-pool event. As well as aiding interpretation of the observations, results from this study are expected to inform the design of future generations of operational forecasting systems

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David M. Schultz, Lance F. Bosart, Brian A. Colle, Huw C. Davies, Christopher Dearden, Daniel Keyser, Olivia Martius, Paul J. Roebber, W. James Steenburgh, Hans Volkert, and Andrew C. Winters

Abstract

The year 1919 was important in meteorology, not only because it was the year that the American Meteorological Society was founded, but also for two other reasons. One of the foundational papers in extratropical cyclone structure by Jakob Bjerknes was published in 1919, leading to what is now known as the Norwegian cyclone model. Also that year, a series of meetings was held that led to the formation of organizations that promoted the international collaboration and scientific exchange required for extratropical cyclone research, which by necessity involves spatial scales spanning national borders. This chapter describes the history of scientific inquiry into the structure, evolution, and dynamics of extratropical cyclones, their constituent fronts, and their attendant jet streams and storm tracks. We refer to these phenomena collectively as the centerpiece of meteorology because of their central role in fostering meteorological research during this century. This extremely productive period in extratropical cyclone research has been possible because of 1) the need to address practical challenges of poor forecasts that had large socioeconomic consequences, 2) the intermingling of theory, observations, and diagnosis (including dynamical modeling) to provide improved physical understanding and conceptual models, and 3) strong international cooperation. Conceptual frameworks for cyclones arise from a desire to classify and understand cyclones; they include the Norwegian cyclone model and its sister the Shapiro–Keyser cyclone model. The challenge of understanding the dynamics of cyclones led to such theoretical frameworks as quasigeostrophy, baroclinic instability, semigeostrophy, and frontogenesis. The challenge of predicting explosive extratropical cyclones in particular led to new theoretical developments such as potential-vorticity thinking and downstream development. Deeper appreciation of the limits of predictability has resulted from an evolution from determinism to chaos. Last, observational insights led to detailed cyclone and frontal structure, storm tracks, and rainbands.

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G. K. Grice, R. J. Trapp, S. F. Corfidi, R. Davies-Jones, C. C. Buonanno, J. P. Craven, K. K. Droegemeier, C. Duchon, J. V. Houghton, R. A. Prentice, G. Romine, K. Schlachter, and K. K. Wagner
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R. Benoit, C. Schär, P. Binder, S. Chamberland, H. C. Davies, M. Desgagné, C. Girard, C. Keil, N. Kouwen, D. Lüthi, D. Maric, E. Müller, P. Pellerin, J. Schmidli, F. Schubiger, C. Schwierz, M. Sprenger, A. Walser, S. Willemse, W. Yu, and E. Zala

Recent developments in numerical modeling and computer technology will soon allow for limited-area production-type numerical weather prediction at a resolution of 1–2 km. This advance opens exciting prospects for the prediction of airflow and precipitation phenomena in and around mountainous regions, by improving the representation of the underlying topography, and by explicitly simulating (rather than parameterizing) moist convection.

During the Special Observation Period (SOP; 7 Sept–15 Nov 1999) of the Mesoscale Alpine Programme (MAP) the Canadian Mesoscale Compressible Community Model (MC2) has been run in real time at a horizontal resolution of 3 km on a computational domain of 350 × 300 × 50 grid points, covering the whole of the Alpine region. An overview of the model configuration and performance will be presented along with simulation and validation results from selected MAP cases. Some critical aspects that require particular attention in future research will also be addressed.

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