The correlation between Meteosat water vapour (WV) data and positive anomalies of potential vorticity (PV) is investigated using a case study involving cyclogenesis in the Alpine region and a north-west African depression. Correlation analyses are performed at different isobaric surfaces in the middle and upper troposphere for two latitudinal belts (25–35° N – ‘southern’, and 35–52.5°N – ‘northern’). A significant correlation is obtained at 500 hPa (correlation coefficient range: 0.58–0.64). Linear regression models are fitted to the samples from the northern and southern belts. On the basis of these results, the use of WV data as a tool for detection of errors in model-derived PV fields is discussed. It is concluded that, though some of the discrepancies between PV fields and WV images could be due to errors in the model analysis or prognosis, some mismatches are possibly due to the variable sensitivity of the WV channel because of the changes in pressure over the commonly used PV fields. It is suggested that the comparison of WV data with the field of positive PV anomalies at 500 hPa might be another approach to help solve the problem of verifying and validating numerical output.

Road-surface cooling during three contrasting frontal passages was examined for various sites with widely varying exposure across Devon. Delays in road cooling were dependent on exposure, topography, thermal inertia, precipitation and surface wetness. Scattered cloud and cloud bands complicate the spatial variability, but for winter maintenance, worst-case scenarios of complete cloud clearance produce important guidelines for prioritising salting routes.

Methods to calculate extreme wind speeds are described and reviewed, including ‘classical’ methods based on the generalized extreme value (GEV) distribution and the generalized Pareto distribution (GPD), and approaches designed specifically to deal with short data sets. The emphasis is very much on the needs of users who seek an accurate method to derive extreme wind speeds but are not fully conversant with up-to-date developments in this complex subject area. First, ‘standard’ methods are reviewed: annual maxima, independent storms, r-largest extremes with the GEV distribution, and peak-over-threshold extremes with the GPD. Techniques for calculating the distribution parameters and quantiles are described. There follows a discussion of the factors which must be considered in order to fulfil the criterion that the data should be independent and identically distributed, and in order to minimize standard errors. It is commonplace in studies of extreme wind speeds that the time series available for analysis are very short. Finally, therefore, the paper deals with techniques applicable to data sets as short as two years, including simulation modelling and methods based on the parameters of the parent distribution.

The NERC radar facility located near Aberystwyth (52.4° N, 4.1° W) was established for remote sensing of the structure and dynamics of the mesosphere, stratosphere and troposphere; hence the name MST radar. The facility records scatter and reflection from metre-scale irregularities in refractive index produced by atmospheric waves and turbulence. The measurement of the three-dimensional wind field at tropospheric and lower stratospheric heights represents the primary capability of the facility and is expected to be particularly useful in short-range local weather forecasting operations. At the same time, the temporal and height resolution achievable lend themselves to studies of smaller-scale, time-varying phenomena, such as waves and turbulence. Particular attention has been paid to the characteristics and vertical propagation of mountain waves and inertia-gravity waves, and estimates made of the vertical flux of horizontal momentum associated with such waves, known to play a significant role in the large-scale atmospheric circulation. The turbulence associated with the dissipation of these waves, and also generated in the vicinity of jet streams, has also been examined. Quasi-specular reflection of radar waves associated with changes in temperature gradients has helped monitor the height and sharpness of the tropopause and also the structure of frontal zones. The technical capabilities of the facility and the continuity of data provided by its all-weather and extended operation have been shown to be well suited to both mesoscale meteorological and research applications.

This paper deals with ‘mushroom-shaped’ patterns that may be observed in water vapour (WV) imagery. Its associated conceptual model has been adapted from the ‘T’ configuration. For this model some remarkable properties of WV imagery have been used: the spatial and temporal continuity of atmospheric structures, the more detailed upper-level circulation features and the possibility of observing and monitoring relationships between them. Using this model, and from the kinematic viewpoint, it is possible to infer some features such as deformation zones, rotation centres (vorticity centres) and relative maxima of wind in the upper-middle-level flows. Some examples are presented to show the usefulness of this conceptual model for the interpretation of WV imagery, the diagnosis of upper-middle-level flow and the evaluation of the numerical model.

The significance of the atmospheric stability approach in models for estimating evapotranspiration is studied by considering the errors encountered when simply adopting neutral instead of actual stability conditions. Maize, cotton and wheat crops grown in 10 hectare fields (central Greece) were used to estimate actual and maximum evapotranspiration for both stability considerations, employing a model based on the equations of Shuttleworth & Wallace (1985). The required meteorological data were recorded in each field continuously, whereas the leaf area index and the plant height were measured weekly. Results show that when estimation of cumulative evapotranspiration losses is attempted, the choice of the atmospheric stability approach is of no importance (error ≤ 2%), as over- and under-estimation of evapotranspiration are counterbalanced when stable and unstable situations are (under the climatic conditions of experimentation) successively interchanged during the cultivating period. When, however, daily estimations are required, the actual stability conditions have to be taken into account, as the daily evapotranspiration rate estimated by regarding the atmospheric stability as neutral may deviate from the corresponding ‘actual’ rate by as much as 50 – 80%.

A regional numerical weather prediction system, consisting of an analysis scheme, a forecast model and a variety of interpretation, visualization and military application tools, named ‘BLM’ has been routinely operating at the German Military Geophysical Office (GMGO) since 1984. At that time, the grid domain was confined to Central Europe and parts of the North Atlantic, where a polar stereographic map projection was appropriate. Owing to increasing user demands, the system had to be made relocatable to provide numerical forecasts at any location of the globe. The relocatable version, called ‘RBL’, is still formulated on a stereographic plane, which is now defined using a rotated geographical system with an artificial north pole right in the middle of the domain. This approach is well known. However, most of the current numerical weather prediction systems are defined on a horizontal latitude-longitude grid, so that the equator in rotated coordinates runs through the centre of the forecast area. The main advantages of the horizontal coordinate transformation used by the ‘RBL’ are numerical stability for a large and economical time step, which enables the model to be run on a computer with limited power, and an invariant map factor all over the globe, which eases the interpretation of the model results for different locations.

The performance of three automatic depression tracking schemes developed by Terry & Atlas (1996), Murray & Simmonds (1991a) and Konig et al. (1993) when applied over one month are assessed. The schemes respectively identify depressions by (a) locating the innermost closed contour in a PMSL field, (b) finding maxima in the curvature in a bi-cubic spline fitted to the PMSL data and (c) performing a grid point search to identify a minimum in a PMSL field. The largest number of depressions was found by the Murray and Simmonds scheme, with the Atlas and Terry scheme finding the least. The Murray and Simmonds scheme also found the largest number of tracks, with the other two having comparable numbers of tracks. Two possible explanations for the differences in the number of tracks are considered. Firstly, the case where one or more centres identified by one scheme as corresponding to a single track are not found by the other schemes. Secondly, the case where a single track found by one scheme is split into two or more tracks by the other schemes. All three techniques had a similar latitude of cyclogenesis, although the Atlas and Terry and Konig et al. schemes found more lows at high latitudes as a result of using data on a latitude/longitude grid. The longest mean track length was found with the Konig et al. scheme. A comparison of the PMSL fields with satellite imagery shows that the major NWP centres have trouble producing reliable analyses around the Antarctic because of the lack of data.