The aim of data-impact studies at the UK Met. Office is to investigate how observations affect the accuracy of model forecasts. Results from such experiments provide useful evidence on which to base the design of observational networks. This project, using a case study approach, investigated the relative benefit of different observation types within The Met. Office's Mesoscale Model domain on forecasts of three-hourly precipitation accumulation over the UK up to 12 hours ahead. The method used assesses the impact of assimilating single observation types, or a limited combination of types, where impact is measured against a control forecast obtained after a dummy assimilation using no observations. In experiments for 13 case studies, the observation types that most frequently provided a beneficial impact when presented alone to the assimilation were sonde data, surface data and data from the Moisture Observation Processing System (MOPS).

The diagnosis of a vigorous dry intrusion over the central Mediterranean is performed using water-vapour images from METEOSAT. This dry intrusion was located on the rear side of a cold front (propagating from Italy to Greece) and played an important role in the onset of thunderstorms over the western Greek coasts. A combination of satellite imagery and potential vorticity analyses showed that the dry air originated in the lower-stratospheric and higher-tropospheric layers. The interaction of the dry air with the moist air masses within the warm conveyor belt ahead of the cold front (overrun of warm air by low equivalent potential temperature air) produced a potentially unstable region over the area of reported thunderstorms.

This paper illustrates a key lesson related to most uses of long-range climate forecast information, namely that effective weather-related decision-making requires understanding and integration of weather information with other, often complex factors. Northern Illinois University's heating plant manager and staff meteorologist, along with a group of meteorology students, worked together to assess different types of available information that could be used in an autumn natural gas purchasing decision. Weather information assessed included the impact of ENSO events on winters in northern Illinois and the Climate Prediction Center's (CPC) long-range climate outlooks. Non-weather factors, such as the cost and available supplies of natural gas prior to the heating season, contribute to the complexity of the natural gas purchase decision. A decision tree was developed and it incorporated three parts: (a) natural gas supply levels, (b) the CPC long-lead climate outlooks for the region, and (c) an ENSO model developed for DeKalb. The results were used to decide in autumn whether to lock in a price or ride the market each winter. The decision tree was tested for the period 1995-99, and returned a cost-effective decision in three of the four winters.

The coastal climates of France have been studied using temperature and precipitation data from 1961 to 1990. It is found that the coastlines of the French Atlantic (and adjacent seas) and Mediterranean show many similar climatic features, but the coastal orientation and nearby topography cause some local climatic peculiarities. The relatively flat topography near the coasts of the North Sea, English Channel and the Atlantic result in a fairly homogeneous coastal climate. In these coastal regions there are days of frost each year, but the precipitation amount is usually not high, especially in summer on the Atlantic coast. On the other hand, the mountainous areas of Provence and the Riviera, which are adjacent to the Mediterranean, cause a juxtaposition of varied coastal climates. Large temperature and rainfall differences are observed, especially between the capes and the heads of gulfs, but also between the narrow and wide gulfs. During the warm months, some sea breezes cause coastal thermal peculiarities, which depend on the orientation of the prevailing flow.

A climatological study of the occurrence of freezing precipitation at ground level is presented. It is based on reports of freezing precipitation in SYNOP messages from the three winters covering the years 1995-1998. The geographical domain covered is Western and Central Europe, extending northwards to Scandinavia and southwards to North Africa. The types of freezing precipitation selected are freezing rain, freezing drizzle and ice pellets. The frequency of occurrence of each of these types of freezing precipitation was computed. The results show that freezing precipitation is not an uncommon phenomenon in Europe, with the combined frequency of the various types of freezing precipitation (either drizzle, rain or ice pellets) being 0.5 to 1% of all SYNOP messages that include an observation of the current weather in Central Europe. The surface air temperature intervals associated with freezing precipitation have been studied. An attempt was then made to determine the mode of formation of freezing precipitation (either by melting of snow crystals in a layer of positive temperatures or by a process of coalescence or diffusional growth) by studying the thermodynamic soundings associated with freezing precipitation events.

Forecasting the occurrence and strength of the nocturnal jet is important to aviation, especially to balloonists and small aircraft. Jets also have implications for pollution transportation and the clearance of low cloud and fog. A one-dimensional model showed that the main mechanism for the development of the nocturnal jet was a hybrid between an inertial oscillation in a layer within and above the jet core and a quasi-steady jet within the inversion. The model, supported by observations, suggests that a nocturnal inversion is required to produce a jet and that the strength of the jet maximum is approximately 1.3 times the geostrophic wind. A wind profile minimum is also predicted to occur when the inertial oscillation reaches a minimum in the stress-free layer and this was observed using a tethered balloon flown at Cardington.

The influence on the propagation of coastally trapped disturbances (CTD) of large gaps in the bounding coastal mountains is studied via application of the Colorado State University Regional Atmospheric Modeling System (RAMS). By large gap it is meant that the width is of order 25–50% of the Rossby radius. In order to isolate the effects of the gaps, idealised simulations are used. Thus, RAMS is configured with a straight north-south coastline with a ramp-shaped coastal mountain range in which a gap is inserted and a gravity current-like CTD generated through cooling the low level atmosphere at the southern end of the domain. Radiative, land surface and cloud parameterisations are turned off to avoid sea breeze generation and other thermal effects and to focus on the dynamical effect of the gaps. Four simulations are conducted: no gap in the coastal mountain barrier, an insertion of a narrow (80 km) gap, a wide (160 km) gap, and an idealised island in the barrier. It is found that the gaps influence CTD propagation by effectively stalling the propagation at their mouth for a period and subsequently weakening the gravity current-like features (vertical extent, strength and abruptness of the wind and stratification changes at the leading edge). The idealised island has a less obvious impact since it is bounded by relatively narrow straits and does not allow communication with the model interior like the gaps do. Although lack of data and the nature of the idealised simulation prevents a detailed assessment of the simulations against real cases, qualitative comparisons with observed events in western North America and south-eastern Australia are made. Application of these results to air quality and forecasting issues is discussed.

The development of a hurricane-like vortex over the Mediterranean Sea was studied using (mainly) the UK Met. Office Unified Model. The Mediterranean cyclone formed in the morning of 15 January 1995 over the sea between Greece and Sicily. Strong convection was observed prior to its genesis. During the longest part of the cyclone's lifetime, strong surface fluxes and, as a result, deep convection existed in its vicinity. Its track was influenced by the surface fluxes and the flow in the wider region. The forecast of the mesoscale and limited-area models reproduced the general characteristics of the actual system as they appeared at the surface and upper-air charts and at the satellite imagery. The investigation of the cyclone's characteristics gave strong evidence (including an ‘eye’ and a warm core) to support the initial assertion that it was similar to tropical cyclones and some polar lows. Baroclinic instability does not seem particularly important, although the cyclone formed at the edge of a baroclinic zone. A numerical experiment showed the vortex did not develop in the absence of surface heat and moisture fluxes. Another experiment showed that sensible and latent heat fluxes were equally important in its development.

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