The use of the SSM/I sea ice concentration data together with a sea surface temperature (SST) analysis in a numerical weather prediction (NWP) model is described. The ice concentration data provide a useful representation of the SST freezing-point isotherm while the SST field allows a quality control of the sea ice data. A land-sea mask, an orography field and a climatological SST field are also used to improve the analysis of surface temperature in some inland seas and lakes. A monthly mean sea ice concentration climatology has been created from 13 years of the ECMWF re-analysis ice concentration dataset which was also derived from satellite multi-channel microwave radiometer data. Data assimilation experiments have shown a small positive impact in the southern hemisphere model forecast scores when this new surface field is used in the NWP model.

On 11–12 November 1987, a frontal system associated with an intense narrow cold frontal rainband was observed over the Channel between England and France. This case constitutes intensive observational period 2 (IOP2) of the FRONTS 87 experiment. It was an ideal example of upper-level and low-level jet coupling with substantial ascending motions and precipitation. The frontal system is analysed, in this paper, using the mesoscale non-hydrostatic model CSU/RAMS. The model uses a two-way interactive nested grid structure and full cloud microphysics parameterisation at various levels of complexity. The model results are compared with previous numerical investigations of this case as well as with observations obtained from the experimental network deployed during the campaign. The model succeeded in representing the frontal structure at meso- and convective scales, taking into account the large scale forcing; thus it is an important tool for the study of secondary cyclogenesis observed during the FASTEX campaign.

An expert system (MEDEX) for predicting the gale-force onset, continuation, and cessation of seven major wind types within the Mediterranean basin has been designed, developed, and tested. The six wind types consist of the bora (flowing through both the Adriatic and Aegean Seas), etesian, levante, mistral, sirocco and westerly (poniente and vendaval). Except for the sirocco, these winds result from synoptic situations that lead to topographical channelling. MEDEX is rule-based and incorporates fuzzy logic to handle both objective and subjective inputs, the latter being a unique application of fuzzy logic. MEDEX has approximately 330 fuzzy rules, covering the seven winds in both winter and summer seasons. While MEDEX has been designed as a nowcasting tool (0–12 h), it can be applied to any future time for which forecasting charts (consisting of surface pressure and 500 mb height fields) are available. Inputs consist of objective pressure gradients in addition to subjective interpretations of various synoptic features. These inputs, as well as the corresponding rules, were developed, tuned, and verified over a two-year period during which the weather conditions from 539 days were individually analysed. Ground truth verification was produced primarily from over-water Special Sensor Microwave/Imager (SSM/I) wind speed measurements but also included observations as available, model predictions, and official Navy wind warnings. Evaluations of MEDEX performance for both onset and cessation of winter and summer winds are presented. In addition, comparisons with forecast statistics for the Navy's regional weather centre in Rota, Spain show that MEDEX has comparable forecasting skill.

The University of New South Wales (UNSW) high-resolution numerical weather prediction model (HIRES) has been used to forecast wind direction and speed at a height of 10 m over the Sydney to Hobart race yacht area for the past three years. The model is run routinely, three times daily, at the New South Wales Regional Office of the Australian Bureau of Meteorology in Sydney. One of these model runs is out to seven days ahead and its boundary conditions are provided by the Bureau of Meteorology's global model. For the 1996 Sydney to Hobart yacht race the model was run at 25 km resolution out to five days ahead and was subjected to detailed verification by one of the authors (KLB), who participated in the race aboard the yacht AMP Wild Oats and carried out an observational program during the race. The model winds were verified on a six-hourly basis utilising instrumentation on the yacht. The yacht carried wind sensors, which are situated on top of the yacht's mast at a height of 17.5 m above the water. The authors were interested in both the wind directional trends and the wind speeds forecast by the model. It was found that the model gave overall guidance of good quality but was particularly accurate early in the race when a major wind change known locally as a Southerly Buster occurred just after the start of the race. Later in the race from about the fourth day, the quality of the forecasts decreased in accuracy. An improved version of the model, run at higher resolution (10 km), will be verified again in the next race, in December 1997.

A mesoscale numerical weather prediction model and its associated diagnostics are evaluated to gauge their ability to forecast convection and the convective environment for ten case-study days. The diagnostic indices and parameters evaluated are those used to assess atmospheric instability, mesoscale forcing, inhibition and low-level moisture supply. In addition, bulk Richardson number and helicity diagnostics are examined for convection organisation. Convection over mountainous regions is not considered. The case-study days are grouped according to their synoptic type in order to evaluate the usefulness of the forecast diagnostics. In the evaluation it is found that by combining the CAPE (convective available potential energy) and low-level relative humidity diagnostics one can delineate quite large areas where convective triggering is possible and outside which convection will not develop. Organisation of the mesoscale convective systems is also considered. It is found that forecast strong low-level shears tend to be well correlated with observed linear convective systems. A set of stability indices are evaluated quantitatively and qualitatively by comparing the forecast stability indices with observations at a sounding location. CAPE and a simple index based on a moist temperature difference between two layers (Adedokun2) are identified as the most useful forecast stability indices for all of the cases.

This paper describes a new instrument designed to measure cloud or fog total water content mounted on a tethered balloon facility. The instrument is an evaporator type, which is inexpensive and relatively easy to construct. The device measures total water content with an uncertainty of 0.3 g kg–1, and a resolution of 0.03 g kg−1, at a frequency of 1 Hz. This typically gives a spatial resolution of 5–20 m and allows turbulent statistics to be calculated. Tethered balloons provide an ideal platform for the instrument to study low cloud, allowing very detailed measurement of cloud structure to be made. Some typical data are presented as an illustration.

The utility of a drought severity index, based on accumulated monthly precipitation deficits and a three-monthly initiation and termination rule, for assessing the variability in the timing and severity of the drought hazard between four selected locations in Devon and Cornwall since 1957 is documented. The robustness of the drought events identified is then assessed by changing the index formulation rules. Results indicate that, using a six-monthly, rather than a three-monthly, rule, drought sequences are more difficult to initiate and terminate and are more persistent with a longer mean duration. The drought severity indices are then used to define a hierarchy of significant drought events, isolated in terms of the number of indices simultaneously exceeding an arbitrarily defined severity. Seven Class I droughts, regional in extent, are then discussed; and circulation contrasts between events highlighted using aggregated Lamb weather types. Two drought sub-types emerge, distinguishable according to whether the controlling anticyclone had a greater tendency to be located to the north and east of the UK or to the south and west. A comparative analysis of the ongoing 1995-drought, in terms of its persistence, and rapidity of onset and cessation, is also conducted.

Two case studies in which elevated thunderstorms played an important role in enhancing precipitation totals are discussed. During the period 20–22 February 1993, elevated thunderstorms over Iowa produced a mesoscale band of snow with amounts in excess of 25 cm across north central Iowa. Diagnosis of the environment revealed an elevated layer of convective instability between 700 and 540 hPa above a well-defined frontal zone. Elevated, upright convection resulted from the release of the convective instability as air parcels, ascending isentropically over the frontal zone, reached saturation. A second case, in which heavy rain (greater than 50 mm in 24 h) fell over parts of Oklahoma, Kansas and Missouri during the period 27–28 April 1994, is also examined. Once again, elevated thunderstorms resulted from the ascent of an elevated layer of convective instability over a strong baroclinic zone. Positive CAPE values are found by lifting air parcels having the maximum equivalent potential temperature in the lower portion of the troposphere, whereas no available energy is diagnosed for surface parcels. While both cases strongly resemble the climatology of elevated thunderstorms, these case studies suggest that convective instability aloft released by isentropic ascent, rather than frontogenetical forcing in the presence of weak symmetric stability, can result in heavy precipitation.

The CERES-Rice version 3.0 crop growth simulation model was calibrated and evaluated for the agroclimatic conditions of the state of Kerala in India. Genetic coefficients were developed for the rice crop variety Jaya and used for the model evaluation studies. In four experiments using different transplanting dates during the virippu season (June to September) under rainfed conditions (i.e. no irrigation), the flowering date was predicted within an error of four days and date of crop maturity within an error of two days. The model was found to predict the phenological events of the crop fairly well. The grain yield predicted by the model was within an error of 3% for all the transplanting dates, but the straw yield prediction was within an error of 27%. The high accuracy of the grain yield prediction showed the ability of the model to simulate the growth of the crop in the agroclimatic conditions of Kerala. It can be concluded from this study that the model can be used for making various strategic and tactical decisions related to agricultural planning in the state.

During December 1993 a microwave radiometer belonging to the Central Aerological Observatory (CAO) Moscow was operated at the Meteorological Research Unit, Cardington, UK. The radiometer has a single channel with a central frequency of 61.0 GHz, bandwidth of 2 GHz and sensitivity 0.04 K for a 1 s integration time. The antenna (beam-width 6°) was scanned from nadir to horizontal in 9° steps. Calibration was done every scan cycle using an electrically switched load. The information potentially available from such a radiometer is discussed. The measurements made at Cardington are compared with in situ observations made by radiosondes and by the Meteorological Office's tethered balloon system. Microwave brightness temperatures calculated from the radiosonde ascents are compared with observations. These were found to agree with a mean difference of 0.1 K and with a standard deviation of < 0.3 K. Similarly retrieved temperature profiles up to 500 m were found to agree with the radiosonde profiles within 1 K rms. During the comparison period there were no strong low-level inversions to test fully the radiometer's retrieval capability.