A system has been developed and made operational at the Met Office for the real-time diagnosis of soil state and surface hydrology. It is based on the Met Office Surface Exchanges Scheme (MOSES) modified to take account of unresolved soil and topographic heterogeneity when calculating surface runoff by incorporating a Probability Distributed Moisture (PDM) scheme developed by the Centre for Ecology and Hydrology. The implementation of MOSES-PDM in the Met Office's Nimrod nowcasting system is described. High resolution soil characteristics and land cover data, together with Nimrod's analyses of precipitation amount and type, cloud cover and near-surface atmospheric variables are used to drive MOSES-PDM. Hourly values of snowmelt, runoff, net surface radiation, evaporation, potential evaporation, soil temperature, soil moisture and soil moisture deficit are calculated on a 5 km grid.

A scheme for the detection of fog and low stratus over land during daytime based on data of the MODIS (Moderate Resolution Imaging Spectroradiometer) instrument is presented. The method is based on an initial threshold test procedure in the MODIS solar bands 1–7 (0.62–2.155 μm). Fog and low stratus detection generally relies on the definition of minimum and maximum fog and low stratus properties, which are converted to spectral thresholds by means of radiative transfer calculations (RTC). Extended sensitivity studies reveal that thresholds mainly depend on the solar zenith angle and, hence, illumination-dependent threshold functions are developed. Areas covered by snow, ice and mid-/high-level clouds as well as bright/hazy land surfaces are omitted from the initial classification result by means of a subsequent cloud-top height test based on MODIS IR band 31 (at 12 μm) and a NIR/VIS ratio test. The validation of the final fog and low stratus mask generally shows a satisfactory performance of the scheme. Validation problems occur due to the late overpass time of the TERRA platform and the time lag between SYNOP and satellite observations. Apparent misclassifications are mainly found at the edge of the fog layers, probably due to over-or underestimation of fog and low stratus cover in the transition zone from fog to haze.

A model to deduce cloudiness from automatic measurements of synoptic and road weather stations during winter nights is introduced. The (height-adjusted) cloud amount of each station can be determined from near-surface temperature measurements and precipitation information only. By using the difference between the air temperature at surface level and the air temperature 2 m above ground, a correlation coefficient is achieved of up to 0.91 between the calculated cloud amount and observations. The model is applied to a dense network in central Switzerland in order to obtain a two-dimensional cloud map by interpolation. With the help of a tracking algorithm, the displacement of cloud patterns is nowcasted. The procedure was tested using data from winter 2003–4. It works successfully with a forecast range up to about 90 minutes. The results are used to predict a change in surface temperature (in cases with changing cloudiness), and thus allow nowcasting of slippery roads or ground frost.

A ten-year climatology has been prepared of dust days for central eastern Australia, from January 1995 to February 2004, using reports from the Australian Bureau of Meteorology's observational network. It was found that there had been a total of 55 dust days during this ten-year period. Of these, 43 dust days were associated with cold fronts: 24 were classified as embedded in the zonal westerlies in the Great Australian Bight (GAB) or in westerlies derived from low pressure systems in the GAB, whereas the remaining 19 were associated with cold fronts over eastern Australia where high pressure systems in the GAB generated strong post-frontal south to southeast winds. Three detailed case studies of dust storm generation and transport were modelled using a sophisticated coupled wind erosion prediction system. The model predictions in all three cases were found to be broadly consistent both with satellite images highlighting dust and the synoptic observations that reported dust. Antecedent land surface conditions are found to be critically important in generating dust, and their representation in the integrated prediction system is subsequently important in providing skilful model predictions of dust transport.

Four different models for the prediction of frost events caused by air cooling due to thermal radiation are presented in this paper. Three of them are based on polynomial functions that simulate different types of temperature variations in comparison with an exponential one. The fourth, a post-processing method based on Kalman filters, is proposed for those cases where a systematic type of bias has emerged.

The influence of a strong low-level temperature inversion on the occurrence of lee waves, rotors and hydraulic jumps has been investigated using high resolution numerical model simulations. The aim of the work is to develop tools for forecasting hazardous winds downstream of mountains. Two-dimensional simulations were conducted for a range of inversion heights and strengths and a fixed hill shape; lee waves, rotors and hydraulic jumps were found to occur. The flow type depends largely on the ratio of mountain height to inversion height and the upstream Froude number. A flow regime diagram based on these two parameters has been constructed and suggests that rotors could be forecast using upstream profiles, which are generally readily available from numerical weather prediction models.

The applicability of the regime diagram for two-dimensional flow to flows over real terrain has been tested using three-dimensional simulations of flows over East Falkland, South Atlantic, under a range of upstream conditions. The flow type is found to be determined largely by the upstream profiles of wind and temperature, and the maximum height of orography directly upstream, indicating that the flow regime diagram can be used to predict flow type downstream of such terrain. Various three-dimensional flow phenomena occur, such as flow channelling through gaps, and could be taken into account to improve the information available from the regime diagram.

The basic condition for using precipitation data from raingauges and radars is data quality control. This aspect is important for comparing and using rainfall data, for example in models. In the scope of the EU-project VOLTAIRE (Validation of multisensors precipitation fields and numerical modelling in Mediterranean test sites) rain data from Cyprus have been analysed. Different quality control methods have been applied to the rainfall data of 158 raingauges and the data of 11 events (in 2002 and 2003) of the C-Band radar in Kykkos. The first results of the use of ground clutter algorithms for radar data in Cyprus are presented in the paper.

A physically-based 1-D heat and mass transfer model was tested to estimate the heat fluxes of an asphalt surface. The model was run for two summer months for a road test site in southwest Sweden. Parameters for thermal properties, surface runoff, radiation and turbulent transfer were obtained from a description of the road stratification and from the literature. Coefficients of determination (r2) 0.94, 0.93 and 0.97 were obtained when simulated results were compared with observations of net radiation, heat flow below the surface and surface temperature respectively, all with slope coefficients close to unity. In addition, simulation results elucidated the role of water vapour transport through the asphalt-soil profile and its effect on the latent heat flow from the surface. Problems were identified with closure of the heat balance in measurements based on discrepancy between simulated and observed sensible heat flux.