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.

Wind farming has grown rapidly in Europe over the last decade. All European countries with shallow coastal waters and strong mean coastal wind speeds are planning or already constructing offshore wind farms. Large parts of the North Sea and Baltic Sea are being investigated to see whether they are suitable for offshore wind parks. In this study we show how satellite images taken by space-borne radar sensors can be used to determine mesoscale wind fields and thus help in the task of planning offshore wind farms. High-resolution synthetic aperture radar (SAR) images acquired by the European remote sensing satellite ERS-2 showing single wind turbines are investigated. The methods for retrieving high-resolution wind fields from SAR images are explained and statistical comparisons of wind speed and wind direction at the two offshore wind park sites in the North Sea, Horns Rev and Butendiek, are given.

A flood study of the Himalayan rivers has been attempted on the basis of gauge/discharge data from 27 sites on major tributaries of the Ganga between 1986 and 1999. This study has shown that the Ghaghra, Gandak, and Kosi are the most important tributaries of the Ganga and their flood waters should be harnessed for developmental purposes to save the Indo-Gangetic plains from the recurrence of yearly floods. It was also found that some of the sites on these rivers have recorded floods more than 100 to 150 times during the 14-year period.

Formation of frost on paved surfaces presents a potential hazard to the motoring public in cold climates. Temperatures of the paved surface are not measured routinely by the National Weather Service and are not part of public forecasts of winter conditions, yet highway maintenance personnel must make frost suppression and anti-icing decisions based on expectations of future paved-surface temperatures. The Road Weather Information System measures road surface, air and dew-point temperatures, road surface conditions, and wind data at numerous locations in the state of Iowa and reports the data in real-time to maintenance offices. A model based on simple concepts of moisture flux to the surface was developed that uses data from roadway weather stations or forecasts of dew-point temperature, air temperature, surface temperature and wind speed to calculate frost accumulation on bridge decks in Iowa. The analysis showed that the model has sufficient accuracy to be used as an operational tool for assessing frost accumulation on bridge decks. A logistical regression procedure was developed to determine the probability that a maintenance worker will observe frost for a given calculated frost depth.

In this study an attempt is made to investigate the upper-tropospheric downstream development over north-west Europe, which leads to surface cyclogenesis in the central Mediterranean. A case study is analysed to demonstrate that the upper-tropospheric downstream development could be closely related to the upper-tropospheric frontogenesis that appears upon the north-eastern flank of a blocking high. The frontogenesis is characterised by a jet streak within a strongly baroclinic zone and a tropopause folding associated with cold stratospheric air intrusion into the troposphere. According to this interpretation, the eddy ageostrophic divergence of eddy geopotential fluxes (dispersion and spreading of eddy kinetic energy), other than friction dissipation and barotropic conversion to the mean flow, is mainly responsible for the loss of kinetic energy from a decaying depression of synoptic scale that has passed the mature stage. This dispersed eddy kinetic energy accumulates in the vicinity of the aforementioned jet streak where it is transferred downstream and further triggers the generation or rejuvenation of a new disturbance.

Measurements and estimates of evaporation have been routinely carried out for over a century. In the United Kingdom the most well-known method of estimating evaporation and its derivatives such as soil moisture deficit (SMD) is the Meteorological Office Rainfall and Evaporation Calculation System (MORECS) which is based on the Penman-Monteith equation. Direct measurements of evaporation have become less common in recent years. A comparison of measurements and estimates of evaporation and SMD at a single site and over two catchment areas is presented in this paper. Measurements of actual evaporation from a grassed lysimeter have been achieved for the period 1995-98 and they agree well with catchment losses over the same time period in two natural catchments, the Upper Brue and Upper Cary. In contrast to this, MORECS estimates of actual evaporation are different by an average of +20% for the Upper Brue catchment and +27% for the Upper Cary catchment. The largest difference is nearly +40% for the Upper Cary catchment. Similarly, there are substantial differences in the estimates of SMD, often up to 50 mm. When local rainfall is used together with MORECS estimates of potential evaporation the discrepancy is reduced but significant differences remain during much of the growing season, although improvements do occur. The results from this paper will be of great importance to growers, water resource planners, engineers who give flood warnings, and to river flow modellers.

This study considers night-time urban air and road surface temperature variations under different synoptic weather conditions. Five years of winter weather data from a single weather station were classified into groups with similar air-mass characteristics. Using a two-stage process of principal components analysis followed by cluster analysis, the resulting clusters were used to sort paired air and road-surface temperature observations recorded at 2100hr from the West Midlands road weather information network into days with similar meteorological controls. The six resulting clusters have good between-group variation and feature a dominant wind direction. The results show that the difference between air and road surface temperatures is significant except during strong westerly conditions. Days featuring clear and anticyclonic weather showed the greatest between-site variation and evidence of the urban heat island effect. Under these conditions, road surface temperatures were warmer than the air - the opposite of what one would normally expect in rural areas. Those days with wet and windy weather were found to have the least between-site variation, excepting that due to altitude. The presence of a high vapour flux in the urban environment literally dampens the temperature response.

In strong baroclinic flows, small errors in the initial conditions may grow rapidly and lead to large forecast errors even at very short ranges. It is therefore of crucial importance that these initial conditions are determined carefully. In this paper we use two different but complementary methods to improve the operational analysis of a storm that caused loss of life and much material damage in France on 27 December 1999. The first method is based on the adjoint technique: an adjoint model-run provides the sensitivity of one aspect of the forecast (the cost function) to the initial conditions. We then apply a correction whose shape is the sensitivity so that the corrected initial state better fits the observations. The second method is based on the Quasi-Geostrophic Potential Vorticity (QGPV) inversion. A misfit between the model analysis and satellite pictures (water vapour Meteosat images, for example) leads to the subjective modification of some of the features of the potential vorticity field. The analysis is modified using the wind and temperature corrections provided by the inversion of the QGPV correction. Both methods significantly improve the forecast and may provide valuable guidance for forecasters in real time.

Hong Kong International Airport (HKIA) is located in a coastal environment and is occasionally affected by sea breezes. High air temperatures relative to sea surface temperature, coupled with light winds or moderate east to northeasterly winds are favourable conditions for the development of a sea breeze at the airport. In the afternoon of 27 April 2000, sea breezes affected the airport and a sea breeze front was formed by convergence between the westerly sea breeze and strengthening easterlies. An aircraft landing at HKIA encountered the sea breeze front and reported wind shear with airspeed gain of up to 15 kt at a height of 30 m (100 feet) above ground. Surface wind observations and Doppler velocity data of a Terminal Doppler Weather Radar (TDWR) allow quantitative analysis of the wind shear event. Shortly after the wind shear event, the TDWR also captured interesting interactions between the sea breeze front and an area of passing showers. This paper presents a quantitative analysis of the wind shear event and observations of the sea breeze interactions by the TDWR.

‘Koshava’ is a gusty wind of moderate to strong intensity, blowing from a south-easterly direction, over Serbia, Romania and Bulgaria. It is caused by the interaction between the synoptic circulation and the orography of the Carpathian and the Balkan mountains. This paper analyses wind data from an area where the influence the ‘Koshava’ wind is high during periods of maximum duration of ‘Koshava’. The focus of the paper is the examination of urban and suburban effects on ‘Koshava’ and the correlation between the instantaneous maximum wind speed and the hourly mean values. Various empirical distributions are fitted to the daily maximum wind speed and the best fit for each season and site is proposed. The vertical structure of the lower troposphere above a suburban site during the longest period of consecutive days of ‘Koshava’, which occurred in January and February 1972, is considered.

A new method of constraining divergent attenuation corrections for weather radar systems is presented. This was motivated by the need for reliable attenuation corrections when making quantitative precipitation estimates using a small, mobile X-band radar at short range. The approach is suitable for systems requiring attenuation correction in real-time and requires no auxiliary data. An outline of the literature on attenuation and its correction for single-polarisation weather radar is presented. The traditional form of correction is known to be problematic due to the divergence, which may occur in its estimation. The form of constraint presented is based on the representation of attenuation correction originally derived for space-borne radar configurations. This method determines when divergent estimates will occur and allows a more realistic application of the constrained correction. Examples of attenuation correction applied to X-band radar observations in comparison with ground clutter returns are presented, showing good agreement and hence good absolute calibration. Selected profiles are then used to determine the influence of measurement errors on attenuation estimation in the context of this representation. The paper concludes with a discussion of the practical limitations and considerations in applying an attenuation correction to quantitative weather radar rainfall estimates.

Two methods for estimating ten-day rainfall totals from Meteosat infra-red imagery were compared for the April-June 1996 ‘long rains’ of Kenya in an area covering the eastern highlands and the Tana and Athi river basins. One of these (the Bristol ‘B4’ method) was then used for rainfall estimation for the whole of Kenya, for November 1996 and the other, the TAMSAT ‘Cold Cloud Duration’ (CCD) method was used to estimate rainfall for the whole of Kenya for November 1997 to April 1998. April-June 1996 was an unusual season with very few large rainstorms. For this comparison period the B4 method gave better estimates of actual rainfall than the TAMSAT method because it used a variable cold cloud threshold temperature and ongoing calibration against rain gauge data. Comparison of ten-day CCD totals with rainfall for the 1997-1998 period indicated that using the TAMSAT method gave best rainfall estimates for the arid and semi-arid areas of eastern and northern Kenya and for months other than the main rainy season months of November and April. Both methods could be used successfully to identify periods with well below or well above average rainfall even over highland areas, and they are therefore useful for providing food security early warnings.

A workstation-based NWP scheme is described which allows forecasters to reposition and change in intensity features such as fronts and depressions, and thus alter model fields in a realistic way. The technique is based on distortion of either geopotential height or potential vorticity (PV). In the latter case, the PV is calculated from geopotential distribution, distorted, then inverted to yield a modified, yet consistent and dynamically balanced, set of grid-point data.

This study compares the performance of different models used to assess the local wind near-surface conditions at an agricultural site in Scania, southern Sweden. The methods are: (a) the WASP model (Wind Analysis and Application Program), (b) separate linear regressions of the two wind vector components, (c) a regression model based on vector correlation, and (d) linear regression of scalar wind. Each method was tested with three different data sets over nine months: wind measurements from the nearby Sturup airport SYNOP station, 10 m surface wind and surface geostrophic wind produced by the operational Swedish Mesoscale Analysis system (Mesan). The wind climate estimations were compared with observed winds at the field site, with respect to mean wind speed, wind direction, wind speed frequency distribution and the relative frequency of winds above 6 m s−1. All models performed reasonably well with data from Sturup and Mesan surface wind, but gave less reliable results with the Mesan geostrophic data. The estimated frequency of winds above 6 m s−1 was in general lower than the observed frequency. Overall, best results were obtained with WASP in combination with measurements from Sturup.

A statistical comparison of observations and forecasts using the Regional Atmospheric Modelling System (RAMS) for operations at the Savannah River Site (SRS) in South Carolina is discussed. Simulated and observed values of wind direction and wind speed, collected twice daily for a two-year period from April 1998 to March 2000, are compared in a variety of ways for five different locations in the southeastern United States. Simulated and observed turbulence intensities are also compared for a one-year period beginning in February 1999 for the SRS. Statistical measures used for comparison are the mean absolute error, mean relative error and standard deviation. Directional errors tend to increase with forecast time while speeds tend to be underestimated over the entire forecast range, especially those exceeding 8 m s−1. Seasonally, winds are predicted with greater accuracy during the winter and spring months, when speeds in the southeastern United States are higher and direction is less variable. Vertical turbulence intensity at SRS is slightly underestimated, while horizontal turbulence intensity is slightly overestimated. In general, the statistical measures show that the model produces results with reasonable skill for the wind speed, direction and turbulence.

The optimum sowing dates for soybean cv. Gaurav were derived for Jabalpur, Raipur and Gwalior in the state of Madhya Pradesh in central India. Dates were derived based on two strategies: (a) probabilities of rainfall and temperature events using ClimProb, a PC based software package, and (b) the CROPGRO Soybean v3.0 crop growth simulation model. In Madhya Pradesh, the optimum sowing dates for multiple cropping, with the first crop as soybean under rainfed conditions, are between weeks 25 and 27, while the optimum sowing dates for rainfed mono-cropping are between weeks 28 and 29.

Quasi-geostrophic (Q-G) theory can be used to infer the vertical circulation associated with a synoptic distribution of pressure and temperature in much the same sense as geostrophic theory is used to infer a three-dimensional distribution of the horizontal circulation. For the purposes of subjective analysis, a diagnosis of the vertical circulation and implied surface development can be based on identifying the sign and magnitude of a geostrophic forcing function associated with the particular distributions of geopotential and thickness shown on a conventional 500 hPa analysis chart. Examples of idealised circulation features are used to demonstrate both the strengths and the weaknesses of this approach, with reference to an analysis based on three different methods of evaluating the geostrophic forcing.

This paper investigates a case of convective snow that was observed with an occluded-type cyclone. Cross-section and plan-view analyses show that convection resulted from the release of elevated, potential instability by isentropic uplift associated with an easterly trowal airstream (a western extension of the warm conveyor belt) located between 700 and 850 hPa. Forcing for ascent was supplied by low-level frontogenesis, as well as an intensifying 500hPa tropopause fold and its associated potential vorticity anomaly. The latter not only provided a source of very cold, dense air, but also was responsible for lower tropospheric cyclogenesis and subsequent trowal generation.

Of the full set of observations carried out at weather stations, the ‘present weather’ observation is one of the most difficult to automate. Initially, elements such as rain and fog were automated. However, during the past ten years a new group of sensors, aimed at identifying the type and intensity of precipitation, has emerged. These sensors have come to be called Present Weather Sensors (PWSs). Most of the early tests and intercomparisons have been limited to liquid precipitation. Not much attention has been given to investigating the reasons for disagreement between the primary reference, the professional meteorological observer, and the automated observations. This paper presents an analysis of an intercomparison between observer and one type of PWS (Vaisala FD12P), in conditions of both liquid and frozen precipitation. An interaction model is introduced as a new and improved tool for the quality control of PWSs.

The synoptic situations associated with significant temperature decreases in Buenos Aires are analysed. Knowledge of these situations for the winter season is very useful for the community, especially when they are related to the consumption of natural gas, which is one of the most important sources of energy in Argentina. The rotated principal component method is used to classify the synoptic situations. The significant temperature decreases in winter during the period 1980–1988 occur in five modes at 1000 hPa and three modes at 500 hPa. The relationship between the two levels identifies two dominant patterns. The first has a strong high-pressure system with a northwest-southeast axis and a maximum strength near 80° W, 40° S; this pattern is related to south-southeast winds over Buenos Aires. The second type shows a high-pressure system at 105° W, 48° S and a minimum near 110° W, 30°–35° S; this kind of pattern is associated with persistent blocking situations in the South Pacific which result in persistent winds from the south over Buenos Aires. In addition, a relationship between such situations and the resulting increases in the demand for natural gas in residential areas is shown. This will help in the forecasting of such events in the future.