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.

The United States Defense Threat Reduction Agency (DTRA) DIPOLE EAST 169 experiment took place at White Sands Missile Range (WSMR), New Mexico on 23 September 1999. The objective of the experiment was to validate the DTRA Hazard Prediction and Assessment Capability (HPAC) suite of models in a weakly forced, light wind environment. Specifically, DTRA diagnostic and prognostic models were used to produce wind fields that were passed to DTRA's transport model. The resultant transport calculations were compared with the ground truth data. A release of SF6 was tracked by remote sensors to provide ground truth data. Test day weather conditions were gathered. Data from the WSMR Surface Automated Meteorological Site (SAMS) sites, a mobile profiling system, and from rawinsonde were collected and fed into the models.

A presentation of the general weather conditions that existed on the test day is first reviewed, followed by a discussion of the forecast profiles for the test site as well as a discussion of the weather condition that existed in the calculation domain as evidenced by the surface data. This provides the background conditions for the test such as the cloudy conditions that were experienced and the light winds throughout the domain. Included in this discussion is the presentation of the ground truth data collected by the Fourier Transfer Infrared Spectrometers (FTIRs) used to determine the location of the SF6 cloud centroid. The results of the transport model output based on input from the diagnostic and prognostic models are presented. Finally, the results of the actual SF6 cloud centroid are compared with the modelled results. The differences are discussed and potential causes for the differences considered. However, the overall model performance produced results that show that cloud tracking and calculations were within one grid unit of ground truth. Overall, the models produced reliable results for this limited study on a light wind case.

Long records of hydrological and hydrometeorological variables are of inestimable value for the planning, design and management of water resource systems. Unfortunately, such long series invariably exhibit inconsistencies and non-homogeneities arising from a wide variety of causes - for example, changes in instruments and observation practices, and alterations to the general environment of the instruments themselves. Both non-parametric and parametric statistical tests are available to detect the presence of long-term movements in recorded time series, but the interpretation of results from such testing has often to be carried out in the absence of sufficient station metadata. A case study of precipitation records for 20 stations in Scotland over the period from 1961-99 has demonstrated that the results from statistical testing for inconsistency and non-homogeneity should also be interpreted in the context of prevailing weather systems. In particular, the increases in both annual and winter (Dec-Feb) rainfall totals recorded during this period at sites in western Scotland appear to reflect the changes in the winter atmospheric circulation over the Northern Hemisphere as encapsulated in the North Atlantic Oscillation Index (NAOI). Sustained periods of positive values of the NAOI, albeit less severe than during those of the 1970s and 1980s, are evident in the historical record of this index. The trends in rainfall totals identified could therefore be interpreted as arising from natural variability or even greenhouse gas forcing rather than from any inconsistency and non-homogeneity.

High atmospheric temperature and humidity can cause discomfort and even death among people in high-risk groups. Forecasting such meteorological situations can be a useful tool for public health managers. The predictability of discomfort is analysed using a time series of the relative strain index in Buenos Aires. Other authors have found that index values exceeding the thresholds of both nocturnal and diurnal comfort can increase mortality. This study finds that such situations are associated with index values over 0.20 at 14:00 h. local time (RSI14>0.20). Consequently, the meteorological conditions associated with RSI14≥0.20 are studied. The anomalies of the geopotential monthly mean fields show the passage of a low-pressure system south of 45°S. This system is related to a trough, possibly of frontal character, moving across the province of Buenos Aires, and a simultaneous intensification of the north-western low in Argentina. The synoptic situation is associated with a general northerly flow that brings heat and moisture from the tropics. On 81% of days with RSI14=0.20, winds with a northerly component were recorded. This increases to 84.4% when at least one day with northerly wind is previously observed. The synoptic characteristics are examined using a combination of circulation indices: meridional (R), zonal (Z) and curvature (C). The daily sequence of the weighted averages of the anomalies obtained are ΔR<0, ΔZ>0 and ΔC>0. Some 67% of the days when RSI14=0.20 correspond to this daily index pattern estimated at locations near Buenos Aires.

The interaction between atmospheric and construction conditions and the exothermic, temperature-dependent hydration reactions of the concrete's binding components may produce adverse conditions in curing concrete, thereby reducing the quality of that concrete. Accurate model forecasts of concrete temperatures and moisture would help engineers determine an optimal time to pour, an optimal mix design, and/or optimal curing practices. Existing models of curing concrete bridge decks and road surface prediction models lack realistic boundary conditions. The concrete models contain unnecessarily detailed hydration heat generation mechanisms for a simplified field forecast model. In this paper, a new energy balance model (SLABS), which can be easily adapted to predict road surface conditions, is described and applied to predict the temperatures and moisture of curing concrete bridge decks made with New York State Department of Transportation's Class HP concrete. Highest concrete temperatures occurred at high air temperatures, humidities and initial concrete temperatures and at low cloud cover fractions and wind speeds. Peak concrete temperatures can exceed 60°C. To minimise concrete temperatures and temperature gradient magnitudes, concrete should be placed during the late afternoon or early evening. As a field forecast model for which the meteorological inputs are taken from NGM MOS forecasts, the outputs of SLABS include the peak concrete temperature (to within 2°C of the observed in one application), peak temperature gradient, evaporation rate at the time of placement and several warning messages indicating adverse field conditions.

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.

In Finland, winter road maintenance has been privatised, and the use of reliable predictors to measure the harshness of winter conditions has become even more important. Several indices have been developed to estimate the difficulty of winter conditions; however, many of these indices require relatively complicated input data and are in that sense vulnerable - for example, to changes in the meteorological station network. In the current study the year-to-year variation of the climatological variables describing maintenance costs were compared with the variation in costs. This made it possible to make a qualitative interpretation of the costs. This approach is ‘simple enough’ for its use to be easily accepted both by the contractors responsible for winter road maintenance and also by the administrators responsible for covering the maintenance costs.

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.

The role of thermal advection in development is examined in its different guises, from the frontal cyclone model of the Bergen School, through the ideas of Sutcliffe and Petterssen, to the quasi-geostrophic omega equation, system-relative isentropic analysis and the potential vorticity (PV) perspective. The PV formulation of the omega equation introduced by Hoskins et al. (1985) is used, together with a simple quasi-geostrophic numerical model, to compare system relative isentropic upgliding with full vertical velocity. The PV approach is compared with the application of the traditional omega equation, particularly with regard to cancellation between vorticity/potential vorticity and thermal advection terms. An equivalence is proved between the traditional form of the omega equation and the diagnosis of vertical velocity via ageostrophic motion, with the contribution from thermal advection being identified with the vertical motion that arises from isallobaric ageostrophic motion.

Orographically enhanced rainfall is the dominant climatic feature of the alpine regions of South Island, New Zealand. Most of this rain occurs in strong north-westerly winds although a forecast difficulty exists in that near-saturated north-westerly flows do not always produce heavy rain. Air masses impinging on the Southern Alps are generally carried over the alpine divide in times that are normally regarded as insufficient for clouds to produce precipitation-sized particles. Hence a possible explanation for these observations is that precipitation occurs only when pre-existing clouds advected onshore contain cloud drops that are sufficiently mature to allow for raindrop growth in the available time. A model combining advection of moist air over the topography and IR satellite data was used to show that synoptic cloud is necessary for the formation of heavy rainfall in the Alps.

This paper is concerned with the transformation of an upper level trough into a shear line and a tropopause cyclone or cut-off low. During this event, the isentropic potential vorticity of air columns between fixed isentropes approaching the trough axis at its level of maximum intensity from the west is well conserved over a period of about 12 to 24 hours, but there is a significant repartitioning of the constituent parts, from a relatively large static stability to a relatively large absolute vorticity. This indicates stretching of vertical columns of air. The effect that is inducing the vortex stretching is identified within the context of quasi-geostrophic theory as ‘opposite vector-frontogenesis’. This implies that Q-vectors point in opposite directions, respectively, above and below the level of maximum trough-intensity. Isentropic analysis indicates that the criterion for unstable isentropic downgliding is fulfilled in the upper troposphere in a restricted area, covering several hundred thousand square kilometres, near the base of the trough when and where the cut-off low is formed. This area of unstable isentropic downgliding, to which we assign the term ‘baroclinic downburst’, and which maintains its identity during a period of time in the order of 24 hours, coincides approximately with the dry intrusion as identified on a ‘water vapour’ image. It is argued that unstable isentropic downgliding is in fact necessary in order to realise a cyclone-core characterised by relatively high static stability, relatively high absolute vorticity and relatively high potential vorticity, as is demanded by the invertibility principle for potential vorticity. The events described above are triggered by the interaction of two potential vorticity anomalies.

A three-dimensional numerical model for mountain waves is described and validated against radiosonde observations over the Isle of Arran, south-west Scotland. The model is based on the Boussinesq equations of motion linearised about a background profile of wind, potential temperature and density. The equations are discretised using finite-difference approximations and the time dependence is retained. Solutions are then computed by integrating forward the equations in time until a stationary mountain-wave field is obtained. Data from radiosondes, launched during fieldwork on Arran during February and March 2000, are used to represent the background flow and model simulations of the wave field are compared directly with fluctuations in the ascent rates of the balloons. On average, the model reproduces the observed maximum peak-to-peak vertical velocity to within about 0.6 ms-1. In many cases the accuracy of the simulations is even better than this: the model vertical velocity along the path of the radiosondes agree well with fluctuations in the balloon ascent rate. The sensitivity of the model results to the wave ‘launching height’ is discussed. The model was also used to provide forecasts of wave activity during the field campaign. In this case, rather than using radiosonde data to represent the background flow, data were extracted from a global forecast model. The accuracy of these forecasts is examined and the viability of using such a model to provide routine operational high-resolution mountain-wave forecasts for specific sites is discussed.

The pricing of weather derivatives motivates the need to build accurate statistical models of daily temperature variability.Current published models are shown to be inaccurate for locations that show strong seasonality in the probability distribution and autocorrelation structure of temperature anomalies. With respect to the first of these problems, we present a new transform that allows seasonally varying non-normal temperature anomaly distributions to be cast into normal distributions. With respect to the second, we present a new parametric time-series model that captures both the seasonality and the slow decay of the autocorrelation structure of observed temperature anomalies. This model is valid when the seasonality is slowly varying. We also present a simple non-parametric method that is accurate in all cases, including extreme non-normality and rapidly varying seasonality. Application of these new methods in some realistic weather derivative valuation examples shows that they can have a very large impact on the final price when compared to existing methods.

On 16 May 2000, a low pressure system produced very intense winds over the Río de la Plata (RP) river, giving rise to floods within the region. In order to study this event, the synoptic evolution of the cyclone of 14-17 May 2000 is documented using conventional observations and the NCEP analysed data sets. The synoptic-dynamic overview reveals that the system starts as a usual cyclogenesis on the Chilean coast. The low level cyclone favours a convectively unstable environment within the central Argentinean region triggering convective storms and, consequently, a large amount of latent-heat release. The latent heat and the vorticity advection both play an important role in determining the depth of the system and its movement. Another unusual feature of this event is the north-eastward displacement of the cyclone, which is attributed to the interaction with other low-pressure systems located in the South Atlantic Ocean.

We noted with interest the recent article by Reason & Jackson, which presented a modelling case study of the coastal ridging that occurs along the south-eastern coast of Australia. As the authors and others have pointed out, coastal ridging is a dominant feature of the weather over this region in the warmer months, and also occurs with varying degrees of regularity in other parts of the world. The paper by Reason & Jackson draws to some extent on an earlier publication of ours on the same topic (Speer & Leslie, 1997) and we would like the authors to respond to our comments on a number of alternative interpretations we appear to have of case study they chose. We offer our comments because the topic area is vital to the understanding and prediction of a significant component of the weather over the most populous part of the Australian continent. To that end, we feel that an exchange of views would be of considerable interest to readers of Meteorological Applications, particularly as we and the authors have generated a significant fraction of the published work on this topic, especially for southeastern Australia.

We thank Leslie & Speer (2003) for their comments on our paper (Reason & Jackson 2002). These comments are centred around the possible interpretation of the synoptic evolution of this complex coastal ridging event over southeastern Australia in terms of the original typing scheme proposed by Speer & Leslie (1997). We should like to emphasize at the outset of this Reply that the purpose of Reason & Jackson (2002) was not to provide a synoptic classification of this event or an interpretation in terms of Speer & Leslie's scheme but rather to present a simulation of this complex event with a mesoscale numerical model (in this case, RAMS). Previous RAMS simulations of Australian coastal ridging events by us (Reason et al. 1999; Tory et al. 2001) gave some confidence in the ability of this model to successfully capture the evolution of strong post-frontal coastal ridging. The 30/12/96-1/1/97 event analysed in Reason & Jackson (2002) was not as straightforward a case and it was therefore of some interest to see whether the model could capture the more complex evolution and hybrid nature of this weather system whose synoptic interpretation is difficult and more open to debate.

We discuss how weather forecasts can be used in the pricing of weather derivatives and derive results for the most important types of weather index and contract. We show that calculating the expected payoff of linear contracts on linear indices requires only forecasts of the mean temperature over the contract period. Calculating the expected payoff of linear contracts on non-linear indices requires forecasts of both the mean and the distribution of temperatures, but not of the dependence between temperature distributions on different days. Calculating the expected payoff of non-linear contracts requires forecasts of the full multivariate distribution of temperature over the whole contract. For contracts that extend beyond the end of available forecasts, correlations between the forecast and post-forecast periods must be taken into account when estimating this distribution. We present two methods by which this can be achieved, both of which combine information from climatological models of daily temperature with information from probabilistic forecasts.