The impact of vegetation on the microclimate has not been adequately considered in the analysis of temperature forecasting and modelling. To fill part of this gap, the following study was undertaken.

A daily 850-700 mb layer mean temperature, computed from the National Center for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis, and satellite-derived greenness values, as defined by NDVI (Normalised Difference Vegetation Index), were correlated with surface maximum and minimum temperatures at six sites in northeast Colorado for the years 1989-98. The NDVI values, representing landscape greenness, act as a proxy for latent heat partitioning via transpiration. These sites encompass a wide array of environments, from irrigated-urban to short-grass prairie. The explained variance (r2 value) of surface maximum and minimum temperature by only the 850-700 mb layer mean temperature was subtracted from the corresponding explained variance by the 850-700 mb layer mean temperature and NDVI values. The subtraction shows that by including NDVI values in the analysis, the r2 values, and thus the degree of explanation of the surface temperatures, increase by a mean of 6% for the maxima and 8% for the minima over the period March-October. At most sites, there is a seasonal dependence in the explained variance of the maximum temperatures because of the seasonal cycle of plant growth and senescence. Between individual sites, the highest increase in explained variance occurred at the site with the least amount of anthropogenic influence. This work suggests the vegetation state needs to be included as a factor in surface temperature forecasting, numerical modeling, and climate change assessments.

Over recent years most European countries have experienced destructive precipitation events, many of which have generated floods resulting in the loss of lives and economic damage. Rather surprisingly, Europe has no common database for flood events, which continue to be monitored by different national and/or local authorities, with different criteria. In the light of this situation, the main aim of the present work is to propose a classification scheme for a European-level flood database, in which severe flood episodes are related to meteorological conditions and mesoscale settings.

The meteorological systems relating to the most severe events (22 events) over a five-year period (1992-1996) were analysed using Meteosat imagery and conventional meteorological data. Most of the observed events were related to extra tropical cyclone development. The results are discussed in terms of conceptual models currently used for extra tropical cyclones: the structure and dynamics of Atlantic cyclones can very often be interpreted in terms of well- known conceptual models, while characteristics of Mediterranean systems are not fully rendered by current descriptions. Our study underpins the need to improve the knowledge of Mediterranean systems, possibly introducing new conceptual models and/or refining the current ones.

It is part of British sailing and forecasting folklore that the wind speed increases in the Dover Straits when there is an established wind-flow - westerly/south-westerly or easterly/north-easterly - along the English Channel. However the underlying mechanism of the phenomenon is unclear. We have used the Met Office ‘New Dynamics’ mesoscale model to perform a case study on an occasion when this phenomenon was observed in the Channel but not forecast well by the operational model, UM 4.5. Results are presented showing the sensitivity of forecasts to horizontal resolution (down to 2km) and to vertical resolution. We probe the physical mechanism of the Channel jet by altering the surrounding orography and the land or sea surface roughness. Both the orography and the surface roughness are shown to influence the jet formation.

A Cloud Resolving Model (CRM) is used to simulate the showery convective activity that occurred over the United Kingdom on 13 April 1999, which was notable for observations of strong gusts. Simulations based on radiosonde profiles from both Herstmonceux and Hemsby are shown to give good agreement with available observations for this case. In particular, there is good agreement with the observed cloud top heights, precipitation amount and type, and strength of convective gusts. An algorithm to predict the maximum convective gust strength is shown to be consistent with values measured from the model. Further analysis of the model gust strengths shows that they could be approximated to a Gaussian distribution and that the mean and standard deviation of the Gaussian can be related to the maximum gust strength and the mean wind. A simple algorithm for predicting the probability distribution of the gust strengths measured in the model gives reasonable qualitative agreement with the measured probabilities and it is suggested that this demonstrates the utility of the CRM in developing and evaluating probabilistic forecasting tools for convective weather.

This paper reports the results of the aerosol lidar experiments that have been performed at the Indian Institute of Tropical Meteorology (IITM), Pune (18.54°N, 73.85°E, 559 m amsl), a tropical station in India. The lidar-observed cloud macro-physical parameters (cloud-base and cloud-ceiling heights, vertical thickness, etc.) and polarisation characteristics and their association with surface-generated aerosols at the experimental site are presented and discussed. The correspondence among the lidar-derived aerosol distributions, meteorological parameters and south-west (SW) monsoon (June-September) activity over Pune during 12 successive SW monsoon seasons (1987-98) including two pairs of contrasting seasons of 1987-8 and 1993-4 is also examined. The results indicate an association between variations in aerosol loading in the boundary layer during the pre-monsoon season (March-May) and precipitation intensity during the ensuing monsoon season. Moreover, the decrease in aerosol content from pre-monsoon to monsoon season is found to follow the SW monsoon season total precipitation. Thus the results suggest that (i) the IITM lidar can also be a useful remote sensor for aerosol characterisation studies from polarisation measurements, and some important physical properties of clouds in the lower atmosphere over the station, and (ii) there exists a correspondence between boundary-layer aerosol content and SW monsoon precipitation over Pune, which is explained in terms of the type of aerosols and the environmental and meteorological processes, particularly during pre-monsoon and monsoon months prevailing over the experimental station.

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.

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.

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.

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.