The application of kites as a platform for meteorological measurement within the boundary layer is examined. Kites are shown to fulfil a useful role within this research area, particularly when a highly mobile, lightweight and inexpensive instrumental platform is required. They are ideal for use at remote sites, or sites where terrain limits access for more conventional profiling equipment. The development of a reliable, tethered radiosonde atmospheric profiling system (TRAPS) is described, comparing the relative performance of two different sizes of two different types of kite. Basic kite aerodynamics are explained as well as a number of practical problems which have to be overcome or accommodated. Appropriate kite selection for given wind conditions and experimental requirements is suggested, providing an operational windspeed of between 2 and 14 m s-1. Finally, experimental results from three diverse remote sites illustrate practical applications of the profiling system, investigating firstly the structure of surface inversions over an ice shelf in the Antarctica, secondly flow over hills in Cumbria and thirdly convective boundary-layer structure on the Isle of Arran, Scotland.

There have been significant reforms in New Zealand of government management in general and of the science and transport sectors in particular. The impact of the reforms on the provision of meteorological services is discussed as an example of the application of the general reform thrust to a specialist technical area. The eventual outcome was the establishment of Meteorological Service of New Zealand Ltd (MetService) as a commercial company, trading in the weather forecasting market but remaining under Crown ownership. At the same time the National Institute of Water and Atmosphere (NIWA) was established. It includes the climatic responsibilities and much of the scientific research component of the former NZMS. It too operates commercially and is Crown owned. Unlike MetService, NIWA is not required to return a dividend to its owners. The procedures leading to the establishment of these new organisations, their mode of operation and their initial successful performance are described.

Three severe hail- and windstorms, which occurred in Northern Switzerland, have been investigated. The storms produced hailswaths of 70-170 km in length and 12-25 km in width. Compact tracks of severe wind damages within the hailswaths are documented from public reports. These tracks have a length of 12-25 km. The damage was produced by straight-line winds rather than by tornadoes. Volume-scan Doppler radar data of the storms are available in time steps of 5 min. Radar signatures, such as low-level convergence and shear, mid-level vorticity, and high-level divergence, were attributed to the damage tracks at the ground. The low-level radar signatures allow the deduction of the time of occurrence of the damage tracks with a precision of some minutes.

Striking similarities in the evolution of the three stormes were found. The storms developed in the foothills of the Alps and the Jura mountains and propagated towards the plains of the Swiss midland. The storms can be classified as ‘high-precipitation’ supercell storms, known as producers of severe straight-line winds in the USA. Meso(anti)cyclonic vortex signatures were seen 40-55 min in advance of the heavy wind damage at the ground. The damage tracks were associated with explosive secondary cellular growth aloft. The physical explanation of this behaviour is that the first cells with mid-level rotation produced a gust front outflow that was accelerated to damaging strength at the time when the secondary cellular growth was initiated. The operational implication is that the nowcasting of severe and damaging winds can be improved by considering mid-level rotation in an early stage of the evolving storms.

A physical discussion of vorticity and its change by stretching and tilting is presented. The discussion naturally leads on to the conservation of quasi-geostrophic potential vorticity, q, and the flow associated with anomalies in q. The extension to full potential vorticity (PV) and the associated conservation and inversion properties is made. The use for illustrating and understanding synoptic developments of maps of PV on isentropic surfaces and potential temperature (θ) on PV surfaces is shown by considering two recent examples in the North Atlantic-Europe regions. The first is a summer situation and the second is the major blocking in January 1996. Concluding comments about the role of diabetic processes and other possible uses of these diagnostics are made.

The combined use of the European Centre for Medium-range Weather Forecasts (ECMWF) high-resolution operational model, at T213 spectral triangular truncation and with 31 vertical levels (T213L31), and the Ensemble Prediction System (EPS), during cases of intense Mediterranean storms, is studied. In particular, it is discussed how EPS products can be used to provide a measure of confidence in the high-resolution precipitation forecast. Three case studies (two extreme events plus one false-alarm case) are analysed. The first event took place between 21 and 22 October 1994 over Greece, where heavy rainfall led to local flash-floods in many areas that cost the loss of 12 lives, and caused significant property damage. The second event occurred in northwest Italy (and some parts of southern France) exactly two weeks later. More than 60 lives were lost and the cost of damage was enormous. For both these cases, the EPS probability values for precipitation occurrence supported the medium-range T213L31 prediction, which proved to be successful. A third case is also investigated, where the high-resolution forecast suggested heavy rainfall over northern Italy but was not supported by the EPS. The T213L31 prediction for this case was poor. Finally, the reliability of EPS probability predictions is discussed. It is shown that probability forecasts of low-level temperature and wind show considerable skill. Reliability statistics of clusters of large-scale flow show considerable skill as well. Probabilistic precipitation predictions are less skilful, possibly due to the EPS model resolution.

Weather radar images were used in an investigation of the rainfall events which gave rise to various damaging UK floods in the 1990s. Radar data of sufficient quality were available for seven events from an initial list of 26. The relative temporal and spatial rainfall profiles of each event are displayed and summarised, along with the antecedent catchment wetnesses. Catchment rainfall totals were obtained from daily raingauges. These characteristics are compared with their equivalents in a standard method of flood estimation, the design event method of the Flood Studies Report. The results highlight the complexity of rainfall fields. The wide variety of event characteristics includes some large deviations from their counterparts in the design event. Six of the catchments were modelled with the design event method in order to assess the significance of the deviations in terms of the estimated flood peaks. The rainfall depth and duration and the antecedent catchment wetness were found to have a significant influence within the design event method. The temporal profile was of lesser importance, for most of the events studied.

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.

A whole-farm-level decision model is used to examine the impact of the type of decisions producers make on the value of seasonal climate forecasts. Results suggest that precipitation forecasts directed towards crop mix and applied nitrogen level decisions would have the largest economic value. Further, the results show that the economic value of climate forecasts cannot be obtained by examining only a small set of decision types. Rather, all decision types must be modelled to value seasonal precipitation forecasts correctly. This occurs because in response to seasonal climate forecasts changes in one type of decision may override the need to change other decision types. Finally, forecasts of precipitation during the crop tasselling and grain filling stages may be more valuable than precipitation forecasts for earlier crop growth periods.

Surface precipitation-rate estimates derived from radar data are potentially of considerable value to high-resolution Numerical Weather Prediction (NWP) models. This paper describes a scheme developed to assimilate precipitation rates derived from the UK weather radar network into the UK Met. Office Mesoscale Model, with the aim of improving the analysis and forecast of precipitation. It is based on 'latent heat nudging', in which the model profiles of latent heating are scaled by the ratio of observed and model precipitation rates. This causes the model to adjust so that the diagnosed precipitation rate agrees more closely with observations.

The assimilation algorithm is outlined, and the results of a trial of the scheme are described. The scheme brings an increase in forecast skill for precipitation distribution in the first six to nine hours of the forecast, a conclusion supported both by objective verification against radar data and subjective assessment of 14 forecasts. The main benefit was found to occur in frontal cases. The scheme was implemented operationally on 16 April 1996.

A divergence-based procedure for diagnosing development, based on a two-layer model of the atmosphere, is discussed. It is pointed out that thinking in terms of vorticity advection and thermal advection is directly equivalent to considering divergence due to advective and isallobaric components of the ageostrophic wind. It is shown that such an approach is similar in basis to the quasi-geostrophic omega equation, but it is argued that it is more suitable for subjective application. However, simply estimating vertical velocity or instantaneous pressure tendency from synoptic charts is of questionable accuracy and little operational use, especially since such diagnostics are much more reliably obtained from NWP models. It is necessary to think in terms of the development and movement of features in the flow of the upper as well as the lower troposphere, and their vertical interactions.

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.

The physics of climate

An International Conference on ‘The Physics of Climate’' will be held at the Royal Society, London, UK, on 29 and 30 October 1997. The themes of the conference are:

[bull ] Radiation, clouds, humidity and aerosols.

[bull ] Climate, chaos, predictability and entropy.

[bull ] Interactions between the cryosphere, land, atmosphere and biosphere.

[bull ] Climate models, observations and detection of climate change.

Information about the conference is available from: Royal Meteorological Society, 104 Oxford Road, Reading RG1 7LL, UK.

Fax: +44 118 956 8571

E-mail: [email protected]

Experimental model forecasts conditions every 10 kilometres

The National Weather Service (NWS) in the USA has begun testing a new experimental computer model that will assist forecasters to predict weather conditions across the western United States with more detail than ever before. The model, known as the Eta-10, allows forecasters to monitor the development of weather systems down to a 10-km grid.

With the aid of a generalised frontogenesis vector and an expanded Q-vector diagnosis, the frontogenetic and frontolytic effects in numerical model analysed winds are determined for the case of a typical cyclogenesis over the North Atlantic. These effects, when occurring in the geostrophic part of the flow, give rise to circulations transverse to the frontal zone and the resulting vertical motions are of importance for the dynamics of the cyclone. The forcing of the transverse circulatory motions, which is given by Q-vector divergences across the isotherms, is computed and compared with the forcing resulting from divergences along the frontal zone. The analyses for 850 and 500 hPa show that frontogenesis dominates at the beginning of the cyclogenesis as well as at its final stage, whereas in the meantime different patterns result for both levels. A significant feature in this case was the development of a new frontal zone north of the deepening low through interaction with the parent low further north. Comparison of the different parts of the omega forcing reveals that the forcing through Q-vector divergences along the isotherms is the stronger one during the mature stage, but that the transverse circulations contribute significantly to the total forcing. This is especially true for the cold front in the lower troposphere and for the newly developing frontal zone in both levels. At the final stage the influence of the transverse circulatory forcing is even more important and determines to a great extent the shape of the total forcing. The outcome of this study corresponds very well with the findings from other investigations and with the fields derived from a vortex model.

Meteorology at Reading

The Department of Meteorology at the University of Reading has recently moved into a new building, which was opened by the Chancellor, Lord Carrington, on April 14th, 1997. The building allows the Department, the NERC Centre for Global Atmospheric Modelling, and the Meteorological Office/University Joint Centre for Mesoscale Meteorology to come together in the same building for the first time.

There are several extant techniques for diagnosing the dynamics and structure of synoptic-scale flow. The essence of three of these approaches (the Q-field diagnosis, Lagrangian airstreams and the PV-perspective) is outlined, and their relative characteristics illustrated by examining two synoptic cases. It is shown that the three approaches can independently reveal key features of the events, and that the views they offer are complementary.

A brief account is given of the case work undertaken as part of the Summer Study Week on ‘Extra-tropical Cyclones’ held in Reading during July 1996. It is shown that different approximate methods of assessing synoptic-scale vertical velocity can give widely differing results. Computation of the full quasi-geostrophically forced vertical velocity is recommended as a more informative procedure. Forecasts for the track of an explosively deepening north Atlantic cyclone are compared at different lead times. The comparison encompasses raw model forecasts and model forecasts adjusted by groups of case work participants. The groups achieved a large improvement over a model forecast at the 2-3 day lead time, but only marginal improvements thereafter. It is suggested that improved understanding of development mechanisms would enable the forecaster to appreciate better the range of possible outcomes in cyclogenetic situations.