Colocated UHF and VHF wind profiler radars have been operated during the passage of a sharp cold front. The results demonstrate the utility of the profilers, in conjunction with a conceptual model, for identifying and quantifying many of the features commonly associated with such fronts: low-level and upper-level jets, and rearward-sloping warm-conveyor-belt flow with underlying rear-inflow. The UHF radar detected much of the transverse circulation and also the vertical distribution of precipitation, including a possible region of evaporation associated with the dry rear-inflow jet. The VHF radar, in addition to extending the velocity measurements above the level of the precipitation, mapped the tropopause and, in particular, identified a tropopause fold, the position of which corresponded to the origin of the rear-inflow jet detected by both profilers.

An acoustic array antenna which has a narrow main lobe and suppressed sidelobes is studied. The directivity function formed by the multiple point sources is used to estimate the aperture of an array and the spacing between the transducer elements to make a desired beam. Then the aperture tapering based on a Chebyshev distribution and the effect of an acoustic shield are evaluated to suppress the sidelobe radiation. The total performance of the phased array system has been examined by comparison with tower instruments. The field test shows that the measured beam pattern agrees with the calculated one and the sodar-derived winds have a correlation of 0.96 and a standard deviation of differences of about 0.47-0.52 m s-1 compared with in situ measurements. These statistical values are as good as or even better than those from the previous experiment.

This paper describes a new instrument designed to measure cloud or fog total water content mounted on a tethered balloon facility. The instrument is an evaporator type, which is inexpensive and relatively easy to construct. The device measures total water content with an uncertainty of 0.3 g kg–1, and a resolution of 0.03 g kg−1, at a frequency of 1 Hz. This typically gives a spatial resolution of 5–20 m and allows turbulent statistics to be calculated. Tethered balloons provide an ideal platform for the instrument to study low cloud, allowing very detailed measurement of cloud structure to be made. Some typical data are presented as an illustration.

In February and March 1996, the French NWP model Arpège was run to give operational forecasts five days ahead. These forecasts were a part of preparations for FASTEX (Fronts and Atlantic Storm-Track Experiment), whose field phase took place in 1997. The extensive collection and availability of model output during this period gave an excellent opportunity to study in real time the behaviour of different NWP models during the period of the year when strong winds and rapid changes in weather conditions are frequent. In this paper, we study a case where the ECMWF model predicted an incorrect pressure gradient at Northeast Iceland, while Arpège correctly simulated the gradient. The model errors are connected to anomalies in the mid-tropospheric vorticity field, which is in general quite different in the two models. The differences in the vorticity fields are discussed in relation to orographic effects.

In this paper we review work undertaken on sea clutter illustrated by previously unpublished observations made with an S-band radar during a research project in South Wales carried out during the winter of 1976/77. The implications for detection with C-band Doppler radar are considered.

An objective Visibility Analysis/Forecast System has been developed as part of Nimrod, the United Kingdom Meteorological Office (UKMO) very-short-range forecasting system. The analysis blends satellite imagery and surface observations with a one-hour forecast using a variational algorithm. An extrapolation forecast is produced by adding UKMO Mesoscale Model (MM) trends to the analysis values. This is merged with persistence (the analysis) and MM values, to produce the final forecast. Results from eight case studies are presented, showing the potential of the system, which demonstrates significant improvements on the MM predictions (the currently available objective forecast method).

The utility of a drought severity index, based on accumulated monthly precipitation deficits and a three-monthly initiation and termination rule, for assessing the variability in the timing and severity of the drought hazard between four selected locations in Devon and Cornwall since 1957 is documented. The robustness of the drought events identified is then assessed by changing the index formulation rules. Results indicate that, using a six-monthly, rather than a three-monthly, rule, drought sequences are more difficult to initiate and terminate and are more persistent with a longer mean duration. The drought severity indices are then used to define a hierarchy of significant drought events, isolated in terms of the number of indices simultaneously exceeding an arbitrarily defined severity. Seven Class I droughts, regional in extent, are then discussed; and circulation contrasts between events highlighted using aggregated Lamb weather types. Two drought sub-types emerge, distinguishable according to whether the controlling anticyclone had a greater tendency to be located to the north and east of the UK or to the south and west. A comparative analysis of the ongoing 1995-drought, in terms of its persistence, and rapidity of onset and cessation, is also conducted.

Time series of daily precipitation, at Valentia, Ireland, are examined using conditioned chain-dependent processes. The conditioning is carried out using two different indices of large-scale atmospheric circulation. The first conditioning index relates to mean monthly sea level pressure (SLP), conditioned as either above or below the historic monthly mean. The second conditioning index is based on the geostrophic wind direction (GWD), conditioned as being inside or outside the historic storm quadrant for the month. For each conditioning index, there are two conditioned daily models and they differ in the parameters that relate to both occurrence and intensity and therefore to changes in the distribution of monthly total precipitation. The results of conditioning the daily precipitation on SLP are compared with conditioning on GWD. The GWD conditioned model gave similar results to the SLP conditioned model for the wetter months, but performed poorly for the summer season. Combining the two SLP conditioned daily models into an induced model, which is a mixture of the two chain dependent processes, produces a variance for monthly total precipitation that is closer to the observed value than the unconditioned chain-dependent model.

Meteorology education and training Web site

The NWS Training Center (NWSTC) in Kansas City, the Operational Support Facility (OSF) in Norman, and the COMET Program in Boulder, USA, have established meteorology a new education and training Web site:

http://meted.ucar.edu

Warning of turbulence

During test flights aboard a research aircraft over the Rocky Mountains, a lidar sensor seeking clear-air turbulence detected moderate turbulence eight to ten seconds in advance of the plane as it flew 300 miles per hour several times into disturbed air. The lidar could eventually be part of a detection and warning system used on commercial planes to prevent injuries to passengers and crews. The experiment is a collaboration among the National Aeronautics and Space Administration (NASA), National Science Foundation (NSF), and Coherent Technologies Inc. (CTI) of Lafayette, Colorado, USA.

The basic statistics of aerological variables obtained from radiosonde data at Ezeiza, Buenos Aires, Argentina, for 1986 are examined, and their relationships with occurrence and non-occurrence of daily local precipitation are analysed. The resulting samples are statistically different for nearly all the variables. Nevertheless, thermodynamic variables at the 700 mb level and the K-Whiting instability index yield the best correlations with rainfall, though it is noted that they are diagnostic and not prognostic variables. 500 mb relative vorticity at selected gridpoints is included in the analysis, given the effectiveness of this dynamic field to recognise favourable patterns for daily precipitation occurrence. Some experiments using these potential predictors to specify occurrence or non-occurrence of the event within a 24-hour period are presented.

Two case studies in which elevated thunderstorms played an important role in enhancing precipitation totals are discussed. During the period 20–22 February 1993, elevated thunderstorms over Iowa produced a mesoscale band of snow with amounts in excess of 25 cm across north central Iowa. Diagnosis of the environment revealed an elevated layer of convective instability between 700 and 540 hPa above a well-defined frontal zone. Elevated, upright convection resulted from the release of the convective instability as air parcels, ascending isentropically over the frontal zone, reached saturation. A second case, in which heavy rain (greater than 50 mm in 24 h) fell over parts of Oklahoma, Kansas and Missouri during the period 27–28 April 1994, is also examined. Once again, elevated thunderstorms resulted from the ascent of an elevated layer of convective instability over a strong baroclinic zone. Positive CAPE values are found by lifting air parcels having the maximum equivalent potential temperature in the lower portion of the troposphere, whereas no available energy is diagnosed for surface parcels. While both cases strongly resemble the climatology of elevated thunderstorms, these case studies suggest that convective instability aloft released by isentropic ascent, rather than frontogenetical forcing in the presence of weak symmetric stability, can result in heavy precipitation.

The CERES-Rice version 3.0 crop growth simulation model was calibrated and evaluated for the agroclimatic conditions of the state of Kerala in India. Genetic coefficients were developed for the rice crop variety Jaya and used for the model evaluation studies. In four experiments using different transplanting dates during the virippu season (June to September) under rainfed conditions (i.e. no irrigation), the flowering date was predicted within an error of four days and date of crop maturity within an error of two days. The model was found to predict the phenological events of the crop fairly well. The grain yield predicted by the model was within an error of 3% for all the transplanting dates, but the straw yield prediction was within an error of 27%. The high accuracy of the grain yield prediction showed the ability of the model to simulate the growth of the crop in the agroclimatic conditions of Kerala. It can be concluded from this study that the model can be used for making various strategic and tactical decisions related to agricultural planning in the state.

During December 1993 a microwave radiometer belonging to the Central Aerological Observatory (CAO) Moscow was operated at the Meteorological Research Unit, Cardington, UK. The radiometer has a single channel with a central frequency of 61.0 GHz, bandwidth of 2 GHz and sensitivity 0.04 K for a 1 s integration time. The antenna (beam-width 6°) was scanned from nadir to horizontal in 9° steps. Calibration was done every scan cycle using an electrically switched load. The information potentially available from such a radiometer is discussed. The measurements made at Cardington are compared with in situ observations made by radiosondes and by the Meteorological Office's tethered balloon system. Microwave brightness temperatures calculated from the radiosonde ascents are compared with observations. These were found to agree with a mean difference of 0.1 K and with a standard deviation of < 0.3 K. Similarly retrieved temperature profiles up to 500 m were found to agree with the radiosonde profiles within 1 K rms. During the comparison period there were no strong low-level inversions to test fully the radiometer's retrieval capability.