Weather observations from the Chilbolton Advanced Meteorological Radar

Transcription

1 ~ (1999) Temperature The science of climate change. Cambridge University Press, pp Shuttleworth, W. J. (1979) Evaporation. Institute of Hydrology Report No. 56 Spilhaus, A. F. (1935) The transient condition of the human hair hygrometer element. Cambridge, Mass. MIT, Meteorological Course, Professional Notes No. 8, Part 1 Strangeways, I. C. (1998) Back to basics: The met. enclosure : Part 3 - Radiation. Weather, 53, pp Back to basics: The met. enclosure : Part 4 ~ Weather, 54, pp Wylie, R. G. (1968) Resume ofknowledge ofthe properties of the psychrometer. CSIRO, Melbourne, Report NO. PIR-64 Correspondence to: Dr I. C. Strangeways, Terra- Data, PO Box 48, Wallingford, Oxon. 0x10 On. Weather observations from the Chilbolton Advanced Meteorological Radar C. A. D. Kilburn, D. Chapman2, A. J. Illingworth2 and R. J. Hogan2 1 Rutherford Appleton Laboratory, Chilton 2Reading University The Chilbolton Advanced Meteorological Radar (Fig. 1, see front cover) is the largest steerable meteorological radar in the world and has been used for the past 30 years to develop new techniques for probing the atmosphere. The system transmits a high power pulse of microwave energy and detects the return backscatter from meteorological targets such as raindrops, snowflakes, hail and sleet. In this article we describe new results obtained with this radar which give insights into the way that turbulence and billows cause mixing across frontal surfaces, the identification of hail, turbulence and mixing in the convective boundary layer, and, finally, the vertical profile of cloud cover and its comparison with the clouds in operational forecasting models. The Chilbolton system operates at the S- Band frequency of 3 GHz, and is located near the pleasant village of Chilbolton, near Andover (Hampshire). The dominant visual feature of the system is the 25m antenna which produces an exceptionally narrow radar beam only 0.25 degrees wide. Even at a range of 50km, the beam is only 200 m wide. The dish surface is made of aluminium petals which are of a honeycomb-sandwich construction, and can be adjusted to within 3 mm to correctly focus the beam. The antenna is fully steerable in the above-ground hemisphere. The range resolution of the system can be set as low as 75m. The total weight of the steelwork and aluminium petals is around 400 tonnes; this is supported by the reinforced concrete tower which weighs 2000 tonnes. Each transmitted pulse lasts half a microsecond and the power transmitted is 700 kw. Pulses are emitted from the radar 610 times a second and the returned signals are averaged over a period of around a quarter of a second in order to give a picture of the properties of the meteorological targets over a range of distances. Rotating the dish using the electric drive motors on the steelwork as data are being taken produces an atmospheric snapshot in any chosen plane. The term advanced in the radar s name refers to its capabilities in resolution, velocity measurement and target shape measurement. The velocity measurements use the Doppler effect whereby a moving object causes the returned signal to shift in frequency. This is the same effect as the change in the tone of a fire engine as it passes an observer. A striking example of high-resolution airflow measurements can be seen in the October 1998 issue of 352

2 Weather (Chapman et al. 1998) which describes observations of a tornadic storm in the UK. The range resolution of the image on the front cover of that issue was 300m and the angular resolution was 200m. The resolution of the UK rain radar network is normally only 2 km, so this radar can reveal important fine-scale processes which are not resolved using the operational network. The radar has the capability of transmitting with the electric field in either a horizontal or a vertical plane and these transmissions are referred to as having different polarisations. Rain with a lot of small raindrops could give the same radar return as rain with a few large drops but would produce a much larger rainfall rate at the ground. Large raindrops are surprisingly flat as they fall, and the larger the raindrop, the flatter they become. Thus for large raindrops, a horizontally polarised wave gives a larger backscatter than a vertically polarised wave. The ratio of the horizontal and vertical returns is called the differential reflectivity, and is a measure of the mean drop shape and hence their size. Small drops are basically spherical and so the ratio of the returns is unity. This measure of the drop sizes leads to a better rainfall estimate. The radar is used for validation of new ground-based or space-borne rainfall measurement systems, piggybacking of higherfrequency cloud radars and observations of various meteorological phenomena such as fronts, squall lines and convection. In the remainder of this article we will present examples of meteorological phenomena observed with the Chilbolton radar. Kelvin-Helmholtz billows in a warm frontal zone When Chilbolton radar data are analysed, measurements are made of the speed and direction of the raindrops or other scatterers. Since falling rain and cloud droplets will follow the horizontal wind field, this enables the radial component of the wind field to be measured to an accuracy of better than 0.1 m s- in regions of precipitation. In spite of the limitations of having only the line-of-sight velocity component (known as the Doppler velocity), making realistic assumptions such as the incompressibility of air and knowing the broad structure of the weather event being observed can sometimes allow the other components to be derived. An example of the use of velocity information is the following case-study of billows observed in a warm frontal zone. Kelvin-Helmholtz billows may occur in the atmosphere in regions of stratified shear, i.e. where the wind direction or speed changes rapidly with height in a region where warm (less dense) air sits on top of cooler (more dense) air. The effect of the shear is to displace the surface between the cool and warm air into rolls of billows oriented so that their axes lie perpendicular to the shear, similar in some ways to ocean waves which may be generated by the wind. Figure 369) shows a vertical section located just ahead of, and oriented parallel to, a surface warm front that passed over the UK in September The coloured shading shows the Doppler velocity measured by the radar, with positive values indicating air moving away from the radar (towards the west in this case). The blue shading represents cold air below the warm front moving westwards at around 10 m s-. The red shading represents warmer air above the warm front moving eastwards, again with a speed of around 10 m s-l. The wave evident in this figure is due to a train of Kelvin-Helmholtz billows (Chapman and Browning 1997) with a wavelength of 4km, and similar billows are thought to have existed for many hours over much of the length of the front. Further analysis of the velocity data has shed light on the effect and importance of billows such as this. Figure 2(b) (p. 369) shows streamlines that have been derived from the Doppler velocity. These streamlines indicate the motion of air parcels within the plane of the cross-section, and clearly show that the billows mixed air over a vertical depth of around 1 km. Further calculations (Chapman and Browning 1999) have shown that this mixing had a significant impact on the structure of the warm front itself, smoothing the transition between the cold and warm air. The existence and properties of billows such as these may also be important in affecting the transport of pollutants and 353

3 chemical species such as ozone between airmasses in the atmosphere. This has been the subject of a recent experimental campaign using the Met. Office's C-130 aircraft in which further radar observations of Kelvin-Helmholtz billows have been obtained. Billows may also pose a threat to aviation safety, by concentrating the wind shear as shown in Fig. 2(b). An aircraft flying through these billows would experience rapid and substantial changes in head wind, as well as moderate or severe turbulence. Hail observations On 10 July 1995 an intense hailstorm occurred at Tilehurst near Reading (Berkshire) during which 80 observers reported large oblate hailstones falling for a period of up to 5 minutes. Some observers also collected the hailstones, which allowed comparisons to be made with radar data taken during the storm (Smyth et al. 1999). The largest hailstone measured 3.2 cm in the major axis and had an axial ratio of 0.6. Figure 3 (p. 369) shows the radar reflectivity results during the storm. Rain-radar reflectivity has logarithmic units called dbz (decibels reflectivity) which come from the power received by the radar adjusted with a radar constant and a range correction. The resulting value can be directly related to raindrop numbers and sizes. The intense radar echoes with a radar reflectivity of 60dBZ in Fig. 3 could be interpreted as rainfall rates of over 200 mm h-', or alternatively could result from the presence of a few large hailstones with a much lower equivalent rainfall rate. Radar is used operationally to provide warning of flooding so it is important to distinguish between intense echoes due to very heavy rain and those from hailstones. A simple approach is to assume that reflectivities above a certain level must be due to hail, but if this is done some heavy rainfall events could be missed. Hail-detection algorithms that make use of polarisation properties of the radar signal have been suggested which rely on the hailstones randomly tumbling. If this were true one would expect the ratio of the returns for horizontal and vertical polarisation (the differential reflectivity) to be unity, but in this case 354 the horizontal return was larger than the vertical so the algorithm failed. This is consistent with the oblate shape of the hailstones which were collected and suggests that they fell with some degree of alignment. A new technique which combines Doppler and polarisation techniques can be used to identlfy hail. A variable called the differential phase measures the difference in the velocity of the horizontally and vertically polarised radar waves as they pass through precipitation. Heavy rain consists of large oblate raindrops so the horizontally polarised radar wave is slowed down more than the vertical one, and the phase difference between the two waves increases with range. In heavy rainfall there is a unique relationship between the radar reflectivity, the differential phase shift, and the differential reflectivity. This arises because the raindrops have a well defined shape which is a unique function of their size. Hail can be recognised from a comparison of these three variables; the observed phase shift when hail is present is much lower than would be predicted for rain (Smyth et al. 1999). Boundary-layer observations The Chilbolton radar can also sometimes make observations when there is no rain or cloud in the sky. There is a seasonal variation in the strength of these clear-air echoes, peaking in summer. The echoes are much weaker than those from rain, with a radar reflectivity as low as -20dBZ. The echoes are generally caused by turbulence in a convective boundary layer, but insects or airborne particulates from fires are also common targets. Whilst observing a convective boundary layer, even if all of the boundary layer does not cause echo returns, a return from the temperature inversion at the top of the layer is very common. The reason is that the temperature changes cause a change in the refractive index of the air, which when disturbed by turbulence gives an echo. The same turbulence above or below this level gives an echo too weak to be detected. Figure 4 (p. 370) shows an example of such a radar return at 1001 GMT on 13 August 1998 and it can be seen that there are deformations in the echo. The scan is a vertical

4 cross-section of the atmosphere in a direction due south of the radar. The scale of undulation of several kilometres is an appropriate scale for convective plumes caused by heating at the surface. The pulling down, or entrainment, of high refractive index gradient air from the boundary-layer inversion causes the echoes at the plume edges (Battan 1973). The strongest gradients of refractive index are at the top and sides of each convective cell, with gradients decreasing around the lower edges of the cell. The lack of echoes in the centre is due to the lack of refractive-index structure in the same way as in the rest of the boundary layer. The polarisation capability of the radar was used to check on the shape of the scatterers in Fig. 4 and they were found not to be aligned in any particular direction. Echoes in clear air which do have a high differential reflectivity are generally associated with insects. This is because insects have one axis larger than the other which, coupled with assumptions of horizontal flight and common direction between neighbours, explains a radar reflectivity which is dependent on the orientation of the electric field. The absence of this differential reflectivity signal in Fig. 4 confirms that turbulent variations in the refractive index caused the echoes. There is also the possibility that cloud was forming at the top of the plumes. Cloud cover comparisons Two smaller cloud radars, operating at frequencies of 35 and 94GHz, are mounted on the side of the main 25m antenna. These higher frequencies are far less susceptible to contamination by echoes from the ground at short range, making them more suitable for observing clouds. In addition to improving our understanding of cloud processes, continuous vertically pointing observations by these instruments taken over long periods can be used to assess the performance of numerical forecast models in the way that they simulate clouds. Cloud cover is a parameter used in numerical models of the atmosphere which is necessary for radiative-transfer calculations to be accurate, but it has a rather uncertain physical basis. Satellites can provide validation for the total cloud cover throughout the whole depth of the troposphere, but only radar can perform validation at each model level. Three months of near-continuous data were taken between October 1998 and January 1999, and by simply counting up the number of cloudy pixels at each ECMWF (European Centre for Medium-Range Weather Forecasts) model level over an hour-long period, cloud cover was derived which could be directly compared with the model values. The lidar ceilometer at Chilbolton was used to distinguish between rain and cloud. Figure 5 (see back cover) compares a month-long period of observed cloud cover with that from the ECMWF model at the closest grid point to Chilbolton. In general, the largescale features match up reasonably well, although differences in the detail suggest that there is still room for improvement in the way that cloud cover is formulated in the model. A more rigorous statistical analysis of the comparison is currently in progress. We also intend to derive ice-water content from the radar data to compare with both the ECMWF and The Met. Office models. Acknowledgements Hail and billow observations were supported by a grant and studentship from the Natural Environment Research Council (NERC). The cloud observations were supported by NERC and from the European Space Research and Technology Centre and the model data were supplied by ECMWF. The boundary-layer observations were sponsored by The Met. Office. Thanks to the staff at the Chilbolton Observatory, particularly Darcy Ladd for performing the radar observations of the boundary layer. Thanks also to Tim Smyth for the original production of Fig. 3. References Battan, L. J. (1973) Radar obserwhn of the utmosphere. University of Chicago Press Chapman, D. and Browning, K. A. (1997) Radar observations of wind-shear splitting within evolving atmospheric Kelvin-Helmholtz billows. Q.J. R. Meteorol. SOC., 123, pp (1999) Release of potential shearing instability in warm frontal zones. Q. J. R. Meteorol. SOC., 125, pp

5 ~ Chapman, D., Roberts, N. M., Illingworth, A. J. and Browning, K. A. (1998) First radar detection of a misocyclone in a UK tornadic storm. Weather, 53, pp Smyth, T. J., Blackman, T. M. and Illingworth, A. J. (1999) Observations of oblate hail using dual polarization radar and implications for hail- detection schemes. (2. J. R. Meteorol. SOC., 125, pp Correspondence to: Dr C. A. D. Kilburn, Radio Communications Research Unit, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 OQX. A cluster of intense rainfall events in West Berkshire, summer 1999 Stephen Burt Stratfield Mortirner, West Berkshire In the space of less than 12 weeks during the summer of 1999, seven short-period intense rainfall events were recorded at my rainfall station at Stratfield Mortimer in West Berkshire. Five of the seven events reached or exceeded the annual average 5-minute maximum rainfall intensity, six events exceeded the annual average maximum hourly fall, while two events occurred within 24 hours to give the highest daily fall recorded locally for 17 years (57.1 mm on 9 August). In two of the falls the rate of rainfall exceeded 100 mm h-' for several minutes, briefly exceeding 250mmh-' in the short storm of 29 May By mid-august 1999, five of the six highest 60-minute falls since my records began in the village 12 years previously (1 km site move June 1998) had been recorded in the previous 12 weeks - and the resulting summer had been the wettest in Stratfield Mortimer since Depth-duration statistics for all seven events are tabulated in Table 1 (shown as both cumulative rainfall for specified durations, and equivalent rainfall intensities for the same durations) and graphed in Fig. 1. For comparison, the annual average maximum 5-minute rainfall intensity at this site (average 5 years ) is 42mmh-' (3.5mm period fall); the annual average wettest hour* (average 12 years ) is 11.2 mm. The observations were made at my rainfall site at Stratfield Mortimer, some lokm south-west of Reading 356 (Met. Office rainfall station No , National Grid reference SU (41) , altitude 60 m above mean sea-level). Observations are made with a standard Met. Office MkII 5 in raingauge, with autographic records from a Didcot Instruments 0.2 mm capacity tippingbucket raingauge exposed adjacent to the check gauge. The Didcot Instruments raingauge records to an integral logger at l-minute resolution and is also logged at 5-minute intervals by my Davis Instruments automatic weather station (AWS) from which records for other elements are available. All Didcot records shown here have been corrected to the daily totals obtained from the check gauge (and, for the records to 7 June, for a small timing error). A brief description of the events follows in chronological order. Wednesday 19 May 1999 Heavy thunderstorms broke out widely across southern England on 19 May, on the edge of a depression located over northern France. Maximum temperatures had reached "C locally (22.3"C in Stratfield Mortimer). In Berkshire, the storms were notable for both * The wettest hour is any 60-minute duration commencing at any exact 5-minute period (e.g. 15, 20, 25 minutes past the hour, etc.) - not constrained to exact 'clock hours'.