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A. J. Illingworth

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A. J. Illingworth and J. Latham

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A. J. Illingworth and J. Latham

Abstract

A one-dimensional precipitative model of cloud electrification is outlined in which field growth results from the operation of either in inductive or non-inductive mechanism. The cloud is cylindrical, of finite dimensions, and charging is confined to a supercooled zone within which precipitation growth occurs. Account is taken of loss of negative charge arriving at the ground on precipitation and the storage of positive charge carried by the updraft to a level above the charging zone.

The most important conclusion is that previous models of cloud electrification, which have often been extremely elaborate, provide gross overestimates of the rate of field growth because they have assumed a cloud of infinite width. They also predict a “top-hat” distribution of field, which is shown to be quite unrealistic. The present calculations cast serious doubt on the capability of an inductive mechanism, by itself, to produce breakdown fields in the available time.

These calculations also indicate that 1) the retardation of field growth due to the effect of electrical forces on particle velocities is negligible; and that 2) the “lower positive charge” can be produced in the bases of clouds, in some circumstances, without having to invoke an additional charging mechanism.

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A. J. Illingworth and C. J. Stevens

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An instrument for measuring the size and concentration of raindrops is described which has the ability to function equally well in calm conditions and strong winds. Raindrops are detected optically in a shadowgraph-type imaging system. A unique feature of the device is the sensing of drop size as drops enter and leave a cylindrical sample volume. Each drop produces two equally sized pulses, the heights of which are proportional to the drop's diameter and their separation yields the drop's transit time. The drop concentration can be derived from this data without assuming or measuring drop velocities. The instrument has a large sample volume, but a negligible loss of data resulting from two drops being sampled simultaneously. Drops above 300 μm can be detected, but beam divergence limits accurate sizing to drops larger than 400 μm. Laboratory calibration shows that drops greater than 1 mm in diameter may be sized to better than 5%. Comparison tests with a tipping bucket raingage reveal that time integrations of the raindrop size spectra yield rainfall totals generally within 10% of the bulk-measured values. These tests also show that disdrometers which sense raindrop flux win seriously overestimate drop concentrations in windy conditions but that this new device performs satisfactorily for winds of up to 20 m s−1.

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A. J. Illingworth and I. J. Caylor

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The differential reflectivity (Z DR) measures the mean shape of hydrometeors and provides an estimate of the mean size of raindrops Observations of Z DR for rain may be combined with the conventional radar reflectivity factor (Z) and fitted to any two-parameter raindrop size distribution and this information used to derive more accurate rainfall rates. In such work the precise shape of raindrops is a critical parameter. Recently available data suggest that large raindrops are more oblate than previously believed. These new shapes support the idea that Z DR values above 3.5 dB can be attributed to rain. Average values of Z DR as a function of Z obtained in heavy rain by the Chilbolton radar agree very closely with those predicted using the new shapes. Statistics are also presented of the natural variability of raindrop spectra in heavy rain. Analytic expressions are proposed for computing rainfall rate from Z and Z DR.

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J. C. Nicol and A. J. Illingworth

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Radar refractivity retrievals have the potential to accurately capture near-surface humidity fields from the phase change of ground clutter returns. In practice, phase changes are very noisy and the required smoothing will diminish large radial phase change gradients, leading to severe underestimates of large refractivity changes (ΔN). To mitigate this, the mean refractivity change over the field (〈ΔNfield) must be subtracted prior to smoothing. However, both observations and simulations indicate that highly correlated returns (e.g., when single targets straddle neighboring gates) result in underestimates of 〈ΔNfield when pulse-pair processing is used. This may contribute to reported differences of up to 30 N units between surface observations and retrievals. This effect can be avoided if 〈ΔNfield is estimated using a linear least squares fit to azimuthally averaged phase changes. Nevertheless, subsequent smoothing of the phase changes will still tend to diminish the all-important spatial perturbations in retrieved refractivity relative to 〈ΔNfield; an iterative estimation approach may be required. The uncertainty in the target location within the range gate leads to additional phase noise proportional to ΔN, pulse length, and radar frequency. The use of short pulse lengths is recommended, not only to reduce this noise but to increase both the maximum detectable refractivity change and the number of suitable targets. Retrievals of refractivity fields must allow for large ΔN relative to an earlier reference field. This should be achievable for short pulses at S band, but phase noise due to target motion may prevent this at C band, while at X band even the retrieval of ΔN over shorter periods may at times be impossible.

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W. J. Keat, C. D. Westbrook, and A. J. Illingworth

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The copolar correlation coefficient ρ hv has many applications, including hydrometeor classification, ground clutter and melting-layer identification, interpretation of ice microphysics, and the retrieval of raindrop size distributions (DSDs). However, the quantitative error estimates that are necessary if these applications are to be fully exploited are currently lacking. Previous error estimates of ρ hv rely on knowledge of the unknown “true” ρ hv and implicitly assume a Gaussian probability distribution function of ρ hv samples. Frequency distributions of ρ hv estimates are in fact shown to be highly negatively skewed. A new variable, = log10(1 − ρ hv), is defined that does have Gaussian error statistics and a standard deviation depending only on the number of independent radar pulses. This is verified using observations of spherical drizzle drops, allowing, for the first time, the construction of rigorous confidence intervals in estimates of ρ hv. In addition, the manner in which the imperfect collocation of the horizontal and vertical polarization sample volumes may be accounted for is demonstrated. The possibility of using L to estimate the dispersion parameter μ in the gamma drop size distribution is investigated. Including drop oscillations is found to be essential for this application; otherwise, there could be biases in retrieved μ of up to approximately 8. Preliminary results in rainfall are presented. In a convective rain case study, the estimates presented herein show μ to be substantially larger than 0 (an exponential DSD). In this particular rain event, rain rate would be overestimated by up to 50% if a simple exponential DSD is assumed.

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J. C. Nicol, A. J. Illingworth, T. Darlington, and M. Kitchen

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Radar refractivity retrievals can capture near-surface humidity changes, but noisy phase changes of the ground clutter returns limit the accuracy for both klystron- and magnetron-based systems. Observations with a C-band (5.6 cm) magnetron weather radar indicate that the correction for phase changes introduced by local oscillator frequency changes leads to refractivity errors no larger than 0.25 N units: equivalent to a relative humidity change of only 0.25% at 20°C. Requested stable local oscillator (STALO) frequency changes were accurate to 0.002 ppm based on laboratory measurements. More serious are the random phase change errors introduced when targets are not at the range-gate center and there are changes in the transmitter frequency (Δf Tx) or the refractivity (ΔN). Observations at C band with a 2-μs pulse show an additional 66° of phase change noise for a Δf Tx of 190 kHz (34 ppm); this allows the effect due to ΔN to be predicted. Even at S band with klystron transmitters, significant phase change noise should occur when a large ΔN develops relative to the reference period [e.g., ~55° when ΔN = 60 for the Next Generation Weather Radar (NEXRAD) radars]. At shorter wavelengths (e.g., C and X band) and with magnetron transmitters in particular, refractivity retrievals relative to an earlier reference period are even more difficult, and operational retrievals may be restricted to changes over shorter (e.g., hourly) periods of time. Target location errors can be reduced by using a shorter pulse or identified by a new technique making alternate measurements at two closely spaced frequencies, which could even be achieved with a dual–pulse repetition frequency (PRF) operation of a magnetron transmitter.

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B. Vonnegut, A. J. Illingworth, and P. R. Krehbiel

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P. Meischner, C. Collier, A. Illingworth, J. Joss, and W. Randeu

The European Union COST (Cooperation in Science and Technology) action on advanced weather radar systems is described. The associated five-year research project, which began in early 1993, has the objective to develop guideline specifications for a future generation of European operational radar systems. The authors describe the status of the project, the results reached so far in assessing and reviewing the potential improvements to conventional radars, the products and application of Doppler radar data, the contribution of polarimetric radars to the improvement of quantitative precipitation measurements and for nowcasting, and the possible development of electronically scanned systems. Problems to be tackled in the remaining years of the project are assessments of future technological feasibility, market forecasts, and cost/benefit investigations for the varied requirement profiles across Europe. It is intended to generate a high-level specification for the next generation of weather radars in Europe.

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