The Retrieval of Ice Water Content from Radar Reflectivity Factor and Temperature and Its Use in Evaluating a Mesoscale Model

Abstract

Ice clouds are an important yet largely unvalidated component of weather forecasting and climate models, but radar offers the potential to provide the necessary data to evaluate them. First in this paper, coordinated aircraft in situ measurements and scans by a 3-GHz radar are presented, demonstrating that, for stratiform midlatitude ice clouds, radar reflectivity in the Rayleigh-scattering regime may be reliably calculated from aircraft size spectra if the “Brown and Francis” mass–size relationship is used. The comparisons spanned radar reflectivity values from −15 to +20 dBZ, ice water contents (IWCs) from 0.01 to 0.4 g m−3, and median volumetric diameters between 0.2 and 3 mm. In mixed-phase conditions the agreement is much poorer because of the higher-density ice particles present. A large midlatitude aircraft dataset is then used to derive expressions that relate radar reflectivity and temperature to ice water content and visible extinction coefficient. The analysis is an advance over previous work in several ways: the retrievals vary smoothly with both input parameters, different relationships are derived for the common radar frequencies of 3, 35, and 94 GHz, and the problem of retrieving the long-term mean and the horizontal variance of ice cloud parameters is considered separately. It is shown that the dependence on temperature arises because of the temperature dependence of the number concentration “intercept parameter” rather than mean particle size. A comparison is presented of ice water content derived from scanning 3-GHz radar with the values held in the Met Office mesoscale forecast model, for eight precipitating cases spanning 39 h over southern England. It is found that the model predicted mean IWC to within 10% of the observations at temperatures between −30° and −10°C but tended to underestimate it by around a factor of 2 at colder temperatures.

G band atmospheric radars: new frontiers in cloud physics

“…For the PSD, Field et al (2005) developed a parametrization based on in situ measurements from stratiform ice cloud over the British Isles which captures the quasi-bimodal shape of real size spectra and is more realistic than the simple exponential PSD used by Hogan and Illingworth (1999). For simplicity we will initially assume that the relationship between the mass m and maximum dimension D of the particles follows the empirical relationship of Brown and Francis (1995); the results from Hogan et al (2006) and confirm that this is a realistic approximation for many ice clouds. The problem of how to identify cases where the particles are more or less dense is considered in Sect.…”

“…So far we have assumed that the relationship between a particle's mass and diameter may be described by the well-known relationship of Brown and Francis (1995), which has been validated by Hogan et al (2006) and . However, this may not be suitable for all clouds, and indeed Hogan et al (2006) noted that in mixed-phase regions the agreement between radar and in situ data was poor when Brown and Francis densities were assumed.…”

“…However, this may not be suitable for all clouds, and indeed Hogan et al (2006) noted that in mixed-phase regions the agreement between radar and in situ data was poor when Brown and Francis densities were assumed. Likewise, Matrosov (2011) observed DWV values as high as 0.25 m s −1 using 35 and 94 GHz radars.…”

Evaluation of cloud microphysics schemes in simulations of a winter storm using radar and radiometer measurements

“…Smith, 1984;Ferrier, 1994;Hogan et al, 2006;Milbrandt et al, 2008]. Because this study will investigate both the simulated Z e and the Doppler velocity, we begin with the strict definition of radar reflectivity as the sum of the backscattering cross section (s) in a unit volume, = R sN(D)dD.…”

“…(A.1), we get the equivalent radar reflectivity factor, which is applicable to particles of arbitrary habit [also see Eq. 12 in Hogan et al, 2006]: :4) [55] For a hydrometeor species with assumed spherical shape, Z ex is proportional to the 6 th moment of the PSD,…”