JANUARY 2008 HEYMSFIELD ET AL. 135 Testing IWC Retrieval Methods Using Radar and Ancillary Measurements with In Situ Data ϩ ϩ ANDREW J. HEYMSFIELD,* ALAIN PROTAT, RICHARD T. AUSTIN,# DOMINIQUE BOUNIOL, ROBIN J. HOGAN,@ JULIEN DELANOË,@ HAJIME OKAMOTO,& KAORI SATO,& ϩϩ GERD-JAN VAN ZADELHOFF,** DAVID P. DONOVAN,** AND ZHIEN WANG *National Center for Atmospheric Research,## Boulder, Colorado ϩCentre d’E´ tude des Environnements Terrestre et Planétaires, Vélizy, France #Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado @Department of Meteorology, Reading University, Reading, United Kingdom &Center for Atmospheric and Oceanic Studies, Tohoku University, Sendai, Japan **Koninklijk Nederlands Meteorologisch Instituut, De Bilt, Netherlands ϩϩDepartment of Atmospheric Sciences, University of Wyoming, Laramie, Wyoming (Manuscript received 3 October 2006, in final form 26 April 2007) ABSTRACT Vertical profiles of ice water content (IWC) can now be derived globally from spaceborne cloud satellite radar (CloudSat) data. Integrating these data with Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) data may further increase accuracy. Evaluations of the accuracy of IWC retrieved from radar alone and together with other measurements are now essential. A forward model employing aircraft Lagrangian spiral descents through mid- and low-latitude ice clouds is used to estimate profiles of what a lidar and conventional and Doppler radar would sense. Radar reflectivity Ze and Doppler fall speed at multiple wavelengths and extinction in visible wavelengths were derived from particle size distributions and shape data, constrained by IWC that were measured directly in most instances. These data were provided to eight teams that together cover 10 retrieval methods. Almost 3400 vertically distributed points from 19 clouds were used. Approximate cloud optical depths ranged from below 1 to more than 50. The teams returned retrieval IWC profiles that were evaluated in seven different ways to identify the amount and sources of errors. The mean (median) ratio of the retrieved-to-measured IWC was 1.15 (1.03) Ϯ 0.66 for all teams, 1.08 (1.00) Ϯ 0.60 for those employing a lidar–radar approach, and 1.27 (1.12) Ϯ 0.78 for the ϾϪ standard CloudSat radar–visible optical depth algorithm for Ze 28 dBZe. The ratios for the groups employing the lidar–radar approach and the radar–visible optical depth algorithm may be lower by as much as 25% because of uncertainties in the extinction in small ice particles provided to the groups. Retrievals from future spaceborne radar using reflectivity–Doppler fall speeds show considerable promise. A lidar– radar approach, as applied to measurements from CALIPSO and CloudSat, is useful only in a narrow range of ice water paths (IWP) (40 Ͻ IWP Ͻ 100gmϪ2). Because of the use of the Rayleigh approximation at high reflectivities in some of the algorithms and differences in the way nonspherical particles and Mie effects are considered, IWC retrievals in regions of radar reflectivity at 94 GHz exceeding about 5 dBZe are subject to uncertainties of Ϯ50%. 1. Introduction surface and by controlling the loss of thermal energy to Clouds cover approximately 60% of the earth’s sur- space. Because of their height in the atmosphere, ice face, strongly influencing its energy budget by control- clouds have a dominant effect on longwave forcing and ling the amount of solar radiation reaching the earth’s on the earth’s net radiation budget (Hartmann et al. 1992). Details of the ice microphysics—including the ice water content (IWC), ice water path (IWP), extinc- ## The National Center for Atmospheric Research is sponsored tion coefficient in visible wavelengths (␴), and ice par- by the National Science Foundation. ticle shape—significantly affect ice cloud radiative properties. Corresponding author address: Andrew J. Heymsfield, 3450 Cloud satellite radar (CloudSat), with an onboard Mitchell Lane, Boulder, CO 80301. millimeter-wavelength (94.05 GHz) radar, and the E-mail: [email protected] Cloud-Aerosol Lidar and Infrared Pathfinder Satellite DOI: 10.1175/2007JAMC1606.1 © 2008 American Meteorological Society Unauthenticated | Downloaded 10/06/21 06:24 AM UTC JAM2605 136 JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY VOLUME 47 Observation (CALIPSO) satellite, with a dual wave- however, excellent results were obtained when used length (0.532 and 1.064 ␮m) and dual polarization lidar with ancillary lidar or radiometric measurements. Mace system, present new opportunities to characterize the et al. (2005) used a statistical approach to compare microphysical properties of ice clouds on a global Moderate Resolution Imaging Spectroradiometer scale. CloudSat provides data enabling investigators to (MODIS) overpasses of cirrus with ground-based re- quantitatively evaluate the relationship between verti- mote sensing observations. Using retrievals of cloud cal profiles of cloud ice water content and cloud radia- properties, it was found that there was a positive cor- tive properties and to utilize these results to improve relation in the effective particle size, the optical thick- the representation of ice clouds in climate models ness, and the IWP between the satellite- and ground- (Stephens et al. 2002). Because radar provides a mea- based observations, although there were sometimes sig- surement of the equivalent radar reflectivity Ze,itis nificant biases. Hogan et al. (2006b) utilized realistic necessary to develop methods to convert Ze to IWC. 95-GHz radar and 355-nm lidar backscatter profiles Early methods for this conversion used relationships simulated from aircraft-measured size spectra to evalu- between IWC and Ze from ice particle size spectra mea- ate the correction of lidar signals for extinction by ice surements collected in situ (e.g., Heymsfield 1977) and cloud, a potential source of error in the retrieval of at the surface (Sassen 1987). As pointed out by Atlas et IWC from the lidar–radar approach. al. (1995), there is no universal IWC–Ze relationship Although there are in situ validation activities because of large scatter and systematic shifts in particle planned for CloudSat and CALIPSO, there are a num- size from day to day and cloud to cloud. For that rea- ber of issues beyond the retrievals that can lead to er- son, recently developed techniques have retrieved the rors in the retrieved IWC. The lidar–radar approach, IWC using more than radar reflectivity from single ra- combining coincident measurements synergistically, dar alone. These techniques include the use of radar may provide IWC better than could be derived from combined with collocated lidar data (Intrieri et al. 1993; radar alone. However, there are issues related to con- Wang and Sassen 2002a), Ze and cloud optical depth verting lidar backscatter to extinction, although re- derived from an IR radiometer (Matrosov et al. 1998), cently developed algorithms to derive visible extinction Ze and cloud visible optical depth (Benedetti et al. profiles are relatively insensitive to the details of ice 2003), Ze measured at two frequencies (Hogan and Il- microphysics, lidar backscatter-to-extinction ratio, and lingworth 1999), Ze and cloud radiance derived from lidar calibration (Hogan et al. 2006b). Multiple scatter- atmospheric emitted radiance measurements to derive ing is an additional problem, as are attenuation of layer-average IWC for thin cirrus (Mace et al. 1998), Ze CloudSat’s radar beam when the radar reflectivities ex- ϳ and Doppler fall speed (Matrosov et al. 2002; Mace et ceed 5dBZe, spatial averaging scales from space- al. 2002; Delanoë et al. 2007; Sato and Okamoto 2006), borne radar, the difficulty in collocating an aircraft, ra- and Ze and temperature (Liu and Illingworth 2000; dar, and an appropriate cloud, and the large differences Hogan et al. 2006a; Protat et al. 2007). in radar beam volume and the sample volume of an If the goal of spaceborne radar is to provide vertical IWC measurement probe. profiles of IWC and IWP for use in evaluating and In this study, we perform a detailed evaluation of improving the representation of clouds in climate mod- IWC retrieval methods, using test datasets derived els, it is necessary to assess the accuracy and limitations from in situ microphysical measurements. The data of the retrievals. Ground-based remote sensing mea- provided to eight teams included all of the information surements have been used in conjunction with in situ needed for their retrieval methods: vertical profiles of ␴ observations to evaluate retrieved IWC (Matrosov et Ze, , temperature, and Doppler fall speed. However, al. 1995; Wang and Sassen 2002a). The evaluations re- the IWCs were not provided to the teams. Their re- lied on in situ measurements of particle size distribu- trieved IWCs were then compared with the measured tions (PSD) and estimates of ice particle mass and were values. This approach is not subject to the lidar and based on samples from one or two midlatitude cirrus radar issues raised above, which obviously would add to clouds. The IWCs derived in this way are accurate only the error. In section 2, we describe the test dataset and to a factor of 2, so that the evaluations are not conclu- the methodology. In section 3, we evaluate the results, sive. Sassen et al. (2002) used a cloud model with ex- and in section 4, the principal findings are summarized. plicit microphysics to test algorithms for retrieving cir- rus cloud IWC from millimeter-wavelength radar re- 2. Data overview and products provided to contributors flectivity measurements. They found that radar Ze-only approaches suffer from significant problems related to This section provides an overview of the tempera- basic temperature-dependent cirrus cloud processes; tures and microphysical properties encountered during Unauthenticated | Downloaded 10/06/21 06:24 AM UTC JANUARY 2008 HEYMSFIELD ET AL. 137 the cloud penetrations on 19 days used for this study (2007a, hereinafter H07a), their Fig. 3 (case A4, here)]. and describes the methodology used to derive IWC, One case to the right of the star-shaped symbol is from radar reflectivity, and extinction estimates provided to the ARM 2000 IOP (A4).