Comparisons of global cloud ice from MLS, CloudSat,and correlative data sets

摘要

Aura Microwave Limb Sounder (MLS) version 2.2 (V2.2) and CloudSat R04(release 4) ice water content (IWC) and partial-column ice water path (pIWP)measurements are analyzed and compared to other correlative data sets. The MLS IWC,representing an average over —300 x 7 x 4 km3 volume, is retrieved at 215-268 hPawith precision varying between 0.06 and 1 mg/m3. The MLS pIWP products,representing the partial columns over 100 x 7 km2 area with the bottom at —8, —6, and11 km for 115, 240, and 640 GHz, have estimated precisions of 5, 1.5, and0.8 g/m2, respectively. CloudSat, on the other hand, shows a minimum detectablesensitivity of —31 dBZ in the reflectivity measurement at 94 GHz. CloudSat IWC is anaverage over —1.8 x 1.4 x 0.5 km3 volume, and its precision varies from 0.4 mg/m3at 8 km to 1.6 mg/m3 at 12 km. The estimated single-profile precision for CloudSatIWP is —9 g/m2. However, these measurements are associated with relatively largesystematic error, mostly due to uncertainties in the retrieval assumptions aboutmicrophysics, which lead to relatively poor accuracy compared to measurement precision.To characterize systematic differences among various observations and those derived frommodels, we employ the normalized probability density function (pdf) in thecomparisons. CloudSat IWC shows generally consistent slopes of pdf distribution with insitu observations, particularly at —12 km where the in situ data come mostly from long-legflights. Despite similar IWC morphology found between MLS and CloudSatobservations, CloudSat R04 IWC retrieval is higher compared to MLS, especially at14-17 km where the MLS technique is not limited by sensitivity saturation. The MLS andCloudSat IWC pdf's agree well in the overlapped sensitivity range with relative difference<50%, but the difference appears to increase with IWC. MLS and CloudSat cloud icemeasurements are compared with other data sets in terms of monthly map and pdf.Comparisons with European Center for Medium range Weather Forecasting (ECMWF)analyses show that grid box averages of monthly ECMWF IWC are much smaller (by5 x and —20 x) than the same MLS and CloudSat averages. Comparisons of pIWP datafrom CloudSat and passive sensors reveal large uncertainties associated with passivetechniques, such as penetration depth and sensitivity limitation. In particular, retrievalsfrom Moderate Resolution Imaging Spectroradiometer (MODIS) and AdvancedMicrowave Sounding Unit-B (AMSU-B) differ largely in IWP pdf from the CloudSat R04retrieval, showing CloudSat values generally lower (by —5x and —8x, respectively) atIWP = 10-500 g/m2 but higher at IWP > 500 g/m2.