MLS and CloudSat can both measure upper-tropospheric cloud ice water content (IWC). Despite differences in observing techniques, MLS V2.2 and CloudSat R04 IWC retrievals show consistent morphologies but the R04 mean is generally higher by a factor of ~5. Much of the high bias is due to MLS sensitivity degradation at large IWC values. The IWC PDFs agree reasonably well, showing biases less than 50% in the overlapped sensitivity range. At 15-17 km the R04 biases are high against MLS and increase rapidly with IWC. At these altitude, MLS IWC has its best precision and usually is not limited by sensitivity degradation. Comparisons between MLS and R03 (Wu et al., 2008), on the other hand, show rather small biases. These MLS-CloudSat differences warrant a further investigation.
ECMWF monthly mean IWCs are lower compared to MLS V2.2 and CloudSat R04 retrievals by a factor of ~5 and ~20, respectively. The biases between ECMWF and the observations are IWC-dependent, showing that ECMWF is high at small IWCs but low at large IWCs. The PDF of ECMWF IWC drops off too rapidly at the large values. The differences between modeled and observed IWC must be interpreted with caution because cloud and precipitation ice are treated differently in the model whereas observing techniques usually cannot distinguish between the two. Similar model-observation differences are found in the PDF comparisons of GEOS-5.1, MLS and CloudSat IWCs.
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| Figure 1. MLS, ECMWF, and CloudSat IWC maps for 7 July-16 August 2006 at pressure altitudes of 10.7,12, 13.3, 14.7, and 16 km. The maps have the same color scale on a 4°×8° Lat-Lon grid, and a 3-point smoothing is applied to the gridbox averages. The striping distribution in MLS IWC maps at mid-and-high southern latitudes are artifacts of false detection. The ECMWF and CloudSat IWC data are averaged vertically to match the MLS vertical resolution (~4 km) at these altitudes. On the right are scatter plots of the IWC values from the maps, where colors denote latitudes from the equator and lines of the 1:1, 1:5, and 5:1 ratios are shown. |
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| Figure 2. MLS and CloudSat seasonal IWC maps for the period of Dec 2006-Nov 2007 at 147 hPa pressure. |