Special Sensor Microwave/Imager (SSM/I)

Cloud Liquid Water

Overview

Integrated cloud liquid water (CLW) can be retrieved only over ocean due to the low emissivity background of the ocean surface. CLW does impact measurements over land, however, these are difficult to assess due to the varying land emissivity. Although there are many algorithms available for use for the retrieval of CLW from passive microwave measurements, NOAA/NESDIS/ORA has recently developed an algorithm for oceanic CLW using measurements at 19, 37, and 85 GHz.

Use of the 85 GHz measurements allows for the retrieval of extremely low amounts of CLW. It should be noted that there is considerable uncertainty in the retrieved amounts of CLW due to the lack of ground truth. However, the CLW product clearly indicates cloudiness patterns and relative magnitudes.

Two cloud products (ocean only) have been produced, which are summarized in the table below. The first is the mean liquid water path (LWP), which has been computed under cloudy conditions. This gives an indication of the LWP content when clouds are present. The second is the mean cloudiness fraction (CFR), which gives an indication of the persistance and areal coverage of cloudiness. To get the actual mean LWP under all conditions (e.g., clear and cloudy), simply multiply the LWP and CFR values.

The LWP monthly value represents the mean LWP under cloudy conditions. To get the mean monthly LWP under all conditions, simply multiply the LWP by the CFR value at each grid box.

References

Weng F. and N.C. Grody, 1994: Retrieval of liquid and ice water content in atmosphere using Special Sensor Microwave Imager (SSM/I) data, Specialist Meeting on Microwave Radiometry, Rome, IEEE.

Weng, F. and N.C. Grody, 1994: Retrieval of cloud liquid water using the special sensor microwave imager (SSM/I). J. Geophys. Res., 99, 25535-25551.

Weng, F., N.C. Grody, R.R. Ferraro, A.Basist and D. Forsyth, 1997: Cloud liquid water climatology from the special sensor microwave imager. J. Climate, 10, 1086-1096.

Weng, F., N.C. Grody, R.R. Ferraro, Q. Zhao and C.T. Chen, 1997: Global cloud water distribution from Special Sensor Microwave Imager/Sounder and its comparison with GCM simulations. Adv. Space Res., 19, 407-411.