The relationship between surface rainrate and water paths and its implications to satellite rainrate retrieval

Since brightness temperature measured by a passive microwave radiometer reflects the integrated effects of water paths, not surface rainrate, the accuracy of surface rainrate retrieval based on brightness temperatures relies heavily on the natural correlation between surface rainrate and water paths. In this study, we investigate how the water paths are related to surface rainrates using satellite data. It is found that corresponding to a similar surface rainrate ice water path is larger over land than over ocean. In addition, on average, the correlation (R-2) between surface rainrate and ice water path over land (0.36) is also larger than over ocean (0.25). Over ocean, the correlation between surface rainrate and ice water path is greatly reduced where shallow convections exist. The results also show that given a similar surface rainrate, greater values of total water path occur over arid regions than over other land areas while the correlation between surface rainrate and total water path over such regions is smaller. Furthermore, given a similar surface rainrate, over ocean, larger values of liquid water path are observed over the intertropical convergence zone and maritime regions while smaller values appear over regions dominated by shallow convections. The explanation of the above differences in correlations is attempted using the seasonal variability of water paths and the rainfall systems' vertical structures. It is expected that the low correlation between surface rainrate and water paths will be a major obstacle for surface rainrate retrieval using microwave observations. In addition, surface rainrate always possesses a better relationship to liquid water path over ocean (or total water path over land) than to ice water path. Therefore, for surface rainrate retrievals, channels or channel combinations that can capture liquid water signal over ocean (or total water signal over land) are preferable to those solely containing ice water information.