Relationships between TRMM precipitation characteristics and large-scale vertical motions in four reanalysis datasets over the tropical Pacific

In this study, precipitation characteristics observed with the Tropical Rainfall Measuring Mission (TRMM) satellite are examined in relation to large-scale vertical motions in lower (850 hPa) and upper (300 hPa) troposphere over the tropical Pacific for 13 years from 1998 to 2010. Especially, we utilize four reanalysis datasets to document and understand the differences in their behavior.

First of all, spatial correlation coefficients between rainfall and vertical motions are examined on a monthly time scale. Over the eastern Pacific (EPAC), correlations tend to be high at 850 hPa. Correlations tend to be lower with smaller rainfall amounts, which may be because ascent of dry air can exist even in months with little rainfall. On the other hand, over the western Pacific (WPAC), correlations tend to be lower than the EPAC, and higher at 300 hPa than at 850 hPa. Shallow convergence, well correlated with rainfall over the EPAC, is known to be forced by the strong SST gradient, while the warm pool of the WPAC has no similar forcing tied to any specific latitude belt, and therefore heavy rainfall may occur over a wider area, and monthly mean at any given location may include more no-rain days, which result in low correlations.

In addition, there is a large difference in vertical motions at 300 hPa over the EPAC among four reanalysis datasets. For example, some datasets show weak ascent or subsidence over the ITCZ, while other datasets show strong ascent. We will examine this discrepancy among datasets in relation to precipitation systems, which are classified with echo top heights and areas, as well as daily relationships between rainfall and vertical motions.