Assessing the Vertical Latent Heating Structure of the East Pacific ITCZ Using CloudSat CPR and TRMM PR

In the East Pacific intertropical convergence zone (ITCZ), Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) latent heating retrievals suggest a top-heavy structure throughout the year; however, the TRMM PR underestimates light precipitation (< 0.4 mm/h) and the latent heating at low levels associated with this precipitation. In this study, radar observations from both the TRMM and CloudSat satellites are used to assess the vertical latent heating structure in the east Pacific ITCZ (130◦W-90◦W) for 1998-2015. We combined stratiform and deep convective precipitation from the TRMM PR and shallow precipitation from the CloudSat cloud profiling radar (CPR), and applied a simple latent heating look-up table inspired by Schumacher et al. (2004), but modified based on CloudSat echo-top observations. Shallow precipitation contributes around 30% in boreal winter and 15% the rest of the year, and prevails south of the ITCZ throughout the year. A strong seasonal cycle of the latent heating structure is also found, with bottom heaviness during boreal winter and spring and a double latent heating peak with an upper level maximum above 600 hPa in boreal summer and fall. In addition, the ITCZ heating has a meridional slope as a result of the predominance of shallow precipitation in the southern part of the ITCZ and deep convection in the northern part of the ITCZ. The shallow heating in the southern part of the ITCZ is related to low-level moisture convergence within the ITCZ overlain by a stable capping layer. The ITCZ slope is stronger during fall and winter when the Pacific Walker Circulation is stronger. The satellite latent heating retrievals are compared with the total diabatic heating and vertical velocity from three reanalyses (i.e., MERRA2, ERA Interim and NCEP NCAR). Although the meridional slope of ITCZ vertical motion is seen in MERRA2 and NCEP, the strong bottom heaviness suggested by each reanalysis is not consistent with our satellite-based results, especially during the boreal summer and fall.