Assessing the Vertical Velocity of the East Pacific ITCZ

Reanalysis and satellite data sets disagree on the vertical structure of the meridional overturning circulation over the East Pacific (EP) intertropical convergence zone (ITCZ). Recent observations from the Organization of Tropical East Pacific Convection (OTREC) 2019 field campaign show a meridional slope of vertical motion with a shallow mode from 3 degrees to 7 degrees N, over a strong meridional sea surface temperature (SST) gradient, and a deeper mode from 7 degrees to 10.5 degrees N, where the SST is predominantly warm. However, reanalysis fields from the OTREC period show a predominant shallow mode from 3 degrees to 9 degrees N unless OTREC dropsondes are assimilated; then a weak deep mode is found at 8 degrees N. Vertical motion over the OTREC region derived from Global Precipitation Measurement (GPM) precipitation radar latent heating retrievals has the opposite problem in that the deep mode is predominant from 3 degrees to 10 degrees N. A shallow mode is present with the addition of cloud radar data. Plain Language Summary There is no consensus on where air rises most strongly and to what heights over the tropical East Pacific Ocean because there are very few available in situ observations. Thus, we rely on numerical models or satellite observations to make estimates of where air rises and where storms are deepest and strongest. In this study, we show recent field campaign observations that indicate the existence of two modes of upward motion: a shallow mode (i.e., ascending motion up to 4 km AMSL) in the southern part of the main precipitation band just north of the equator, and a deep mode (i.e., ascending motion up to 10 km AMSL) a few hundred km farther north. While numerical models only produce the shallow mode, satellite algorithms suggest only a deep mode. Improvements were made when the numerical models used in situ data for guidance and the satellite algorithms were supplemented with observations from other sensors. In situ observations were crucial to conclude that currently available numerical model and satellite data sets, independently, are problematic for describing the large-scale air motions in the East Pacific.