PNNL

Abstract

The ability of an observationally-constrained cloud-system permitting model (Weather Research and Forecasting; WRF, 4-km grid spacing) and a global climate model (Energy Exascale Earth System Model; E3SM, 1-degree grid spacing) to represent the observed precipitation diurnal cycle over the Amazon basin during the 2014 wet season is assessed. The month-long period is divided roughly equally into days with and without the presence of observed propagating convective systems over the central Amazon. The propagating convective systems are strongly associated with total rain amounts over the basin and also control the observed spatial variability of the diurnal rain rate. When the WRF model is coupled with a 3-D variational data assimilation scheme, many aspects of the spatial variability of the precipitation diurnal cycle over the basin as well as the lifecycle of westward propagating convective systems initiated by the coastal sea-breeze front are similar to satellite and radar estimates of precipitation. Larger errors are produced without data assimilation, suggesting that convection over the basin is sensitive to subtle changes in the large-scale environment. In contrast, a single afternoon peak in rainfall is produced by E3SM for simulations with and without nudging the large-scale winds towards global reanalyses, which indicates that precipitation in the model is largely controlled by local convective activity coupled to diurnal sensible heat fluxes. Despite the differences in simulated precipitation between WRF and E3SM, both models produce differences in zonal wind profiles between days with and without propagating convective systems that are similar to data collected by U.S. DOE Atmospheric Radiation Measurement (ARM) facility during the Green Ocean Amazon (GoAmazon2014/5) campaign and other operational radiosondes. Easterly wind speeds between 2 and 4 km above ground are stronger on days with propagating convective systems; however, easterly wind speeds over the central basin and within 1 km of the ground are stronger on days without propagating convective systems. An analysis of the perturbed environmental conditions over the basin from WRF indicate that the stronger easterly winds aloft transport more moisture inland that can increase convective instability and prolong the lifetime of convective systems. The decelerated near-surface easterly winds on days with propagating convective systems is likely associated with the westerly inflow as a component of the local circulation driven by these systems.