Can the WRF model reproduce the observed temporal and spatial scales of the STD transition in the Amazon rainforest?

Description

Understanding and modeling atmospheric convection remains challenging because clouds form and organize at a wide range of spatial and temporal scales and depend on complex interactions between surface and atmospheric dynamics and thermodynamics. Moreover, long-term, high-resolution observations are scarce, particularly in the tropics, hindering our understanding of the shallow to deep (STD) convective transition. This explains, in part, why climate models still fail to represent the diurnal cycle of precipitation. One way to better understand the physical mechanisms responsible for the development and organization of convection is to use atmospheric models at cloud-resolving spatial resolutions. However, inferences from models require validation with observations.

In this study, we perform high-resolution simulations of the STD transition over the central Amazon rainforest using the Advanced Research WRF (WRF-ARW) regional model at 9 km, 3 km, 1 km, and 200 m resolutions. We used the European Center for Medium-Range Weather Forecasts (ECMWF) reanalysis data (ERA-5) to initialize the model, with a horizontal resolution of 0.25 deg, and the lateral boundary condition was updated every hour. Our focus is on the spatial and temporal scales of precipitable water vapor (PWV) convergence, which we validate using observations from the Amazon Dense GNSS network (Adams et al., 2013).

We followed the criteria proposed by Adams to identify the time of the STD transition: 1) a drop of at least 50 K in cloud top temperature (CTT) within 2 hours, reaching a CTT of 235 K or lower, and 2) a sharp increase in precipitable water vapor within about 1 hour of minimum CTT. We then calculated the correlation of PWV as a function of the distance (in km) and time before the STD transition (in hours).

By enhancing the spatial resolution, we found high convergence of moisture immediately before the deep convection, and the water vapor anomalies decreased as distance increased from the center of the convection. The water vapor convergence and correlation decay time scales are approximately 4 hours and 3.5 hours, respectively, consistent with the findings of Admas et al. (2013, 2017). Our preliminary results indicate that only the highest resolution was able to capture to correct scales, and hence represent the STD transition.