Abstract
Abstract. How convection couples to mesoscale vertical motion and what determines these motions is poorly understood. This study diagnoses profiles of area-averaged mesoscale divergence from measurements of horizontal winds collected by an extensive upper-air sounding network of a recent campaign over the western tropical North Atlantic, the Elucidating the Role of Clouds-Circulation Coupling in Climate (EUREC4A) campaign. Observed area-averaged divergence amplitudes scale approximately inversely with area-equivalent radius. This functional dependence is also confirmed in reanalysis data and a global, freely evolving simulation run at 2.5 km horizontal resolution. Based on the numerical data it is demonstrated that the energy spectra of inertia gravity waves can explain the scaling of divergence amplitudes with area. At individual times, however, few waves can dominate the region. Nearly monochromatic tropospheric waves are diagnosed in the soundings by means of an optimized hodograph analysis. For one day, results suggest that an individual wave directly modulated the satellite-observed cloud pattern. However, because such immediate wave impacts are rare, the systematic modulation of vertical motion due to inertia–gravity waves may be more relevant as a convection-modulating factor. The analytic relationship between energy spectra and divergence amplitudes proposed in this article, if confirmed by future studies, could be used to design better external forcing methods for regional models.
Highlights
The effects of large-spatial-scale and long-timescale processes on convection are well understood, such as shallow convection prevailing in geographic locations of freetropospheric subsidence associated with the Hadley cell (e.g., Albrecht et al, 1995; Norris, 1998; Wood and Hartmann, 2006)
One aim of the present study is to interpret the properties of mesoscale divergence variability in the context of the tropospheric background wave spectrum, which we diagnose from the ERA5 reanalysis (Hersbach et al, 2020) and a global 2.5 km horizontal-resolution simulation initialized on the starting date of the campaign
This study extended the analysis of recent novel measurements of mesoscale divergence variability (Bony and Stevens, 2019) by diagnosing vertical profiles of mesoscale divergence in the extensive sounding network of the 2020 EUREC4A field campaign
Summary
The effects of large-spatial-scale and long-timescale processes on convection are well understood, such as shallow convection prevailing in geographic locations of freetropospheric subsidence associated with the Hadley cell (e.g., Albrecht et al, 1995; Norris, 1998; Wood and Hartmann, 2006). Stephan et al (2020a) confirmed such features in soundings from the 2006 Tropical Warm Pool–International Cloud Experiment (TWP-ICE; May et al, 2008), which took place near Darwin, Australia They tested if the characteristics of divergence variability might be consistent with the gravity wave dispersion relation and concluded that gravity waves may serve to explain the temporal, vertical, and horizontal variability observed in area-averaged horizontal divergence. One aim of the present study is to interpret the properties of mesoscale divergence variability in the context of the tropospheric background wave spectrum, which we diagnose from the ERA5 reanalysis (Hersbach et al, 2020) and a global 2.5 km horizontal-resolution simulation initialized on the starting date of the campaign.
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