The interaction between moist diabatic processes and the atmospheric circulation in African Easterly Wave propagation

Abstract

An objective tracking algorithm is used to characterize the three‐dimensional structure of African Easterly Waves (AEWs) in ERA‐Interim reanalysis and a Met Office Unified Model (UM) simulation. A special focus is dedicated to the coupling of dynamical aspects of the wave and moist convection. The relation between baroclinic features of the wave and latent heating is explored. Latent heating at and slightly ahead of the wave trough is found to reinforce and sustain the anomalous wave circulation through potential vorticity (PV) generation and vortex stretching. The coupling of moist processes and the circulation takes place mainly through moisture convergence at lower mid‐tropospheric levels, between 850 and 500 hPa. These findings are confirmed and examined in more detail in a case‐study of a strong AEW based on high‐resolution UM simulations. PV tracers are used to investigate how different moist diabatic processes invigorate the wave. Again moisture anomalies are found to be the main contributors to generating small‐scale convergence centres and updraughts ahead of the trough at mid‐tropospheric levels. Although buoyancy effects are ultimately responsible for the convective uplift, the results suggest that mesoscale circulations associated with the AEW dynamics are crucial in creating the small‐scale moist static instabilities and vortices which are essential for the AEW maintenance. Boundarylayer mixing and advection from the northern Sahel may create pockets of high‐PV air around the trough in some instances, but this mechanism of wave sustainment needs further investigation.

The Met Office Unified Model Global Atmosphere 7.0/7.1 and JULES Global Land 7.0 configurations

“…The first problem was that prior to March 2016, the Met Office analysis at 850 hPa had a tendency to be warm compared to radiosonde observations due to the details of the bias correction scheme used in assimilating satellite radiances. This has since been addressed by introducing a variational bias correction scheme (VarBC), which brings the analysed global mean 850 hPa temperature both closer to that of other NWP centres and closer to that of radiosonde observations (Ujiie et al, 2017a). The combination of a warm analysis and a climatological cold bias in the operational GA6.1/GL6.1 meant that the model's mean 850 hPa temperature would drift towards the radiosonde observations with an increasing forecast range.…”

The Met Office Global Coupled Model 3.0 and 3.1 (GC3.0 and GC3.1) Configurations

“…The frequency of AEWs is now comparable to reanalyses (not shown), however, Atlantic tropical cyclones require not only the correct frequency of waves, but also the correct distribution of vorticity and accurate positioning and strength of the African Easterly jet. We believe that the combination of these elements is still not sufficiently well simulated to provide a reliable frequency of Atlantic tropical cyclones (Tomassini et al, ).…”

African Easterly Wave Dynamics in Convection‐Permitting Simulations: Rotational Stratiform Instability as a Conceptual Model

“…To relate rotational stratiform instability as described here, to current versions of stratiform instability (e.g., Kuang, ; Mapes, ), the variability of convective inhibition, CAPE, and midtropospheric moisture needs to be examined and related to the initiation and upscale growth of MCSs in the vicinity of AEWs. While it has been argued that CAPE and moisture are important to the AEW‐convection interaction (Janiga & Thorncroft, ; Tomassini, ; Tomassini et al, ), other factors such as synoptic‐scale ascent and wind shear also likely play a role (Janiga & Thorncroft, ; Kiladis et al, ). Through some combination of these factors, we expect that the dynamic response to convection not only strengthens the initial wave but promotes new convection.…”