This study investigates momentum transport in shallow cumulus clouds as simulated with the Dutch Atmospheric Large Eddy Simulation (DALES) for a 150 3 150 km2 domain east of Barbados during 9 days of EUREC4A. DALES is initialized and forced with the mesoscale weather model HARMONIE-AROME and subjectively reproduces observed cloud patterns. This study examines the evolution of momentum transport, which scales contribute to it, and how they modulate the trade winds. Daily-mean momentum flux profiles show downgradient zonal momentum transport in the subcloud layer, which turns countergradient in the cloud layer. The meridional momentum transport is nontrivial, with mostly downgradient transport throughout the trade wind layer except near the top of the surface layer and near cloud tops. Substantial spatial and temporal heterogeneity in momentum flux is observed with much stronger tendencies imposed in areas of organized convection. The study finds that while scales < 2 km dominate momentum flux at 200 m in unorganized fields, submesoscales O(2-20) km carry up to 50% of the zonal momentum flux in the cloud layer in organized fields. For the meridional momentum flux, this fraction is even larger near the surface and in the subcloud layer. The scale dependence of the momentum flux is not explained by changes in convective or boundary layer depth. Instead, the results suggest the importance of spatial heterogeneity, increasing horizontal length scales, and countergradient transport in the presence of organized convection.

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The characterization of systematic forecast errors in lower-tropospheric winds is an essential component of model improvement. This paper is motivated by a global, long-standing surface bias in the operational medium-range weather forecasts produced with the Integrated Forecasting System (IFS) of the European Centre for Medium-Range Weather Forecasts (ECMWF). Over the tropical oceans, excessive easterly flow is found. A similar bias is found in the western North Atlantic trades, where the EUREC4A field campaign provides an unprecedented wealth of measurements. We analyze the wind bias in the IFS and ERA5 reanalysis throughout the entire lower troposphere during EUREC4A. The wind bias varies greatly from day to day, resulting in root mean square errors (RMSEs) up to 2.5 m s−1, with a mean wind speed bias up to −1 m s−1 near and above the trade inversion in the forecasts and up to −0.5 m s−1 in reanalyses. These biases are insensitive to the assimilation of sondes. The modeled zonal and meridional winds exhibit a diurnal cycle that is too strong, leading to a weak wind speed bias everywhere up to 5 km during daytime but a wind speed bias below 2 km at nighttime that is too strong. Removing momentum transport by shallow convection reduces the wind bias near the surface but leads to stronger easterly near cloud base. The update in moist physics in the newest IFS cycle (cycle 47r3) reduces the meridional wind bias, especially during daytime. Below 1 km, modeled friction due to unresolved physical processes appears to be too strong but is (partially) compensated for by the dynamics, making this a challenging coupled problem.@en

Simulations of a marine cold air outbreak with the mesoscale weather model HARMONIE suggest that parameterized shallow convective momentum transport acts to diminish circulations that accompany cellular cloud structures. In this study we test this hypothesis in the trade-wind region where various type of shallow cumulus cloud patterns occur using a hierarchy of model simulations of the EUREC4A campaign. We select a ten-day period within EUREC4A and run the Dutch Atmospheric Large-Eddy Simulation (DALES) on a 150 km x 150 km domain with a resolution of 100 m. Its boundaries are forced hourly with dynamical tendencies from the mesoscale weather model HARMONIE. HARMONIE is also run on a 3200 km x 2000 km domain with 2.5 km resolution, in runs with shallow convective momentum transport on and off. First, we validate the model output with observations. Observations from EUREC4A suggest that in early February, deeper cumulus mediocris and larger cloud structures are associated with a different profile of eddy momentum flux divergence than days with shallower cumulus humilis. Second, we analyze the profiles of eddy momentum flux associated with turbulence, convection, and mesoscale flows. We show the momentum budget associated with different patterns of cloud organization and, lastly, we evaluate whether momentum transport has a significant influence on shallow cloud organization.@en

Profiles of eddy momentum flux divergence are calculated as the residual in the momentum budget constructed from airborne circular dropsonde arrays ((Formula presented.) 220 km) for 13 days during the EUREC (Formula presented.) A/ATOMIC field campaign. The observed dynamical forcing averaged over all flights agrees broadly with European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecasting System (IFS) forecasts. In the direction of the flow, a mean flux divergence (friction) exists over a 1.5-km deep Ekman layer, and a mean flux convergence (acceleration) is present near cloud tops. The friction is countergradient between 1 and 1.5 km, where vertical wind shear exceeds the observed thermal wind. From the frictional profile, a 10-m momentum flux of (Formula presented.) 0.1 N (Formula presented.) m (Formula presented.) is derived, in line with Saildrone turbulence measurements. A momentum flux divergence in the cross-wind direction is pronounced near the surface and acts to veer the wind, opposing the friction-induced cross-isobaric wind turning. Weaker friction and upper-level acceleration of easterly flow are observed when stronger winds and more vigorous convection prevail. Turbulence measurements on board the SAFIRE ATR-42 aircraft and the Uncrewed Aircraft System (UAS) RAAVEN reveal pronounced spatial variability of momentum fluxes, with a non-negligible contribution of mesoscales (5–30 km). The findings highlight the nontrivial impact of turbulence, convection, and mesoscale flows in the presence of diverse cloud fields on the depth and strength of the frictional layer.

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Most cumulus parametrizations today make use of a simple conceptual model of convection, called the mass-flux approach. This approach depicts convection as an ensemble of updrafts and downdrafts occurring within a model grid-box. The aim of this study is to determine convective mass-fluxes and their constituents on the scale of a 100 km GCM grid-box from a C-band polarimetric radar and thereafter investigate the relative role of area fraction and vertical velocity in determining the shape and magnitude of bulk mass-flux profiles. We make use of observational estimates of these quantities spanning 13 wet seasons in the tropical region of Darwin. Following a bulk approach, the results show that the distribution of mass-flux is positively skewed and its mean profile peaks at 4 km. This is the result of constant area fractions and increasing vertical velocities below that level. Above 4 km, in-cloud vertical velocity plays a marginal role compared to the convective area fraction in controlling mass-flux profiles.

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