A short overview

Marine precipitation convection

My research is primarily centered around convective clouds, in particular the myriad of shallow cumulus and cumulus congestus that are ubiquitous over oceans, but also over land. Shallow cumulus and congestus are not as deep as the cumulonimbus clouds that produce heavy rainfall, but similar to their deep counterparts, they organize themselves into a variety of shapes, and grow as deep as 4 km. When they do so, they coincide with gusty winds, and they produce rainfall at a rate that is significant for water and energy budgets over tropical oceans. Shallow cumuli are also crucial to the vertical distribution of heat, moisture and momentum in the lowest kilometers of the atmosphere: the atmospheric boundary layer. This is in turn is important for atmospheric circulations, such as the Hadley and Walker circulations, but also (tropical) cyclones, and, on smaller scales, the mesoscale circulations that are thought to accompany the aggregation of deep convection.

Because trade-wind cumuli are so numerous, they make the dark ocean surface a little brighter. As such, they have a net cooling effect on climate. Biases in low level cloudiness in subtropical regions in models can therefore lead to biases in the energy budget of the Earth system – the uncertainty in future climate predictions by climate models is the most pressing example of how deficiencies in the representation of these small clouds in global models can have far-reaching effects.

Current projects

ENW M2 (NWO): Clouds as momentum pumps for the ocean

Marine convective clouds, especially when they rain, can lead to large wind variability, even at the ocean surface. We investigate the influence of such wind heterogeneity on the exchange of momentum between the atmosphere and ocean, the wind stress, using ultra-high resolution satellite observations of ocean surface roughness (SAR), combined with simulations (LES) and local lidar measurements.

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VIDI (NWO): Tracing convective momentum transport in complex cloudy atmospheres

The transport of momentum through convective circulations is broadly called convective momentum transport (CMT). CMT is parameterized in weather and climate models and its uncertainty hampers accurate prediction of wind. We use novel wind lidar and cloud radar observations and large-domain large-eddy simulations to derive momentum fluxes driven by convection, and use these to validate models.

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ERC Starting Grant CloudBrake

The frictional layer in the trades

Central to the ERC Starting Grant CloudBrake project was the question: how does trade-wind convection influence the large-scale wind? Does it accelerate or decelerate the flow, and at what height levels? To answer these questions we analyzed the momentum budget observed over a 200 km circular area during the EUREC4A field study in Jan-Feb 2020. Interpreting the residual in the budget as the action of turbulence, convection and mesoscale flows, we find that the wind is decelerated over a layer on average ~ 1.5 km deep, but that friction-induced ageostrophic wind turning of wind is weak. An acceleration of wind is also observed in the upper cloud layer and trade-inversion. The observations suggest that convection and mesoscale circulations play a key role in the observed momentum transport.

Momentum transport over land

Shallow cumulus convection is very typical over land in the midlatitudes in Spring and Summer. As part of the ERC Starting Grant CloudBrake project, we analyzed observations and simulations of winds, momentum transport and clouds to investigate how cumulus convection influences wind near the surface and aloft. Analyses focused on a 9-year climatology of Cabauw observations and daily limited-domain LES hindcasts to reveal differences in wind profiles and momentum transport with cloud and weather regimes. We also carried out a dual-airplane flight mission measuring the profile of wind with airborne Doppler wind lidar as well as turbulence below and through clouds.

Wind shear effect on clouds

As part of the ERC Starting Grant CloudBrake project we explored the sensitivity of trade-wind convection, cloudiness and its organization to vertical wind shear. Does wind shear limit or enhance convection? And how does moisture aggregation and cloud organization change in the presence of forward or backward shear?

Highlights - Clouds and Climate

Trimodal convection

In nature, moist convection prefers three modes: shallow, congestus and deep cumulus. Using a conceptual model, can we understand what processes help set the depth of convection in tropical circulations?

Warm rain over oceans

Warm rain is that produced by clouds with tops below the freezing level. Observations reveal that warm rain is a significant part of rainfall over global oceans, with implications for atmospheric dynamics.

Vertical structure of low cloud

Global models disagree on how low clouds respond to increasing carbon dioxide concentrations, and observations show how this behavior relates to the vertical structure of low cloud produced by models.