Research interests

  • microphysical controls on tropical circulation
  • conceptual models of convection and circulation
  • organization of convection
  • rain formation in shallow clouds
  • microphysical processes and parameterizations
  • Lagrangian particles in LES
  • cloud parameterization

 

 

Microphysical controls on clouds, radiation, and tropical circulation

High-resolution, large-domain simulations finally allow us to link microphysical processes to cloud controlling factors such as updraft wind speed and mixing. We investigate how understanding of how the tropical climate changes with warming depends on the knowledge of microphysics and its representation in numerical models. A goal is to quantify how microphysical process assumptions influence heat budgets in the tropics, the response of clouds to warming, and their effect on tropical circulation.

 

 

Conceptual model of a shallow circulation

In numerical simulations of radiative convective equilibrium, convection aggregates by spontaneous clustering of randomly distributed convective cells into organized mesoscale convection despite homogeneous boundary conditions. One mechanisms that has been proposed to initiate and maintain convective organization is a low-level radiative cooling - circulation feedback. I have developed a conceptual model to investigate how low-level cooling in non-convective areas affects the boundary layer structure and its dynamics. In the dry convective two-column model a shallow circulation develops due to differential radiative cooling in neighboring boundary layers. The model suggests that the strength of such a circulation is comparable to the strength of a circulation driven by surface temperature differences of a few Kelvins.

 

 

A Lagrangian perspective on warm rain formation

Despite the ever increasing grid resolution, understanding precipitation remains one of the major challenges in numerical weather prediction as well as climate modeling. To investigate warm rain microphysical processes on a particle-based level, I have developed a Lagrangian drop (LD) model to simulate raindrop growth in shallow cumulus. The LD model is part of the UCLA-LES and represents all relevant rain microphysical processes such as evaporation, accretion and selfcollection among LDs as well as dynamical effects such as sedimentation and inertia. Sensitivities of the LD model are small compared to the uncertainties in the assumptions of commonly used bulk rain microphysics schemes.

I have applied the LD model to study the development of the raindrop size distribution in shallow cumulus clouds and show that the shape of the raindrop size distribution depends on the stage of the lifecycle of the cloud. The study suggests that two-moment schemes with a diagnostic parameterization of the shape parameter, i.e., a local closure in space and time, are not sufficient, especially when being applied across different cloud regimes. One way to overcome this issue may be a prognostic shape parameter, i.e., a triple-moment warm-rain microphysics scheme.

Using the LD model, I have also investigated the growth process of raindrops and the role of recirculation of raindrops for the formation of precipitation in shallow cumulus. Recirculation of raindrops is found to be common in shallow cumulus, especially for those raindrops that contribute to surface precipitation. The fraction of surface precipitation that is attributed to recirculating raindrops differs from cloud to cloud but can be as large as 50 %. This implies that simple conceptual models of raindrop growth that neglect the effect of recirculation disregard a substantial portion of raindrop growth in shallow cumulus.

 

 

Lagrangian drops in LES: a shallow cumulus test case

Simulated growth of raindrops in a shallow cumulus cloud. Shown are the trajectories of those raindrops that reach the ground. The color of the trajectories corresponds to the ambient cloud water mixing ratio and the size of the drops is proportional to the simulated size of the raindrops. Most of the raindrops form in the updraft of the cloud. The raindrops grow the faster, the higher the ambient cloud water is and the more collisions among the raindrops take place.