Ann Kristin Naumann

A Lagrangian drop model to study warm rain microphysical processes in a shallow cumulus

In this study, we introduce a Lagrangian drop (LD) model to study warm rain microphysical processes in shallow cumulus. The approach combines Large-Eddy Simulations (LES) including a bulk microphysics parameterization with an LD model for raindrop growth. The LD model is one-way coupled with the Eulerian 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. To test whether the LD model is fit for purpose, a sensitivity study for isolated shallow cumulus clouds is conducted. We show that the surface precipitation rate and the development of the raindrop size distribution are sensitive to the treatment of selfcollection in the LD model. Some uncertainty remains for the contribution of the subgrid-scale turbulence to the relative velocity difference of a pair of LDs, which appears as a factor in the collision kernel. Sensitivities to other model parameters such as the initial multiplicity or the initial mass distribution are small. Overall, sensitivities of the LD model are small compared to the uncertainties in the assumptions of the bulk rain microphysics scheme, and the LD model is well suited for particle-based studies of raindrop growth and dynamics. This opens up the opportunity to study effects like recirculation, deviations from terminal fall velocity and other microphysical phenomena that so far were not accessible for bin, bulk, or parcel models.

A Conceptual Model of a Shallow Circulation Induced by Prescribed Low-Level Radiative Cooling

AbstractA conceptual bulk model for a dry, convective boundary layer with prescribed horizontally homogeneous and heterogeneous low-level radiative cooling rates is developed. For horizontally homogeneous radiative cooling, the response of the system to varying its prescribed parameters is explored and formulated in terms of nondimensional parameters. Large-eddy simulations with prescribed radiative cooling rates match the results of the bulk model well. It is found that, depending on the strength of the surface coupling, the height of the boundary layer (BL) either increases or decreases in response to increasing radiative BL cooling. Another property of the system is that, for increasing surface temperature, the BL temperature decreases if the prescribed radiative BL cooling rates are strong. This counterintuitive behavior is caused by the formulation of the entrainment rate at the inversion. Heterogeneous radiative BL cooling is found to cause a circulation induced by pressure deviations between the ar...

Earth System Science Frontiers: An Early Career Perspective

AbstractThe exigencies of the global community toward Earth system science will increase in the future as the human population, economies, and the human footprint on the planet continue to grow. This growth, combined with intensifying urbanization, will inevitably exert increasing pressure on all ecosystem services. A unified interdisciplinary approach to Earth system science is required that can address this challenge, integrate technical demands and long-term visions, and reconcile user demands with scientific feasibility. Together with the research arms of the World Meteorological Organization, the Young Earth System Scientists community has gathered early-career scientists from around the world to initiate a discussion about frontiers of Earth system science. To provide optimal information for society, Earth system science has to provide a comprehensive understanding of the physical processes that drive the Earth system and anthropogenic influences. This understanding will be reflected in seamless pre...

Evolution of the Shape of the Raindrop Size Distribution in Simulated Shallow Cumulus

AbstractIn this paper, the evolution of the raindrop size distribution (RSD) is investigated for two isolated shallow cumulus clouds that are modeled with large-eddy simulations. For a two-moment bulk rain microphysics scheme that assumes the RSD to follow a gamma distribution, it is shown that the evolution of the rainwater content of an individual shallow cumulus cloud—in particular, its subcloud-layer rainwater amount and its surface precipitation rate—is highly sensitive to the choice of the shape parameter of the gamma distribution.To further investigate the shape of the RSD, a Lagrangian drop model is used to represent warm rain microphysics without a priori assumptions on the RSD. It is found that the shape parameter is highly variable in space and time and that existing closure equations, which are established from idealized studies of more heavily precipitating cases, are not appropriate for shallow cumulus. Although a relation of the shape parameter to the mean raindrop diameter is also found fo...

Shallow circulations: relevance and strategies for satellite observation

Shallow circulations are central to many tropical cloud systems. We investigate the potential of existing and upcoming data to document these circulations. Different methods to observe or constrain atmospheric circulations rely on satellite-borne instruments. Direct observations of the wind are currently possible at the ocean surface or using tracer patterns. Satellite-borne wind lidar will soon be available, with a much better coverage and accuracy. Meanwhile, circulations can be constrained using satellite observations of atmospheric diabatic heating. We evaluate the commonalities and discrepancies of these estimates together with reanalysis in systems that include shallow circulations. It appears that existing datasets are in qualitative agreement, but that they still differ too much to provide robust evaluation criteria for general circulation models. This state of affairs highlights the potential of satellite-borne wind lidar and of further work on current satellite retrievals.

Cloud structures and rain formation in the atmospheric boundary layer

Wechselwirkungen dynamischer, thermodynamischer und mikrophysikalischer Prozesse auf sehr unterschiedlichen raumlichen und zeitlichen Skalen bestimmen sowohl das Erscheinungsbild von Wolken als auch die Niederschlagsbildung. Diese Dissertation untersucht den Einfluss kleinraumiger Variabilitat auf flache Konvektion und auf eisfreie Niederschlagsbildung mithilfe von hochauflosenden, numerischen Simulationen. Zunachst werden Simulationen unterschiedlicher Wolkenregime verwendet, um die Regimeabhangigkeit einer Wolkenparametrisierung, die auf Wahrscheinlichkeitsdichtefunktionen basiert, zu untersuchen. Dabei zeigt sich eine Regimeabhangigkeit in den Wahrscheinlichkeitsdichtefunktionen, die durch eine Anderung in den Schliesungsgleichungen der Wolkenparametrisierung berucksichtigt werden kann. Die neuen Schliesungsgleichungen verwerfen die Annahme einer strengen Antisymmetrie der Aufwinde im Cumulusregime gegenuber den Abwinden im Stratocumulusregime und reduzieren den Fehler der Wolkenparametrisierung im Cumulusregime in A-priori-Tests. Des Weiteren wird zur Untersuchung mikrophysikalischer Prozesse der Niederschlagsbildung ein Lagrangesches Tropfenmodell entwickelt, das alle relevanten Prozesse des eisfreien Regentropfenwachstums explizit simuliert: Akkreszenz von Wolkenwasser, Selbsteinfang von Regentropfen, Verdunstung und Sedimentation. Eine Sensitivitatsstudie ergibt, dass die Bodenniederschlagsmenge und die Regentropfenverteilung von der Darstellung des Selbsteinfangs in dem Lagrangeschen Modell abhangt. Weitere Simulationen zeigen, dass Unsicherheiten in der Formulierung des Lagrangeschen Modells deutlich kleiner sind als die inharenten Unsicherheiten in einer klassischen momentenbasierten mikrophysikalischen Parametrisierung. Eine Untersuchung der Entwicklung der Regentropfenverteilung in einzelnen Cumuluswolken mit dem Lagrangeschen Tropfenmodell ergibt, dass die Form der Regentropfenverteilung von dem Stadium der Wolke in seinem Lebenszyklus abhangt. Existierende Schliesungsgleichungen, die in momentenbasierten mikrophysikalischen Parametrisierungen verwendet werden und die fur starker regnende Wolken entwickelt wurden, sind nicht in der Lage diese Abhangigkeit wiederzugeben. Das Lagrangesche Tropfenmodell ermoglicht auserdem eine Analyse der Wachstumsgeschichte von Regentropfen. Simulationen eines Cumuluswolkenfeldes zeigen, dass die Zirkulation von Regentropfen -- ein Prozess, der in den momentenbasierten mikrophysikalischen Parametrisierungen grosskaliger Modellen nicht dargestellt wird -- in flachen Cumuluswolken weitverbreitet ist und erheblich zum Bodenniederschlag beitragt. Dynamical, thermodynamical and microphysical interactions on vastly different spatial and temporal scales determine the structure of clouds and the formation of precipitation. This thesis investigates the effect of small-scale variability on the representation of shallow clouds in large-scale models and on particle-kinetic processes that lead to the formation of precipitation without the occurrence of ice particles. Using large-eddy simulations of different shallow cloud regimes, a cloud parametrisation for large-scale models that is based on probability density functions is revisited. A regime dependent characteristic behaviour of the probability density functions is found, which can be taken into account by relaxing the strict antisymmetry of the original closure equations and allow cumulus updrafts to be more vigorous than stratocumulus downdrafts. In a priori tests the new set of closure equations reduces the error of the cloud parametrisation for the shallow cumulus regime. To investigate warm rain microphysical processes on a particle-based level, a Lagrangian drop model is developed that explicitly includes all relevant processes for raindrop growth such as accretional growth from cloud water, selfcollection among raindrops, evaporation and sedimentation. A sensitivity study reveals that the amount of surface precipitation and the slope of the raindrop size distribution are sensitive to the representation of selfcollection in the Lagrangian drop model. The uncertainties in the formulation of the Lagrangian drop model are found to be clearly smaller than uncertainties inherent in a bulk rain microphysics parametrisation. The Lagrangian drop model is applied to study the development of the raindrop size distribution in individual shallow cumulus clouds. The shape of the raindrop size distribution depends on the stage of the lifecycle of the cloud and closure assumptions currently used in bulk rain microphysics parametrisations, which have been developed for more heavily precipitating cases, are not able to capture this dependence. Furthermore, the Lagrangian drop model allows us to analyse the growth histories of raindrops. Recirculation of raindrops -- a process that is not represented by bulk rain microphysics parametrisations in large-scale models -- is found to be common in shallow cumulus and to contribute distinctly to the surface precipitation.