A Lagrangian Study of Precipitation-Driven Downdrafts*

Meteorologische Zeitschrift, 2014

This study combines observations, large-eddy simulations (LES), and direct numerical simulations (DNS) in order to analyze entrainment and mixing in shallow cumulus clouds at all relevant spatial scales and, additionally, to verify the results by the multiple methods used. The observations are based on three flights of the CARRIBA campaign which are similar to the classical BOMEX case used for LES. Virtual flights in the LES data are used to validate the observational method of line measurements. It is shown that line measurements overrepresent the cloud core, and it is quantified how derived statistics depend on small perturbations of the flight track, which has to be taken in account for the interpretation of airborne observations. A linear relation between fluctuations of temperature and liquid water content has been found in both LES and observations in a good quantitative agreement. However, the constant of proportionality deviates from purely adiabatic estimates, which can be attributed to cloud edge mixing. The cloud edge is compared in detail in observations and LES, which agree qualitatively although the LES cloud edge is smoother due to the model's resolution. The resulting typical amplitudes of the turbulence fields from this comparison are compared with the large-scale forcing model which is used in a series of DNS which study the mixing below the meter scale, which show that LES does not resolve the intermittency of small-scale turbulence.

Atmospheric Research, 2006

Airborne measurements of updraft speeds, liquid water content and other microphysical parameters were measured in convective storms during two mid-latitude summers in North Dakota, USA, and in two tropical locations (the continental Amazon region in tropical South America and Kwajalein in the tropical central Pacific Ocean). Data representing 0.5km averages of data collected from a large number of clouds sampled during the field seasons are examined. In the North Dakota clouds, updraft values of a given magnitude were found about as often as downdrafts of similar magnitude in midlevels of the clouds, while lower levels favored downdrafts and upper levels favored updrafts. Drafts in tropical clouds were much weaker and favored updrafts in mid to upper regions of the clouds (the lower levels were not sampled). In two temperature regions (− 5 to − 12°C and − 12 to − 20°C), there was little difference in the frequency of occurrence of liquid water content values in the North Dakota clouds, while in the tropical clouds, higher liquid water contents were less likely to be found in the colder temperature intervals. The particle concentrations measured by FSSP instruments were similar in lower and midlevels of the North Dakota clouds, and decreased in upper levels. In the tropical clouds, the concentrations were similar in the mid and upper regions of the clouds. These results are discussed in terms of previous measurements of midlatitude and tropical clouds.

Journal of Advances in Modeling Earth Systems, 2018

A Lagrangian Particle Dispersion Model is embedded into large eddy simulations to diagnose the responses of shallow cumulus convection to a small‐amplitude large‐scale temperature perturbation. The Lagrangian framework allows for a decomposition of the vertical momentum budget and diagnosis of the forces that regulate cloudy updrafts. The results are used to shed light on the parameterization of vertical velocity in convective schemes, where the treatment of the effects of entrainment as well as buoyancy‐induced and mechanically induced pressure gradients remains highly uncertain. We show that both buoyancy‐induced and mechanically induced pressure gradients are important for the vertical momentum budget of cloudy updrafts, whereas the entrainment dilution term is relatively less important. Based on the analysis of the dominant force balance, we propose a simple model to derive the perturbation pressure gradient forces. We further illustrate that the effective buoyancy and dynamic p...

Geophysical Research Letters, 2012

A new framework for diagnosing mixing in cumulus convection is proposed, where the vertical evolution of the cloud mass flux distribution as a function of given parcel properties is viewed as a mapping process. Matrices that characterize this mapping process are constructed using a Lagrangian particle dispersion model embedded in a Large Eddy Simulation. Extending previous use of LES-diagnosed bulk entrainment and detrainment rates to test bulk plume models, the construction of these mapping matrices from LES provides a reference, against which multiple-parcel models can be compared. The framework is further applied to diagnose the response of shallow cumuli to a small temperature perturbation.

Monthly Weather Review, 2003

All convection parameterizations in models of the atmosphere include a decision tree to decide on at least the occurrence, and often the type, of convection in a model grid volume. This decision tree is sometimes referred to as the ''trigger function.'' This study investigates the role that the decision-making processes play in the simulation of convection in the European Centre for Medium-Range Weather Forecasts global forecast model.

Journal of the Atmospheric Sciences, 1998

Large-eddy simulations that incorporate a size-resolving representation of cloud water are used to study the effect of heavy drizzle on PBL structure. Simulated surface precipitation rates average about 1 mm day Ϫ1 . Heavily drizzling simulations are compared to nondrizzling simulations under two nocturnal PBL regimes-one primarily driven by buoyancy and the other driven equally by buoyancy and shear. Drizzle implies a net latent heating in the cloud that leads to sharp reductions in both entrainment and the production of turbulent kinetic energy by buoyancy (particularly in downdrafts). Drizzle, which evaporates below cloud base, promotes a cooler and moister subcloud layer that further inhibits deep mixing. The cooling and moistening is in quantitative agreement with some observations and is shown to favor the formation of cumuli rising out of the subcloud layer. The cumuli, which are local in space and time, are responsible for most of the heat and moisture transport. They also appear to generate a larger-scale circulation that differs dramatically from the regularity typically found in nonprecipitating stratocumulus. Time-averaged turbulent fluxes of heat and moisture increase in the presence of precipitation, suggesting that drizzle (and drizzle-induced stratification) should not necessarily be taken as a sign of decoupling. Because drizzle primarily affects the vertical distribution of buoyancy, shear production of turbulent kinetic energy mitigates some of the effects described above. Based on large-eddy simulation the authors hypothesize that shallow, well-mixed, radiatively driven stratocumulus cannot persist in the presence of heavy drizzle. In accord with some simpler models, the simulated case with heavy precipitation promotes a reduction in both liquid-water path and entrainment. However, the simulations suggest that timeintegrated cloud fraction may increase as a result of drizzle because thinner precipitating clouds may persist longer if the boundary layer does not deepen as rapidly. These somewhat more complicated dynamics have important implications for a number of hypotheses suggesting that changes in aerosol concentrations, when metabolized by stratocumulus, have a significant effect on climate.

Journal of Geophysical Research: Atmospheres, 2019

The impact of precipitation in shallow cumulus convection on the moisture variance and third‐order moments of moisture is investigated with the help of large‐eddy simulations. Three idealized simulations based on the Rain in Cumulus over the Ocean field experiment are analyzed: one nonprecipitating, on a smaller domain, and two precipitating cases, on a larger domain with different initial profiles of moisture. Results show that precipitation and the associated cloud organization lead to increased generation of higher‐order moments (HOM) of moisture compared to the nonprecipitating case. To understand the physical mechanism and the role of individual processes in this increase, budgets of HOM of moisture are studied. Microphysics directly decreases the generation of HOM of moisture, but this effect is not dominant. The gradient production term is identified as the main source term in the HOM budgets. The influence of the gradient production term on moisture variance is further exami...

Journal of the Atmospheric Sciences, 2005

This paper is the first in a two-part series in which the life cycles of numerically simulated shallow cumulus clouds are systematically examined. The life cycle data for six clouds with a range of cloud-top heights are isolated from an equilibrium trade cumulus field generated by a large-eddy simulation (LES) with a uniform resolution of 25 m. A passive subcloud tracer is used to partition the cloud life cycle transport into saturated and unsaturated components; the tracer shows that on average cumulus convection occurs in a region with time-integrated volume roughly 2 to 3 times that of the liquid-water-containing volume. All six clouds exhibit qualitatively similar vertical mass flux profiles with net downward mass transport at upper levels and net upward mass flux at lower levels. This downward mass flux comes primarily from the unsaturated cloud-mixed convective region during the dissipation stage and is evaporatively driven. Unsaturated negatively buoyant cloud mixtures domina...

Acta Geophysica, 2011

A b s t r a c t This paper presents application of the EULAG model combined with a sophisticated double-moment warm-rain microphysics scheme to the model intercomparison case based on RICO (Rain in Cumulus over Ocean) field observations. As the simulations progress, the cloud field gradually deepens and a relatively sharp temperature and moisture inversions develop in the lower troposphere. Two contrasting aerosol environments are considered, referred to as pristine and polluted, together with two contrasting subgridscale mixing scenarios, the homogeneous and the extremely inhomogeneous mixing. Pristine and polluted environments feature mean cloud droplet concentrations around 40 and 150 mg −1 , respectively, and large differences in the rain characteristics. Various measures are used to contrast evolution of macroscopic cloud field characteristics, such as the mean cloud fraction, the mean cloud width, or the height of the center of mass of the cloud field, among others. Macroscopic characteristics appear similar regardless of the aerosol characteristics or the homogeneity of the subgrid-scale mixing.

Journal of the Atmospheric Sciences, 2009

Aircraft observations of shallow to moderately deep cumulus clouds are analyzed with the purpose to describe the typical horizontal structure of thermodynamic and kinematic parameters near the cumulus margin, from the cloud center into the ambient clear air. The cumuli were sampled in a broad range of environments in three regions: the tropical Atlantic Ocean in winter, the Sonoran Desert during the monsoon, and the arid high plains of Wyoming in summer. The composite analysis of 1624 cumulus penetrations shows that the vertical mass flux, temperature, buoyancy, the buoyancy flux, and the turbulent kinetic energy all tend to reach a minimum near the cloud edge. Most of these variables, and also the liquid water content, the droplet concentration, and the mean droplet size generally decrease in value from within the cumulus towards the cloud edge, slowly at first and rapidly within ~100 m of the cloud edge. These findings provide evidence for significant evaporative cooling in entraining and detraining eddies in the cloud margin. This cooling explains the tendency for downward accelerating, buoyantly driven subsidence in the cloud margin.