Implications of Warm Rain in Shallow Cumulus and Congestus Clouds for Large-Scale Circulations

AbstractThe latest configuration of the Hadley Centre Global Environmental Model version 3 (HadGEM3) contains significant changes in the formulation of warm rain processes and aerosols. We evaluate the impacts of these changes in the simulation of warm rain formation processes using A-Train observations. We introduce a new model evaluation tool, quartile-based Contoured Frequency by Optical Depth Diagrams (CFODDs), in order to fill in some blind spots that conventional CFODDs have. Results indicate that HadGEM3 has weak linkage between the size of particle radius and warm rain formation processes, and switching to the new warm rain microphysics scheme causes more difference in warm rain formation processes than switching to the new aerosol scheme through reducing overly produced drizzle mode in HadGEM3. Finally, we run an experiment in which we perturb the second aerosol indirect effect (AIE) to study the rainfall-aerosol interaction in HadGEM3. Since the large changes in the cloud droplet number concentration (CDNC) appear in the AIE experiment, a large impact in warm rain diagnostics is expected. However, regions with large fractional changes in CDNC show a muted change in precipitation, arguably because large-scale constraints act to reduce the impact of such a big change in CDNC. The adjustment in cloud liquid water path to the AIE perturbation produces a large negative shortwave forcing in the midlatitudes.

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Dynamics of Subsiding Shells in Actively Growing Clouds with Vertical Updrafts

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-19-0018.1 ◽

2020 ◽

Vol 77 (4) ◽

pp. 1353-1369 ◽

Cited By ~ 2

Author(s):

Vishnu Nair ◽

Thijs Heus ◽

Maarten van Reeuwijk

Keyword(s):

Shell Thickness ◽

Large Scale ◽

Cloud Environment ◽

Initial State ◽

Cumulus Clouds ◽

Mean Velocity ◽

Flow Parameters ◽

Shallow Cumulus ◽

Order Of Magnitude ◽

Self Similar

Abstract The dynamics of a subsiding shell at the edges of actively growing shallow cumulus clouds with updrafts is analyzed using direct numerical simulation. The actively growing clouds have a fixed in-cloud buoyancy and velocity. Turbulent mixing and evaporative cooling at the cloud edges generate a subsiding shell that grows with time. A self-similar regime is observed for first- and second-order moments when normalized with respective maximum values. Internal scales derived from integral properties of the flow problem are identified. A self-similarity analysis using these scales reveals that contrary to classical self-similar flows, the turbulent kinetic energy budget terms and velocity moments scale according to the buoyancy and not with the mean velocity. The shell thickness is observed to increase linearly with time. The buoyancy scale remains time invariant and is set by the initial cloud–environment thermodynamics. The shell accelerates ballistically with a magnitude set by the saturation value of the buoyancy of the cloud–environment mixture. In this regime, the shell is buoyancy driven and independent of the in-cloud velocity. Relations are obtained for predicting the shell thickness and minimum velocities by linking the internal scales with external flow parameters. The values thus calculated are consistent with the thickness and velocities observed in typical shallow cumulus clouds. The entrainment coefficient is a function of the initial state of the cloud and the environment, and is shown to be on the same order of magnitude as fractional entrainment rates calculated for large-scale models.

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Responses of Shallow Cumulus Convection to Large-Scale Temperature and Moisture Perturbations: A Comparison of Large-Eddy Simulations and a Convective Parameterization Based on Stochastically Entraining Parcels

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-11-0279.1 ◽

2012 ◽

Vol 69 (6) ◽

pp. 1936-1956 ◽

Cited By ~ 29

Author(s):

Ji Nie ◽

Zhiming Kuang

Keyword(s):

Large Scale ◽

Potential Temperature ◽

Large Eddy Simulations ◽

Cumulus Parameterization ◽

Temperature Perturbation ◽

Cumulus Convection ◽

Positive Temperature ◽

Shallow Cumulus ◽

Scale Temperature ◽

Large Eddy

Abstract Responses of shallow cumuli to large-scale temperature/moisture perturbations are examined through diagnostics of large-eddy simulations (LESs) of the undisturbed Barbados Oceanographic and Meteorological Experiment (BOMEX) case and a stochastic parcel model. The perturbations are added instantaneously and allowed to evolve freely afterward. The parcel model reproduces most of the changes in the LES-simulated cloudy updraft statistics in response to the perturbations. Analyses of parcel histories show that a positive temperature perturbation forms a buoyancy barrier, which preferentially eliminates parcels that start with lower equivalent potential temperature or have experienced heavy entrainment. Besides the amount of entrainment, the height at which parcels entrain is also important in determining their fate. Parcels entraining at higher altitudes are more likely to overcome the buoyancy barrier than those entraining at lower altitudes. Stochastic entrainment is key for the parcel model to reproduce the LES results. Responses to environmental moisture perturbations are quite small compared to those to temperature perturbations because changing environmental moisture is ineffective in modifying buoyancy in the BOMEX shallow cumulus case. The second part of the paper further explores the feasibility of a stochastic parcel–based cumulus parameterization. Air parcels are released from the surface layer and temperature/moisture fluxes effected by the parcels are used to calculate heating/moistening tendencies due to both cumulus convection and boundary layer turbulence. Initial results show that this conceptually simple parameterization produces realistic convective tendencies and also reproduces the LES-simulated mean and variance of cloudy updraft properties, as well as the response of convection to temperature/moisture perturbations.

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Aerosol effects on shallow cumulus cloud fields in idealised and realistic simulations

10.5194/egusphere-egu2020-18231 ◽

2020 ◽

Author(s):

George Spill ◽

Philip Stier ◽

Paul Field ◽

Guy Dagan

Keyword(s):

Large Scale ◽

Trade Wind ◽

Quasi Equilibrium ◽

Cumulus Cloud ◽

Shallow Convection ◽

Shallow Cumulus ◽

Synoptic Conditions ◽

Large Eddy ◽

Transient Forcing ◽

Periodic Domains

Shallow cumulus clouds interact with their environment in myriad significant ways, and yet their behavour is still poorly understood, and is responsible for much uncertainty in climate models. Improving our understanding of these clouds is therefore an important part of improving our understanding of the climate system as a whole.Modelling studies of shallow convection have traditionally made use of highly idealised simulations using large-eddy models, which allow for high resolution, detailed simulations. However, this idealised nature, with periodic boundaries and constant forcing, and the quasi-equilibrium cloud fields produced, means that they do not capture the effect of transient forcing and conditions found in the real atmosphere, which contains shallow cumulus cloud fields unlikely to be in equilibrium.&#160;Simulations with more realistic nested domains and forcings have previously been shown to have significant persistent responses differently to aerosol perturbations, in contrast to many large eddy simulations in which perturbed runs tend to reach a similar quasi-equilibrium.&#160;Here, we further this investigation by using a single model to present a comparison of familiar idealised simulations of trade wind cumuli in periodic domains, and simulations with a nested domain, whose boundary conditions are provided by a global driving model, able to simulate transient synoptic conditions.&#160;The simulations are carried out using the Met Office Unified Model (UM), and are based on a case study from the Rain In Cumulus over the Ocean (RICO) field campaign. Large domains of 500km are chosen in order to capture large scale cloud field behaviour. A double-moment interactive microphysics scheme is used, along with prescribed aerosol profiles based on RICO observations, which are then perturbed.We find that the choice between realistic nested domains with transient forcing and idealised periodic domains with constant forcing does indeed affect the nature of the response to aerosol perturbations, with the realistic simulations displaying much larger persistent changes in domain mean fields such as liquid water path and precipitation rate.&#160;

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Drizzle in Stratiform Boundary Layer Clouds. Part II: Microphysical Aspects

Journal of the Atmospheric Sciences ◽

10.1175/jas3530.1 ◽

2005 ◽

Vol 62 (9) ◽

pp. 3034-3050 ◽

Cited By ~ 114

Author(s):

R. Wood

Keyword(s):

Boundary Layer ◽

Size Distribution ◽

Large Scale ◽

Cloud Droplet ◽

Horizontal Structure ◽

Boundary Layer Clouds ◽

Droplet Concentration ◽

Warm Rain ◽

Lognormal Size Distribution ◽

Accretion Rates

Abstract This is the second of two observational papers examining drizzle in stratiform boundary layer clouds. Part I details the vertical and horizontal structure of cloud and drizzle parameters, including some bulk microphysical variables. In this paper, the focus is on the in situ size-resolved microphysical measurements, particularly of drizzle drops (r > 20 μm). Layer-averaged size distributions of drizzle drops within cloud are shown to be well represented using either a truncated exponential or a truncated lognormal size distribution. The size-resolved microphysical measurements are used to estimate autoconversion and accretion rates by integration of the stochastic collection equation (SCE). These rates are compared with a number of commonly used bulk parameterizations of warm rain formation. While parameterized accretion rates agree well with those derived from the SCE initialized with observed spectra, the autoconversion rates seriously disagree in some cases. These disagreements need to be addressed in order to bolster confidence in large-scale numerical model predictions of the aerosol second indirect effect. Cloud droplet coalescence removal rates and mass and number fall rate relationships used in the bulk microphysical schemes are also compared, revealing some potentially important discrepancies. The relative roles of autoconversion and accretion are estimated by examination of composite profiles from the 12 flights. Autoconversion, although necessary for the production of drizzle drops, is much less important than accretion throughout the lower 80% of the cloud layer in terms of the production of drizzle liquid water. The SCE calculations indicate that the autoconversion rate depends strongly upon the cloud droplet concentration Nd such that a doubling of Nd would lead to a reduction in autoconversion rate of between 2 and 4. Radar reflectivity–precipitation rate (Z–R) relationships suitable for radar use are derived and are shown to be significantly biased in some cases by the undersampling of large (r > 200 μm) drops with the 2D-C probe. A correction based upon the extrapolation to larger sizes using the exponential size distribution changes the Z–R relationship, leading to the conclusion that consideration should be given to sampling issues when examining higher moments of the drop size distribution in drizzling stratiform boundary layer clouds.

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A-Train estimates of the sensitivity of warm rain likelihood and efficiency to cloud size, environmental moisture, and aerosols

10.5194/acp-2020-754 ◽

2020 ◽

Author(s):

Kevin M. Smalley ◽

Anita D. Rapp

Keyword(s):

Cloud Water ◽

Cumulus Cloud ◽

Power Law Distribution ◽

Water Path ◽

Precipitation Efficiency ◽

Warm Rain ◽

Shallow Cumulus ◽

Rain Drop ◽

Increasing Precipitation ◽

Water Contents

Abstract. Precipitation efficiency has been found to play an important role in constraining the sensitivity of the climate through its role in controlling cloud cover, yet understanding of its controls are not fully understood. Here we use CloudSat observations to identify individual contiguous shallow cumulus cloud objects and compute the ratio of cloud water path to rain water path as a proxy for warm rain efficiency (WRE). Cloud objects are then conditionally sampled by cloud-top height, relative humidity, and aerosol optical depth (AOD) to analyze changes in WRE as a function of cloud size (extent). For a fixed cloud-top height, WRE increases with extent and environmental humidity following a double power-law distribution, as a function of extent. Similarly, WRE increases holding environmental moisture constant. There is surprisingly little relationship between WRE and AOD when conditioned by cloud-top height, suggesting that once rain drop formation begins, aerosols may not be as important for WRE as cloud size and depth. Consistent with prior studies, results show an increase in WRE with sea surface temperature. However, for a given depth and SST, WRE is also dependent on cloud size and becomes larger as cloud size increases. Given that larger objects become more frequent with increasing SST, these results imply that increasing precipitation efficiencies with SST are due not only to deeper clouds with greater cloud water contents, but also the propensity for larger clouds which may have more protected updrafts.

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Investigating the Diurnal Evolution of the Cloud Size Distribution of Continental Cumulus Convection Using Multiday LES

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-18-0084.1 ◽

2019 ◽

Vol 76 (3) ◽

pp. 729-747 ◽

Cited By ~ 8

Author(s):

Thirza W. van Laar ◽

Vera Schemann ◽

Roel A. J. Neggers

Keyword(s):

Power Law ◽

Cloud Cover ◽

Large Scale ◽

Correlation Coefficients ◽

Scale Model ◽

Cloud Base ◽

Size Distributions ◽

Total Cloud Cover ◽

Surface Precipitation ◽

Shallow Cumulus

Abstract The diurnal dependence of cumulus cloud size distributions over land is investigated by means of an ensemble of large-eddy simulations (LESs). A total of 146 days of transient continental shallow cumulus are selected and simulated, reflecting a low midday maximum of total cloud cover, weak synoptic forcing, and the absence of strong surface precipitation. The LESs are semi-idealized, forced by large-scale model output but using an interactive surface. This multitude of cases covers a large parameter space of environmental conditions, which is necessary for identifying any diurnal dependencies in cloud size distributions. A power-law exponential function is found to describe the shape of the cloud size distributions for these days well, with the exponential component capturing the departure from power-law scaling at the larger cloud sizes. To assess what controls the largest cloud size in the distribution, the correlation coefficients between the maximum cloud size and various candidate variables reflecting the boundary layer state are computed. The strongest correlation is found between total cloud cover and maximum cloud size. Studying the size density of the cloud area revealed that larger clouds contribute most to a larger total cloud cover, and not the smaller ones. Besides cloud cover, cloud-base and cloud-top height are also found to weakly correlate with the maximum cloud size, suggesting that the classic idea of deeper boundary layers accommodating larger convective thermals still holds for shallow cumulus. Sensitivity tests reveal that the results are only minimally affected by the representation of microphysics and the output resolution.

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Large-scale vertical motion and its influence on cloudiness

10.5194/egusphere-egu2020-4379 ◽

2020 ◽

Author(s):

Geet George ◽

Bjorn Stevens ◽

Sandrine Bony ◽

Marcus Klingebiel

Keyword(s):

Large Scale ◽

Vertical Motion ◽

Cumulus Clouds ◽

Shallow Cumulus ◽

Integrated Water Vapour ◽

Thermodynamics And Dynamics ◽

Lifting Condensation Level ◽

The Impact ◽

First Time

This study uses measurements from the Elucidating the Role of Clouds-Circulation Coupling in Climate, EUREC4A and the second Next-Generation Aircraft Remote Sensing for Validation, NARVAL2 campaigns to investigate the influence of large-scale environmental conditions on cloudiness. For the first time, these campaigns provide divergence measurements, making it possible to explore the impact of large-scale vertical motions on clouds. We attempt to explain the cloudiness through the varying thermodynamics and dynamics in the different environments.&#160; For most of the NARVAL2 case-studies, cloudiness is poorly related to thermodynamical factors such as sea-surface temperature and lower tropospheric stability. Factors such as integrated water vapour and pressure velocity (&#969;) at 500 hPa and 700 hPa can be used to distinguish between actively convecting and suppressed regions, but they are not useful in determining the variation in cloudiness among suppressed cases. We find that &#969; in the boundary layer (up to &#8764;2 km) has a more direct control on the low-level cloudiness in these regions, than that in the upper layers. We use a simplistic method to show that &#969; at the lifting condensation level can be used to determine the cloud cover of shallow cumulus clouds. Thus, we argue that cloud schemes in models should not rely only on thermodynamical information, but also on dynamical predictors.

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Aerosol Effects on Clouds and Precipitation over Central Europe in Different Weather Regimes

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-18-0110.1 ◽

2018 ◽

Vol 75 (12) ◽

pp. 4247-4264 ◽

Cited By ~ 4

Author(s):

Christian Barthlott ◽

Corinna Hoose

Keyword(s):

Central Europe ◽

Environmental Conditions ◽

Large Scale ◽

Cloud Condensation Nuclei ◽

Convective Available Potential Energy ◽

Small Scale ◽

Aerosol Effect ◽

Warm Rain ◽

Synoptic Forcing ◽

Warm Rain Process

Abstract The response of clouds to changes in the aerosol concentration is complex and may differ depending on the cloud type, the aerosol regime, and environmental conditions. In this study, a novel technique is used to systematically modify the environmental conditions in realistic convection-resolving simulations for cases with weak and strong large-scale forcing over central Europe with the Consortium for Small-Scale Modeling (COSMO) model. Besides control runs with quasi-operational settings, initial and boundary temperature profiles are modified with linear increasing temperature increments from 0 to 5 K between 3 and 12 km AGL to represent different amounts of convective available potential energy (CAPE) and relative humidity. The results show a systematic decrease of total precipitation with increasing cloud condensation nuclei (CCN) concentrations for the cases with strong synoptic forcing caused by a suppressed warm-rain process, whereas no systematic aerosol effect is simulated for weak synoptic forcing. The effect of increasing CCN tends to be stronger in the simulations with increased temperatures and lower CAPE. While the large-scale domain-averaged responses to increased CCN are weak, the precipitation forming over mountainous terrain reveals a stronger sensitivity for most of the analyzed cases. Our findings also demonstrate that the role of the warm-rain process is more important for strong than for weak synoptic forcing. The aerosol effect is largest for weakly forced conditions but more predictable for the strongly forced cases. However, more accurate environmental conditions are much more important than accurate aerosol assumptions, especially for weak large-scale forcing.

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