Entrainment of aerosols and their activation in a shallow cumulus cloud studied with a coupled LCM–LES approach

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;

Download Full-text

Multiscale Models for Cumulus Cloud Dynamics

Journal of the Atmospheric Sciences ◽

10.1175/2010jas3380.1 ◽

2010 ◽

Vol 67 (10) ◽

pp. 3269-3285 ◽

Cited By ~ 7

Author(s):

Samuel N. Stechmann ◽

Bjorn Stevens

Keyword(s):

Turbulent Mixing ◽

Apparent Magnitude ◽

Scale Analysis ◽

Cumulus Cloud ◽

Multiscale Models ◽

Cloud Droplets ◽

Shallow Cumulus ◽

Cloud Dynamics ◽

The Difference ◽

Balanced Dynamics

Abstract Cumulus clouds involve processes on a vast range of scales—including cloud droplets, turbulent mixing, and updrafts and downdrafts—and it is often difficult to determine how processes on different scales interact with each other. In this article, several multiscale asymptotic models are derived for cumulus cloud dynamics in order to (i) provide a systematic scale analysis on each scale and (ii) clarify the nature of interactions between different scales. In terms of scale analysis, it is shown that shallow cumulus updrafts can be described by balanced dynamics with a balance between source terms and ascent/descent; this is a cloud-scale version of so-called weak-temperature-gradient models. In terms of multiscale interactions, a model is derived that connects these balanced updrafts to the fluctuations within the balanced updraft envelope. These fluctuations describe parcels and updraft pulses, and this model encompasses some of the multiscale aspects of entrainment. In addition to this shallow cumulus model, to provide a broad picture of general cumulus dynamics, multiscale models are also derived for other scales; these include models for parcels and subparcel turbulent mixing and models for deep cumulus. Broadly speaking, the differences between the shallow and deep cases convey the notion that shallow cumulus dynamics are parcel dominated, whereas deep cumulus dynamics are updraft dominated; this is largely due to the difference in the apparent magnitude of the background temperature stratification. In addition to their use in guiding theory, the multiscale models also provide a framework for multiscale numerical simulations.

Download Full-text

The Route to Raindrop Formation in a Shallow Cumulus Cloud Simulated by a Lagrangian Cloud Model

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-16-0220.1 ◽

2017 ◽

Vol 74 (7) ◽

pp. 2125-2142 ◽

Cited By ~ 18

Author(s):

Fabian Hoffmann ◽

Yign Noh ◽

Siegfried Raasch

Keyword(s):

Drop Size ◽

Cloud Model ◽

Cloud Droplet ◽

Liquid Water Content ◽

Cumulus Cloud ◽

Turbulent Environments ◽

Shallow Cumulus ◽

History Of ◽

The Difference ◽

Single Droplets

Abstract The mechanism of raindrop formation in a shallow cumulus cloud is investigated using a Lagrangian cloud model (LCM). The analysis is focused on how and under which conditions a cloud droplet grows to a raindrop by tracking the history of individual Lagrangian droplets. It is found that the rapid collisional growth, leading to raindrop formation, is triggered when single droplets with a radius of 20 μm appear in the region near the cloud top, characterized by large liquid water content, strong turbulence, large mean droplet size, broad drop size distribution (DSD), and high supersaturations. Raindrop formation easily occurs when turbulence-induced collision enhancement (TICE) is considered, with or without any extra broadening of the DSD by another mechanism (such as entrainment and mixing). In contrast, when TICE is not considered, raindrop formation is severely delayed if no other broadening mechanism is active. The reason for the difference is clarified by the additional analysis of idealized box simulations of the collisional growth process for different DSDs in varied turbulent environments. It is found that TICE does not accelerate the timing of the raindrop formation for individual droplets, but it enhances the collisional growth rate significantly afterward by providing a greater number of large droplets for collision. Higher droplet concentrations increase the time for raindrop formation and decrease precipitation but intensify the effect of TICE.

Download Full-text

Shallow cumulus cloud feedback in large eddy simulations – bridging the gap to storm-resolving models

Atmospheric Chemistry and Physics ◽

10.5194/acp-21-3275-2021 ◽

2021 ◽

Vol 21 (5) ◽

pp. 3275-3288

Author(s):

Jule Radtke ◽

Thorsten Mauritsen ◽

Cathy Hohenegger

Keyword(s):

Cloud Cover ◽

Horizontal Resolution ◽

Cloud Fraction ◽

Cloud Feedback ◽

Cloud Base ◽

Trade Wind ◽

Cumulus Cloud ◽

Shallow Cumulus ◽

Large Eddy ◽

Carry Over

Abstract. The response of shallow trade cumulus clouds to global warming is a leading source of uncertainty in projections of the Earth's changing climate. A setup based on the Rain In Cumulus over the Ocean field campaign is used to simulate a shallow trade wind cumulus field with the Icosahedral Nonhydrostatic Large Eddy Model in a control and a perturbed 4 K warmer climate, while degrading horizontal resolution from 100 m to 5 km. As the resolution is coarsened, the base-state cloud fraction increases substantially, especially near cloud base, lateral mixing is weaker, and cloud tops reach higher. Nevertheless, the overall vertical structure of the cloud layer is surprisingly robust across resolutions. In a warmer climate, cloud cover reduces, alone constituting a positive shortwave cloud feedback: the strength correlates with the amount of base-state cloud fraction and thus is stronger at coarser resolutions. Cloud thickening, resulting from more water vapour availability for condensation in a warmer climate, acts as a compensating feedback, but unlike the cloud cover reduction it is largely resolution independent. Therefore, refining the resolution leads to convergence to a near-zero shallow cumulus feedback. This dependence holds in experiments with enhanced realism including precipitation processes or warming along a moist adiabat instead of uniform warming. Insofar as these findings carry over to other models, they suggest that storm-resolving models may exaggerate the trade wind cumulus cloud feedback.

Download Full-text

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.

Download Full-text

Mechanisms and Model Diversity of Trade-Wind Shallow Cumulus Cloud Feedbacks: A Review

Surveys in Geophysics ◽

10.1007/s10712-017-9418-2 ◽

2017 ◽

Vol 38 (6) ◽

pp. 1331-1353 ◽

Cited By ~ 25

Author(s):

Jessica Vial ◽

Sandrine Bony ◽

Bjorn Stevens ◽

Raphaela Vogel

Keyword(s):

Trade Wind ◽

Cumulus Cloud ◽

Cloud Feedbacks ◽

Shallow Cumulus

Download Full-text

EFFECT OF SOIL MOISTURE ON SHALLOW CUMULUS CLOUD IN LARGE EDDY SIMULATION

Journal of Japan Society of Civil Engineers Ser G (Environmental Research) ◽

10.2208/jscejer.74.i_33 ◽

2018 ◽

Vol 74 (5) ◽

pp. I_33-I_40

Author(s):

Fanny KRISTIANTI ◽

Dzung NGUYEN-LE ◽

Tomohito J. YAMADA

Keyword(s):

Soil Moisture ◽

Large Eddy Simulation ◽

Cumulus Cloud ◽

Eddy Simulation ◽

Shallow Cumulus ◽

Large Eddy

Download Full-text

Use of Radar Chaff for Studying Circulations in and around Shallow Cumulus Clouds

Journal of Applied Meteorology and Climatology ◽

10.1175/jamc-d-13-0255.1 ◽

2014 ◽

Vol 53 (8) ◽

pp. 2058-2071 ◽

Cited By ~ 5

Author(s):

Eunsil Jung ◽

Bruce Albrecht

Keyword(s):

Passive Tracer ◽

Cumulus Cloud ◽

Dipole Antennas ◽

Observation Level ◽

Cumulus Clouds ◽

Cloud Radar ◽

Aerosol Cloud ◽

Shallow Cumulus ◽

Environmental Air

AbstractCirculations in and around cumulus clouds are inferred by using a passive tracer (radar chaff) and an airborne cloud radar during the Barbados Aerosol Cloud Experiment (BACEX). The radar chaff elements used for this experiment are fibers that are cut to a length of about ½ of the radar wavelength to maximize radar returns by serving as dipole antennas. The fibers are packed in fiber tubes and are mounted in a dispenser beneath the wing of the aircraft. The chaff was released near the tops and edges of a growing small cumulus cloud. The aircraft then made penetrations of the cloud at lower levels to observe the chaff signals above the aircraft with the zenith-pointing cloud radar. This study shows that the environmental air above the cloud top descends along the downshear side of the cloud edge and is subsequently entrained back into the same cloud near the observation level. The in-cloud flow follows an inverted letter P pattern. The merits and limitations of the chaff method for tracking circulations in and around small cumuli are discussed.

Download Full-text