Mission Statement

The mission of the Cloud Life Cycle Working Group is to document from observations and modeling and thereby develop understanding of the dynamical, thermodynamical, microphysical, and radiative processes that together determine the evolution of clouds from formation to dissipation, and to translate this understanding into methods for representing cloud processes in numerical weather and climate models.

Objectives

• Identify guiding science questions regarding cloud life cycles that are based on model uncertainties/limitations, that can be addressed using ARM observations, and that support broader programmatic objectives outlined in the ASR Science Plan.
• Facilitate, organize, and maximize the efforts of individual Principal Investigator projects towards answering these broader scientific questions through the formation and organization of subgroups with similar and complementary research goals.
• Identify, prioritize, and help implement the observational campaigns, measurement strategies, and data products that are needed to understand cloud life cycle processes and represent them in models.

Science Questions

Cirrus Clouds

• What are the characteristics of ice supersaturation in the upper troposphere, how do they relate to cirrus clouds and moisture exchange through the tropopause, and how can climate models best represent these processes?
• What is the nature and variability of the particle size distribution in cirrus, and how does it evolve over the cirrus life cycle? What is the radiative impact of small particles?
• What mechanisms in cirrus cloud evolution control the rate at which ice mass sediments and sublimates, and how can these processes be adequately represented in numerical models?
• How do cloud-scale dynamical processes control the evolution of cirrus properties through nucleation, particle growth, sedimentation, and sublimation?
• What degree of complexity is required in cloud property retrieval algorithms, and what minimal set of algorithms can be used, to describe cirrus microphysical properties using ground-based ARM Facility data rigorously enough to calculate their effect on radiation and the cloud’s evolution?
• Why is thin cirrus so commonplace and widespread in the tropics?
• Why do GCM model intercomparison studies find that cirrus do not contribute significantly to the variability among the models? Are cirrus cloud effects adequately represented in GCMs or are all GCMs treating cirrus clouds equally incorrectly?
• What physical mechanisms determine the fractional coverage of anvil clouds (i.e., detrainment from deep convection, wind shear, spreading due to radiative heating, loss of ice mass due to precipitation and sublimation)?
• What are the radiative impacts of cirrus clouds and how do they vary dependent upon cirrus type (i.e. formation mechanism)?
• Can the fractional coverage of anvil cirrus clouds be diagnosed in GCMs or is a prognostic approach required?

Deep Convective Clouds

• What thermodynamic and dynamical conditions determine the triggering and occurrence of deep convection?
• Under what thermodynamic and dynamic conditions does convection organize into mesoscale clusters, and what is the role of mesoscale circulations in determining the properties and lifetime of anvil cirrus?
• What is the vertical and spatial distribution of diabatic heating in convective systems?
• How does entrainment regulate the depth, and strength of convection, and how are these related to the clouds and precipitation they produce and their heating and drying profiles? Do cumulus parameterizations need to portray a spectrum of plumes with different entrainment rates and depths, or can a single plume with varying entrainment/detrainment profiles adequately represent the effect of the ensemble on the environment?
• How does convective heating and drying interact with the large-scale flow to determine the character of tropical dynamical variability?
• How does vertical velocity vary in height, space, time, and strength? How does this variability impact precipitation formation, cirrus production, and convective system lifetime?
• How can we better constrain convective microphysical processes, including those responsible for cirrus, graupel, snow, and rain production, in numerical models?
• How can we better characterize subgrid-scale inhomogeneity in temperature and moisture and incorporate this knowledge into cumulus parameterizations?
• What elements of cumulus parameterizations should be prognostic (e.g. cumulus kinetic energy, mesoscale kinetic energy, anvil cloud fraction), and how can we diagnose these from measurements?
• Should cumulus parameterizations be stochastic, and if so, in what manner?

Low Clouds (warm)

• What factors (in-cloud turbulence levels, particle sizes and sedimentation rates, density gradients) control vertical and horizontal entrainment rates between the clouds and ambient air?
• What relationships among various microphysical and dynamical factors (i.e. particle sizes, cloud thickness, vertical velocity, turbulence, ambient moisture, and aerosol) determine low cloud precipitation onset, rate, and efficiency?
• How do cloud properties (water contents, sizes, number concentrations, cloud thickness, vertical velocities) relate to the near-surface properties (surface fluxes, temperature, moisture, vertical velocity, and aerosol) and their variability?
• How do these cloud properties and process rates relate to the properties of the ambient or overlying air?
• What processes are responsible for cloud transitions, such as those from shallow cumulus to stratocumulus or from shallow convection to deep convection?
• What processes determine the diurnal cycle of marine stratocumulus and continental shallow cumulus clouds?
• Why do climate models over-predict the incidence of precipitation from low clouds and under-predict the incidence of thin (low-liquid water path) low clouds?
• What role does mesoscale inhomogeneity of liquid water or vertical velocity on scales of tens of km play in determining the radiative properties and drizzle rates in low clouds?
• Which of the processes mentioned above determine how stratus, stratocumulus, and cumulus clouds will respond to a climate change?

Low Clouds (cold)

• What factors (such as radiative cooling, moisture advection, surface fluxes, turbulence, and aerosol) determine cloud phase (supercooled liquid only, pure ice, or mixed-phase)?
• What ice nucleation mechanisms are important in cold clouds with and without liquid water?
• What processes (concentrations, vertical velocity, entrainment, riming, etc.) determine precipitation efficiency in cold clouds? What are the relative influences of persistent slow precipitation vs. periodic strong precipitation associated with storms on the Arctic precipitation budget?
• What role do clouds and precipitation play in establishing and limiting stratification in the Arctic lower troposphere?
• What role does Arctic stratus play in the seasonal melt onset, melt duration, and freeze up of sea ice?
• Are recent downward trends in sea ice driven by changes in Arctic clouds, and if so, by microphysical (e.g., phase) changes in locally formed stratus or by changes in clouds formed by poleward advection of moisture in extratropical storms?
• How will Arctic cloud properties respond to climate changes such as decreases in sea-ice concentration, increases in atmospheric temperatures, changes in circulation patterns, etc.?
• What is the appropriate level of complexity to best represent cloud phase in climate models?
• Why do climate models over predict the amount of near surface thin-ice clouds in the cold season?

Midlatitude Storm Clouds

• What role does diabatic heating play in the generation of available potential energy in synoptic storms? Do models reasonably simulate the distribution of diabatic heating and poleward transports of energy within midlatitude storms? Are these issues important for understanding how these storms will respond to climate change?
• How can we measure the tilt, coverage, radiative, and microphysical properties of clouds formed by mesoscale frontal circulations?
• What spatial resolution is adequate for climate models to resolve the important radiative and precipitation properties of mid-latitude storm clouds?
• Do climate models need a parameterization of symmetric instability?
• How can climate models better predict the occurrence of snow vs. rain in winter storms?