GEWEX-CLIVAR: Coordination of U.S. Activities (1998)

NRC, 1998d. The Atmospheric Sciences: Entering the Twenty-First Century. National Academy Press, Washington, DC. pp. 364.

D1. “Surface temperature, moisture, and fluxes are as strongly linked to land surface properties as they are to atmospheric variables. Satellites provide estimates of surface temperature, vegetation, and moisture characteristics that can be converted into estimates of fluxes but require further calibration against in situ measurements of local characteristics.” (pg. 38)

D2. “The effects of possible climate change on the spatial and temporal distribution of floods and droughts, as well as on patterns of precipitation, temperature, and wind, must be better understood to improve water management designs and strategies... Part of the World Climate Research Program, the Global Energy and Water Cycle Experiment (GEWEX), was initiated in 1988 to observe and model the hydrological cycle and energy fluxes in the atmosphere, at the land surfaces and in the upper oceans. GEWEX will significantly increase our understanding of the water-energy cycle and thus provide the basis for a more sophisticated water management system.” (pgs. 44-45)

D3. “The most critical components of a program to address the scientific issues and challenges are the following:

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• To develop a quantitative description of the processes and interactions that determine the observed distributions of water substance in the atmosphere.

The importance of water, whether vapor, liquid, or solid, in climate and weather processes is self-evident, but there are weaknesses in the current ability to specify the atmospheric water cycle. Among these are poor characterization of upper-troposphere water vapor, uncertainties in surface fluxes and precipitation efficiency, poor representation of ensemble effects of cumulus convection on the transport of water and the characterization of precipitation over oceans, and the absence of a comprehensive understanding of the links between the atmospheric water cycle and other components of the hydrological cycle.” (pgs. 64-65)

D4. “Recommended Cloud Physics Research

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• Increase the attention paid to clouds and their interaction with radiation to (1) permit estimation of the coverage and radiative properties of clouds and (2) improve the theoretical understanding of liquid and solid precipitation formation.

• Conduct critical tests of precipitation mechanism theory with increased attention to dynamic consequences for (1) testing models of warm-rain and ice-phase precipitation processes, (2) evaluating the effects of precipitation production and evaporation on the dynamical evolution of storms, and (3) evaluating the importance of precipitation processes on advertent and inadvertent weather modification.

• Develop the ability to predict size distribution of hydrometeors and aerosol populations to (1) determine their joint influence on the Earth's radiation balance, (2) understand their role in sustaining heterogeneous atmospheric chemical reactions and precipitation formation, and

(3) evaluate the influences of microphysical processes on cloud models and the influence of clouds on climate models.

• Investigate the interactions among aerosols, trace chemical species, and clouds; develop and improve the characterization of atmospheric aerosols to (1) characterize cloud condensation nuclei (CCN) activity in chemical global models and (2) develop representations of the radiative effects of aerosols.” (pg. 66)

D5. “Recommended Boundary Layer Meteorology Research

• Understand the structure of cloudy boundary layers to enable characterization of the effects of boundary layer clouds on climate.

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• Improve measurements of exchange of water, heat, and trace constituents at the Earth's surface. This is fundamental information for use in most aspects of tropospheric functioning.

• Understand model interactions of the planetary boundary layer, surface characteristics, and clouds for use in analytical and predictive models of daily temperature cycle, hydrologic studies, and pollution prediction.” (pg. 67)

D6. “Recommended Small-Scale Dynamics Research

• Develop better representations or parameterizations of physical processes occurring on scales smaller than the grid scale in climate models to improve GCM parameterizations.” (pg. 67)

D7. “Three-Dimensional Models of Radiative Transfer in Cloudy Atmospheres

Recent research has shown that the macrophysical (three-dimensional) variations in cloud fields may be as important in determining the radiative properties of the cloud field as are the microphysical characteristics of clouds. It is important that these macrophysical effects on radiative transfer be quantified and included in parameterizations of radiation used in weather and climate models.” (pg. 72)

D8. “Four-Dimensional Distribution of Water in the Atmosphere

Because radiative transfer in the Earth system is inextricably linked to the components of the hydrologic cycle, understanding the hydrologic cycle and the resulting distribution of water vapor is crucial to a complete understanding of radiative interactions. If climate models are to represent radiative transfer properly, they must incorporate improved representations of the hydrologic cycle on scales ranging from cloud-scale and mesoscale processes to the large-scale circulation. The horizontal and vertical distributions of water vapor play critical roles in determining the radiation fluxes and heating rates, often producing preferred regions for cloud development by their dominating influence on the radiation budget.

Improved Understanding of the Roles of Clouds in Climate

Both model simulations and observations have revealed that cloud-radiative interactions play a significant role in climate and climate change. The fundamental physical issues in this regard represent a huge challenge that can be broken down into a number of steps such as (1)

developing more physically based cloud-radiation parameterizations; (2) including the explicit treatment of cloud microphysical and macroscopic properties in climate models; and (3) incorporating the dependence and influence of these properties on the large-scale dynamics and thermodynamics of the models. Upon successful completion of these three steps, an accurate representation of the radiative heating of the atmosphere should emerge.

Explicit efforts should be directed toward using the results of process studies, cloud-scale and mesoscale model simulations, and long-term cloud-scale data, as well as large-scale global data. This will improve our understanding of the roles of clouds in climate and the parameterization of these effects in climate models.” (pg. 73)

D9. “Coverage and Radiative Properties of Clouds

Cirrus and stratocumulus clouds have received particular attention over the past decade because of their important roles in the radiation balance of the Earth. Both have been the subject of intensive field campaigns and many numerical simulations. Although these studies have led to improved understanding of the nature of such clouds, they still have left important problems to be resolved. Among these are the causes of stratocumulus breakup, the quantitative factors determining entrainment into stratocumulus clouds, the factors determining ice concentrations and size distributions in cirrus clouds, and the detailed interactions with radiation in both cloud types. Additionally, for clouds that undergo substantial entrainment, cloud dynamics could strongly modulate cloud drop number concentration (CDNC), thereby helping to describe the relationship with height between cloud condensation nuclei and cloud drop number concentration. Solution to these problems appears to be feasible in the coming decade. In addition, these studies require extension to clouds of the middle troposphere, which often have a more complicated, mixed-phase structure, and to the cirrus clouds of the tropical troposphere.” (pg. 74)

D10. “Focus 2: Develop a Quantitative Description of the Processes and Interactions That Determine the Observed Distributions of Water Substance in the Atmosphere

Precipitation is the source of essentially all fresh water on Earth, so the hydrological cycle is truly “vital” to humans and to most plant and animal life on land. Although precipitation and cloudiness are the most evident results of the cycling of water through the atmosphere, water also has many other effects on weather and climate. Water vapor is the most important greenhouse gas, and variations in cloudiness and ice cover are the primary sources of variability in the albedo of the Earth... The latent heat released or absorbed as water changes phase is the source of energy that drives hurricanes and other severe weather systems. Thus, we cannot understand weather and climate without a good understanding of the distribution of water substance in the atmosphere.

Weaknesses in current specifications of the atmospheric water cycle include poor characterization of upper-tropospheric water vapor, uncertainties in surface fluxes, poor understanding of the factors controlling precipitation efficiency, inability to represent the ensemble effects of cumulus convection on the transport of water, poor characterization of rainfall over the oceans, and the absence of a comprehensive understanding of the links between the atmospheric cycle and other components of the hydrological cycle... Comprehensive international programs to study the hydrological cycle over regional scales appear feasible and are planned.

These expected new results should provide an opportunity to characterize the distribution of water in the atmosphere with new confidence and to relate this distribution to the

underlying interactions with the global hydrological cycle. Such an improved characterization is needed for accurate determination of radiative transfer in the atmosphere, for climate predictions of precipitation amounts and global temperature, and for improved weather forecasts. These studies thus provide a good match between the opportunities presented by new research capabilities and the needs of current research. However, a comprehensive and systematic approach is required if the many factors and processes entering the atmospheric hydrological cycle are to be understood.” (pgs. 102-103)

D11. “Here, ...key [emerging research opportunities] are summarized, and specific recommendations based on them are offered.

1. The fundamental physics of land-air interaction: Basic understanding of the nature of the interaction between atmospheric and land surface processes is at the threshold of major advances and has the potential, when coupled with greatly improved routine measurements of land surface properties, to lead to substantial improvements in understanding and forecasting convection, boundary-layer cloud cover, and regional climate anomalies. The link between soil moisture and precipitation may be the key to improved quantitative precipitation forecasts.

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5. Improved understanding of the hydrological cycle and much better measurements of atmospheric water: Ongoing advances in understanding the control of atmospheric water (in all phases) will lead to much improved understanding of and ability to predict a variety of dynamical systems. Critical physical processes include the control of water vapor by convection and cloud microphysics, and the coupling of the atmospheric boundary layer with the underlying surface. Improved understanding of these processes, together with the advent of much improved techniques for measuring soil properties, atmospheric water vapor, and condensed water, is essential for solving the difficult problem of quantitative precipitation forecasting and will be necessary for adequate modeling of climate as well.” (pg. 171)

D12. “We make the following recommendations, based in part on recognition of the value of the research opportunities summarized above and in part on further deliberations:

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4. Much-improved understanding of land-atmosphere interaction and far better measurements of land surface properties, especially soil moisture, would constitute a major intellectual advance and may hold the key to dramatic improvements in a number of forecasting problems, including the location and timing of the onset of deep convection over land, quantitative precipitation forecasting in general, and seasonal climate prediction. We see a major opportunity that may be exploited by encouraging interactions between hydrologists and atmospheric scientists and by developing new means of routine and comprehensive measurement of soil properties. ” (pgs. 173-174).

D13. “Practical technologies exist for most of the needed observations. Many are planned as part of major observing systems [e.g., the National Aeronautics and Space Administration (NASA) Earth Observing System (EOS)] or major international research programs [e.g., the Global Energy and Water Cycle Experiment (GEWEX), Climate Variability and Prediction Program (CLIVAR), GOALS–Dec-Cen, and the World Ocean Circulation Experiment (WOCE)]. The challenge will be to form the best total, composite, observational system from this diverse set of efforts and to ensure the continuity and geographic coverage of the data in the face of budgetary constraints and pressure to support activities that promise faster results.” (pg. 308)

D14. “The following requirements are essential...:

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3. Develop focused process studies with the objective of addressing key uncertainties associated with boundary-layer processes and vertical convection; improved linkages coupling the atmosphere, oceans, and land surface; and more explicit representation of land surface processes, including vegetation and soil characteristics.” (pg. 316)