The Atmospheric Sciences Entering the Twenty-First Century (1998) / Chapter Skim
Currently Skimming:
Part II: 1 Atmospheric Physics Research Entering the Twenty-First Century
Part II: 1 Atmospheric Physics Research Entering the Twenty-First Century
Pages 61-106
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 61... ...
Part II Disciplinary Assessments
|
From page 63... ...
Major Scientific Goals and Challenges In each of these areas, we generally have a useful understanding of the physical principles involved at the most fundamental level. However, understanding these physical principles alone does not ensure an adequate understanding of observed atmospheric phenomena because the various realizations of these phenomena are inherently complex and result from complicated interactions among physical processes.
|
From page 64... ...
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
|
From page 65... ...
These improvements include, or will include, in situ and remote sensing methods, characterization of precipitation over the oceans, new modeling capability, and comprehensive international programs to study the hydrological cycle at regional scales. · To improve the capability of making critical measurements in support of studies in atmospheric physics.
|
From page 66... ...
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.
|
From page 67... ...
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. · Represent the effects of moist convection in large-scale models to improve models of potentially destructive mesoscale convective thunderstorm supercells.
|
From page 68... ...
techniques from other areas that include pattern recognition, intelligent systems, artificial intelligence, chaos theory, and computer visualization. INTRODUCTION Mission Atmospheric physics seeks to explain, in terms of basic physical principles, atmospheric phenomena that occur on a variety of temporal and spatial scales.
|
From page 69... ...
Growing attention to climate modeling has increased the need to understand radiative transfer in the atmosphere, and recent results demonstrate the influences of radiation on mesoscale weather systems and weather forecasting. The scope of cloud physics has also broadened considerably, with increased attention directed to the roles of clouds in climate, mesoscale meteorology, and atmospheric chemistry.
|
From page 70... ...
It should be noted further that error estimation is itself often an imprecise quantification in atmospheric science because of a lack of understanding of the propagation of error in nonlinear systems and the inability to create repeatable experiments in the atmosphere. Studies in atmospheric physics are improving our understanding of radiative transfer, precipitation formation, transport, and other fundamental processes in the atmosphere.
|
From page 71... ...
Despite the maturation of the discipline, radiative transfer and remote sensing continue to present significant research challenges. Foremost is the continued dichotomy between the modeling of cloud-radiation interactions at the scale of climate models and the understanding of these same processes at the observational and cloud-scale level.
|
From page 72... ...
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.
|
From page 73... ...
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.
|
From page 74... ...
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)
|
From page 75... ...
Definitive tests of all aspects of precipitation formation are lacking, but data are now available to test many of these processes. Predict Size Distributions of Hydrometeors and the Aerosols That Affect Radiative Transfer in the Atmosphere The important links among cloud structure, aerosols, and trace gases have been the subject of intensive study in connection with acid rain, and attention has been devoted to some aspects of the connection between cloud condensation nuclei and cloud microstructure.
|
From page 76... ...
These include ways of representing the radiative effects of cloud droplets or ice crystals; parameterizations of the effects of clouds on the momentum, heat, and water budgets; documentation of the connections between cloud coverage and large-scale conditions such as relative humidity; satellite observations to provide global coverage; and the development of new modeling techniques to represent clouds and liquid water in mesoscale and global models. Global climate models must represent the effects of clouds in terms of largescale variables, whereas the scales at which important processes occur in clouds are often many orders of magnitude smaller.
|
From page 77... ...
Determining the total electrical activity shows promise as a future observational tool for monitoring severe weather such as tornadoes, hail, flash floods, winter storms, and hurricanes. The field of atmospheric electricity traditionally encompasses six areas of research: (1)
|
From page 78... ...
Continued observational, laboratory, and model efforts are needed on the electrification of clouds containing ice, as well as warm clouds, over geographically different parts of the globe. Investigate the Global Electrical Circuit and Lightning as Measures of Stability and Temperature in Climate Change Studies The global electrical circuit may prove useful in monitoring climate change.
|
From page 79... ...
Structure of Cloudy Boundary Layers Boundary layer cloud has a climatically crucial radiative effect. It is also inextricably coupled to the turbulent and convective dynamics of the boundary layer in which it is embedded.
|
From page 80... ...
The challenge of the next few years is to tightly couple observational strategies, numerical modeling experiments, and parameterizations to deal with inhomogeneous and baroclinic boundary layers. Measurements of the Exchange of Water, Heat, and Trace Atmospheric Constituents at the Earth's Surface Surface heat and moisture fluxes are fundamental to the atmospheric heat engine on all length and time scales.
|
From page 81... ...
Interactions of the Planetary Boundary Layer, Surface Characteristics, and Clouds The boundary layer over land is particularly important as the environment for most human endeavors. Boundary layer processes and boundary layer clouds have important regional impacts on the climate over land and must be considered in tandem with land surface processes that exchange heat and moisture with the atmosphere.
|
From page 82... ...
At present and in the near future, the following actions are likely to be important aspects of the study of small-scale atmospheric dynamics. Effects of Moist Convection in Large-Scale Models We have witnessed a long series of field programs focused on the challenge of determining the structure and evolution of convection and the mesoscale systems in which convection is embedded.
|
From page 83... ...
The breakdown of balance and the production of gravity waves are topics that are beginning to be addressed. These studies could lead to better understanding of local severe weather, large-amplitude gravity waves, clear air turbulence, and stratospheric-tropospheric exchange in tropopause-fold areas.
|
From page 84... ...
Calculations of radiative transfer must also consider SGS distributions of cloudiness and other inhomogeneities in the atmosphere and must represent the effects of irregular ice crystals in cirrus clouds in terms of the GCM variables. Boundary layer fluxes are dependent on the nature of the land surface, which is often quite variable within the grid box of a GCM.
|
From page 85... ...
Develop an Ability to Predict the Extent, Lifetimes, and Microphysical and Radiative Properties of Stratocumulus and Cirrus Clouds We are well positioned to improve parameterizations of marine stratocumulus boundary layer cloud from the insights and data gained from past field experiments in the subtropics and a proposed Arctic experiment in 1997. In particular, the connection between the vertical structure of the boundary layer and the type and amount of cloud cover is becoming much better understood.
|
From page 86... ...
Recent research has shown that tropical upper-tropospheric cirrus clouds play more subtle, though possibly critically important, roles in determining the vertical distribution of water vapor throughout the tropical atmosphere and the strength of tropical atmospheric circulation systems; these sensitivities are apparently manifest through a modulation of the static stability and resulting increase in convective motions and moistening of the upper troposphere by evaporation of cirrus cloud ice. It is very important to capture the essence of these tropical uppertropospheric cirrus cloud systems if we are to simulate the climate successfully.
|
From page 87... ...
A practical goal would be to document the role of aerosol particles in ice formation in some of the simpler cloud systems, including widespread cirrus and upslope stratiform clouds, and to learn the origins of particles responsible for nucleating the formation of ice in conditions, including both homogeneous and heterogeneous nucleation. Quantify and Parameterize Surface Effects on Atmospheric Dynamics The boundary layer transfers heat, moisture, and momentum between the surface and the free troposphere, acting as a valve and a reservoir for these quantities.
|
From page 88... ...
The development of lidar techniques for estimating concentrations of trace gases in the boundary layer (e.g., by differential absorption lidar or Raman scattering) offers the opportunity to obtain vertical cross sections of, for example, water vapor or ozone, which could be used to study how these trace species diffuse, especially in cases of horizontal inhomogeneity.
|
From page 89... ...
This may be particularly applicable to addressing problems of horizontally inhomogeneous flow on scales larger than those that can be addressed by a single ground-based scanning lidar or radar. Airborne lidars and millimeter Doppler radars also provide new opportunities for studying heterogeneity in clear and cloudy boundary layers.
|
From page 90... ...
How do cloud physical processes affect the structure and evolution of convective systems? How can gravity wave drag be realistically parameterized into global circulation models?
|
From page 91... ...
This issue remains controversial, but it is one with many opportunities for progress in the next decade. An opportunity to resolve the lively debate between the "convergence causes convection" school and the "instability causes convection" school may lie in examination of the results of existing field experiments such as the Tropical Ocean Global Atmosphere (TOGA)
|
From page 92... ...
There is a need for critical observational data against which to test key aspects of currently accepted theory. For example, the collision efficiency for collisions among water droplets determines the speed of precipitation formation via the warm-rain process, and rates of secondary ice production influence the concentration and sizes of thunderstorm ice crystals that enter anvil regions.
|
From page 93... ...
Because various forms of these parameterizations enter many climate calculations as well, verifying or improving them would have widespread applicability in the modeling of precipitation-producing systems. Determine the Utility of Lightning Observations and Measurements of the Global Electrical Circuit as Proxy Atmospheric Data The lightning location and detection systems that have become operational in the past decade have been an invaluable aid to forecasters in tracking the motion and intensification or decay of storms, particularly in the western United States where radar coverage is incomplete.
|
From page 94... ...
Lightning frequency and the electrical circuit of the globe may also prove valuable as indicators of climate change. There has been great concern and debate over the possibility of global warming due to increases in greenhouse gases, but it is very difficult to obtain reliable measures of changes in global temperature.
|
From page 95... ...
In view of the importance of tropical thunderstorms to the global lightning budget and climate change, it is of particular importance to give added emphasis to clouds in the tropics. In the past decade, there has been considerable progress toward characterizing the peak current, rise time, electric fields, maximum voltages, and other physical properties of the lightning discharge itself.
|
From page 96... ...
Even these lower estimates are comparable to, or larger than, source strengths in the middle and upper troposphere from stratospheric exchange and subsonic aircraft, and the upper estimates are comparable to anthropogenic sources in the boundary layer. Likewise, lightning is reported to produce other chemical species in lesser concentrations, and further studies are needed here as well.
|
From page 97... ...
Examples include multispectral algorithms to infer optical depth, cloud liquid water content, and trace gas concentration from infrared spectrometers. A second opportunity stems from advances made in research applications of remote sensing from the ground.
|
From page 98... ...
Atmospheric Radiation Measurement (ARM) program, the planned launching of the National Aeronautics and Space Administration's (NASA's)
|
From page 99... ...
In radiation, the exploration of three-dimensional radiative transfer in realistic cloud fields and nonspherical scattering problems are becoming feasible and should be fostered. Large eddy simulation models, including explicit drop and aerosol size distributions and even simple chemistry, are a promising tool for exploring climatically important feedbacks such as those between clouds, aerosols, and radiation, as well as for understanding drizzle processes in boundary layer clouds.
|
From page 100... ...
For example, cloud physicists understand how soluble particles in the atmosphere influence the sizes and number of cloud droplets, and chemists understand the fundamental chemical reactions involved in producing soluble particles. Nevertheless, this understanding has not yet led to a predictive capability for the number of soluble particles in the atmosphere and hence for cloud droplet concentrations and sizes, because these processes interact with each other and with radiation, chemical cycles in the atmosphere, global circulation patterns, and the hydrological cycle in ways that are beyond our current understanding.
|
From page 101... ...
Success in predicting radiative transfer for clear skies, new understanding of the structure and lifetime of stratocumulus and cirrus clouds, new appreciation of the roles of aerosols in the atmosphere, the success of cloud-resolving models in representing ensemble influences of clouds, increasingly realistic models of the atmospheric boundary layer, and the development of techniques for inferring hydrometeor and cloud characteristics from satellite observations are all important steps that can support continued and enhanced focus on these and related problems.
|
From page 102... ...
This is an extremely difficult problem, J1 ustifying a multifaceted approach. This approach should include increased attention to the acquisition of long-term, temporally and spatially consistent data sets characterizing the properties of clouds, radiation, water vapor, and trace gases.
|
From page 103... ...
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. Emerging technologies and recent developments now can support a comprehensive new approach to these problems.
|
From page 104... ...
Numerous examples exist in all areas of atmospheric physics.2 In most cases there are good candidate techniques for making the needed measurements, but they await implementation. The instrumentation in current use is seriously out of date and is not taking advantage of modern knowledge and modern technology.
|
From page 105... ...
Required observations beyond our current capabilities include water vapor profiles in the upper troposphere and lower stratosphere, ice water path and content, and precipitation. Although major additional resources are needed for improvements in instrumentation, a modest program, perhaps combining the efforts of university scien
|
From page 106... ...
Many of the components of the research program recommended here address sources of uncertainty in climate prediction. Other benefits will result from improved abilities to predict regional climate and weather.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.