Atmospheric Sciences

tools are more fully described in the section on Paleoclimate. A full understanding of Precipitation Processes requires detailed

knowledge of cloud microphysical processes, including chemistry at the droplet formation stage.At the largest scales, atmospheric scientists study Earth as a system. This requires quantification of the energy inputs and outputs.

The primary input to the system is solar energy, which arrives in the form of electromagnetic radiation (mostly at the wavelengths

associated with visible light). Some fraction of the energy that arrives at the top of the atmosphere is reflected back to space, while

some is absorbed by the atmosphere and the rest is transmitted to the surface. At the surface, solar radiation is generally reflected or

absorbed (transmission is possible in the upper layers of water). When absorbed, the result is an increase in surface temperature

and emission of electromagnetic radiation by the surface. Unlike solar radiation, terrestrial radiation lies within the infrared part of

the electromagnetic spectrum. In other words, the Earth receives energy in the form of light and loses energy in the form of heat.JT Schoof, Southern Illinois University, Carbondale, IL, USA

2013 Elsevier Inc. All rights reserved.

Introduction 1

The atmosphere resembles London for in both there are always far more things going on than anyone could properly attend to.

Lewis Fry Richardson

Introduction

The scope of the atmospheric sciences is the structure and behavior of the atmosphere (Figure 1). As a science, the study of the

atmosphere is grounded in observation, theory, and modeling. As a pioneer of weather forecasting, Lewis Fry Richardson knew very

well the challenges of atmospheric modeling and his quote reflects the complexity of the atmospheres behavior, which results from

interactions between the atmosphere and the other climate system components: the hydrosphere, the cryosphere, the biosphere,

and the land surface. The traditional division in the atmospheric sciences has been between meteorology and climatology. The

former has historically focused on forecasting while the latter has addressed longer-term trends and variability.

Advances in observation associated with better land-based and remote sensing platforms, coupled with improved forecasting

techniques, have resulted in vast improvement in both weather and climate prediction in the past few decades. As an example, the

widespread installation of the WSR-88D Doppler Radar platform in the United States starting in the late 1980s has dramatically

improved identification of severe thunderstorms, enhanced the lead time associated with Severe Weather warnings, and excepting a

few remote regions, provided complete coverage on the contiguous US with radar coverage. At the same time, understanding of

transient systems in both the tropics and mid-latitudes (i.e., cyclones) has improved as satellite platforms have become more

specialized. Furthermore, improved observations from all platforms have been successfully blended (atmospheric scientists use the

term assimilated) into models to produce datasets and forecasts that optimally use all of the available information. While

meteorological research still relies heavily on case studies using observations, the datasets produced from data assimilation

(these are often called reanalysis products) have changed the landscape of climate research, providing serially and spatially

complete, gridded data products of standard atmospheric variables.

During the same period of time, the scientific community has increasingly focused on human impact on the environment.

In the atmospheric sciences, examples include depletion of ozone in the stratosphere (2050 km above the surface), degradation of

air quality in many urban areas, and the continuing increase in the atmospheric concentration of carbon dioxide and other

greenhouse gases which have led to an increase in the global near-surface air temperature of approximately 0.74C in the lastcentury, with accelerated warming in the last 50 years (IPCC 2007). Concerns about future environmental change have also figured

substantively into the atmospheric sciences research agenda, with increasing focus on quantifying the response of the climate

system to various forcing mechanisms, ranging from internally generated variability (for example, El Nino Southern Oscillation,

or ENSO) to volcanic eruptions and anthropogenic forcing from aerosols and greenhouse gases. The increasing use of numerical

models (General Circulation Models, or GCMs, Figures 2 and 3) to investigate these responses has drastically improved our

collective understanding of climate system behavior.

Given the challenges of these advancements, there has been substantive cross-fertilization in the subfields of atmospheric

science and most scientists are working at the intersection of the topics presented in the Earth Systems and Environmental Science

module. For example, Boundary Layer Meteorology is the study of the atmospheric layer near the Earths surface where flow is

influenced by the surface. Boundary layer meteorologists use tools that are also effective for studying Air Quality and regional

Atmospheric Transport. Similarly, atmospheric scientists studying the distant past (paleoclimatologists) often use the tools of

chemists, such as isotope analysis, to infer past climate from environmental proxies (tree rings, corals, deep sea sediments). TheseReference Module in Earth Systems and Environmental Sciences http://dx.doi.org/10.1016/B978-0-12-409548-9.05351-3 1

2 Atmospheric SciencesFigure 1 A limb view of earths atmosphere with the silhouette of Space Shuttle Endeavor re-entering the atmosphere. Image credit: NASA.Some of this terrestrial radiation is absorbed by our atmosphere, producing what is commonly referred to as the greenhouse effect. The

gases responsible for atmospheric absorptionof terrestrial radiation are termedgreenhouse gases and include carbondioxide,methane,

nitrous oxide, water vapor, and others. The term enhanced greenhouse effect is sometimes used to describe the augmentation of this

natural process by humans, principally via combustion of fossil fuels and interference with the natural carbon cycle from land use

change. The greenhouse effect demonstrates the link between atmospheric composition and energy transfer within the earth

atmosphere system, topics which are more fully developed in the sections on Atmospheric Gases and Radiative Transfer.

Aerosols are also central to the global energy balance and therefore global temperatures, as well as in important consideration

for Air Quality. The direct effect of aerosols is to scatter or absorb solar radiation, resulting in a net negative radiative forcing.

However, aerosols also modify clouds, and therefore have a secondary, indirect effect on the global (and regional) radiation

balance. Reducing the uncertainty associated with our understanding of the direct and indirect effects of aerosols is one of the most

important contemporary challenges in the atmospheric sciences. This topic is explored in greater detail in the sections on Aerosols,

Radiative Transfer, and Cloud Physics.

As a result of Earth-sun geometry, the amount of solar radiation arriving at the top of the atmosphere (and hence at the surface)

is not uniform across latitudes. Over the course of a year, the tropics experience a net energy gain, while the polar regions experience

a net energy loss. This net energy differential drives the general circulation of the atmosphere. The resulting Global Circulation

Incomingsolar radiation

Backradiation

MountainsLand

Ocean

3-D grid box(CO2, dust, H2Ov)

Figure 2 A simplified schematic of a general circulation model (GCM). Reproduced from Ruddiman, W. F. (2008). Earths climate : Past, present, andfuture. New York: Scientific American.

Atmospheric Sciences 3FAR

~500 km (T2

1)

~250 km (T4

2)

~180 km (T6

3)

SAR

TARPatterns have a tremendous influence on regional climate characteristics. As a sub-discipline, Climate Dynamics focus on

atmospheric motion, especially the interactions between the atmosphere and other components of the global climate system.

Meteorological and climatological information is now being used in many applications, ranging from agricultural management

to energy demand forecasting. Use of such information in applied contexts requires a strong understanding of regional weather and

climate variability as it relates to larger scales. The subfield of Synoptic Climatology considers the local- to regional-scale climate to

be a function of both local environmental factors (physiography) and the large-scale atmospheric circulation. In many applica-

tions, atmospheric circulation patterns are objectively classified, resulting in groups of days with similar characteristics. The

regional climate can then be studied within these groups to better understand the influence of large scale circulation. These

approaches are described in the Climate Classification section.

The challenges for atmospheric science are many. Further advances in Atmospheric Science have the potential to improve our

understanding of natural hazardous weather events, such as tornadoes, cyclones (both in the tropics and mid-latitudes), floods,

droughts, and blizzards, and thereby limit the impact of these events. Climate attribution studies have now identified human influence

in observed changes in global temperature and precipitation. Understanding the complex response of the Earth-atmosphere system to

the range of natural and anthropogenic drivers of variability and change is likely to remain a key challenge in the coming decades.

Further Reading

IPCC (2007) Summary for policymakers. In: Solomon S, Qin D, Manning M, Chen Z, Marquis M, Averyt KB, Tignor M, and Miller HL (eds.) Climate change 2007: The physical sciencebasis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge/New York: Cambridge University Press.

Le Treut H, Somerville R, Cubasch U, Ding Y, Mauritzen C, Mokssit A, et al. (2007) Historical overview of climate change. In: Solomon S, Qin D, Manning M, Chen Z, Marquis M,Averyt KB, Tignor M, and Miller HL (eds.) Climate change 2007: The physical science basis. Contribution of Working Group I to the Fourth Assessment Report of theIntergovernmental Panel on Climate Change. Cambridge/New York: Cambridge University Press.

Ruddiman WF (2008) Earths climate: Past, present, and future, 2nd edn. New York: W.H. Freeman.

~110 km (T1

06)

AR4

Figure 3 The representation of Northern Europe in climate models demonstrating the increase in model resolution in the last few decades. FAR, SAR,TAR, and AR4 refer to the 1st, 2nd, 3rd, and 4th, Assessment Report of the IPCC, respectively (1990, 1996, 2001, and 2007). Reproduced from Le Treut,H., Somerville, R., Cubasch, U., Ding, Y., Mauritzen, C., Mokssit, A., et al., 2007. Historical overview of climate change. In: Solomon, S., Qin, D.,Manning, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor, M., Miller, H.L. (eds.) Climate change 2007 : The physical science basis. Contribution ofworking group I to the fourth assessment report of the intergovernmental panel on climate change. Cambridge/New York: Cambridge University Press.

Atmospheric SciencesIntroductionFurther Reading