Key Science Questions
The role of clouds and water vapor in climate change is not well understood; yet water vapor is the largest greenhouse gas and directly affects cloud cover and the propagation of radiant energy. In fact, there may be positive feedback between water vapor and other greenhouse gases. Carbon dioxide and other gases from human activities slightly warm the atmosphere, increasing its ability to hold water vapor. Increased water vapor can amplify the effect of an incremental increase of other greenhouse gases.
Achieving ARM's programmatic objectives should lead to the improved treatment of atmospheric radiation in climate models, explicitly recognizing the crucial role of clouds in influencing radiation and the consequent need for accurate descriptions of the presence and properties of clouds in climate models. Key scientific issues which must be resolved to achieve these objectives include:
- What are the effects of atmospheric constituents, particularly clouds, water vapor and aerosols on the radiative flow of energy through the atmosphere and across the Earth's surface?
- What is the nature of the variability of radiation and the radiative properties of the atmosphere on climatically relevant space and time scales?
- What are the primary interactions among the various dynamic, thermodynamic, and radiative processes that determine the radiative properties of an atmospheric column, including clouds and the underlying surface?
- How do radiative processes interact with dynamical and hydrologic processes to produce cloud feedbacks that regulate climate change?
The first three of these questions can be addressed directly through measurements over time (bolstered by short-term "intensive operational periods"), although modeling studies are also required. The fourth question is more difficult to approach, because cloud feedbacks arise from different physical mechanisms and may have varying sensitivity on diurnal, seasonal and inter-annual time scales. Addressing this question effectively requires extensive modeling of variations of simulated clouds on different time scales, which is then evaluated against observations.
Because ARM is a combined measurement and modeling program (including laboratory measurements where appropriate), a central feature of the Program is a set of instrumented field research locales for measuring atmospheric radiation and the properties controlling this radiation, such as the distribution of clouds and water vapor. A principal objective of the permanently instrumented locales is to take continuous measurements of the spectral radiative energy balance profile under a wide range of meteorological conditions. The intent is that the measurements will be sufficiently comprehensive to allow testing of parameterizations through comparisons of field observations with model calculations of the radiation field and associated cloud interactions.
While stating the intent of these measurements is relatively easy, practical and scientific issues arise in implementing such a system. These issues include not only the development of the instrumentation and the interpretation of the data, but also the selection of the geographical locations in which the measurements should be made. The three ARM locales were selected to provide the opportunity to observe a wide range of climatologically important meteorological conditions. ARM's Mobile Facility will further enhance ARM's ability to measure different climate regions.