Climate Processes and Model Validation

The largest uncertainties in global climate models are associated with cloud and water vapor feedback processes. Until now, measurements of water vapor from the atmosphere's upper troposphere have been limited, and as a result accurate modeling of water vapor feedbacks with increasing surface temperature a problem for global climate models. Since its launch in 2002, the AIRS instrument has been able to provide data which has shown a positive correlation between sea surface temperature and water vapor at 250 millibar, indicating a "positive" upper tropospheric water vapor feedback with increased surface warming. Other studies using AIRS data have confirmed that water vapor feedback is positive with increased global warming.

Climate models require validation of the processes that govern atmospheric transport and radiation balance. These models usually use reanalysis from forecast centers. Reanalysis is a method of constructing a high-quality climate data record that combines past observations from different sources to produce a picture of how the Earth's climate has evolved over time. Like the weather models, the water vapor fields in the reanalysis have been limited in their accuracy. AIRS data are able to provide a highly accurate daily source of global three dimensional water vapor fields. 

AIRS data have been used in studies to evaluate the climatology of water vapor fields in the major climate models. Studies have shown that the majority of climate models have considerable errors in the vertical and horizontal distribution of water vapor on an annual climatology. It is believed this is a consequence of compensating errors in the vertical distribution of the water vapor climatology. Drier lower troposphere air is compensated by a wetter upper troposphere in the models to produce an overall correct outgoing longwave radiation (OLR). 

Intraseasonal oscillations of intense rainfall and drought are regular annual processes in Indonesia and the Tropical Western Pacific. Called the Madden Julian Oscillation (MJO), the cycle has a period of about 45 days and occurs in the Northern Hemisphere winter, November - April. The AIRS high resolution vertical and horizontal data of temperature, water vapor and ozone amounts have improved scientist's ability to validate theories governing the MJO. For example, AIRS observations show a warm/moist pre-conditioning of the mid troposphere prior to intense rainfall and a cold/dry pre-conditioning prior to drought; temporal and spatial distributions better match expectations than model predictions. 

The AIRS enables observation of temperature waves including gravity waves and mountain waves. Gravity waves are oscillations in the vertical temperature profile over a horizontal distance. AIRS provides good coverage and horizontal resolution enabling observations in and above the troposphere. Each year Polar Stratospheric Clouds (PSCs) form when the temperature in the stratosphere drops below 195K. Certain types of PSCs convert reservoirs of chlorine to an activated form that destroys ozone. Scientists used AIRS data to identify a case whereby formation of PSC's were caused by of an Antarctic Peninsula mountain wave event.