TROPOSPHERE, AIR QUALITY, & CLIMATE
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Water vapor plays an important role in global climate change through its influence in hydrological and energy cycles, and for its radiative effect as the strongest greenhouse gas. The ability of the atmosphere to hold more water vapor with increasing temperature leads to an important positive feedback for the estimate of global warming in response to the increase in CO2. This feedback is highly sensitive to upper tropospheric water vapor (UTWV). Meanwhile, clouds provide significant radiative forcing to the climate system in their own right.
The Microwave Limb Sounder on Aura satellite provides unprecedented simultaneous measurements of UTWV and cloud ice profiles. Analysis of these datasets has contributed and will continue contributing to our understanding of the dynamics controlling water vapor and cloud variations, and to help quantifying their feedbacks to climate change. In particular, MLS observations contradict the so-called 'Iris hypothesis' which holds that changes in upper tropospheric clouds will give a negative feedback on climate change.
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Numerical models are the primary tools for weather forecast and climate change predictions. Accurate representation of processes involving water vapor, clouds and atmospheric composition has been a challenging problem in models for years. Improvements of weather forecast and climate change predictions require extensive evaluation of model simulations by comparison with global satellite observations. The Aura MLS measurements of water vapor, clouds and other tracer gases provide a unique opportunity to evaluate model performance in the upper troposphere (~8-15km altitude) and to improve model parameterizations of key processes such as convection.
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MLS observations of ozone (O3) and carbon monoxide (CO) provide important information on the chemical and transport processes affecting air pollution in the upper troposphere (~10-15km altitude). CO is a byproduct of combustion associated with both vehicles and industry and with forest fires and domestic fires using for cooking and heating. CO is also produced when other organic molecules in the atmoshpere break down. CO has a lifetime of about a month in the upper troposphere, making it a good marker of recently polluted air. O3 forms down wind of pollution sources, and is the result of the reaction of 'NOx' species (typically emitted from industry and cars, or formed in lightning flashes) with breakdown products from organic species (emitted from industrial and natural sources). Ozone is an important contributor to poor air quality and has a strong daily cycle in the lowest ~2km of the atmosphere. However, its chemical lifetime can be over a month once it is transported to higher altitudes. Descent of ozone-rich air from the stratosphere can increase tropospheric ozone. MLS also measures nitric acid (HNO3) in the upper troposphere, giving information on 'NOx' pollution for which it is an end product.
Air can be transported into the upper troposphere by weather fronts and, particularly rapidly, by the strong convection in thunderstorms. Winds are typically stronger at these higher altitudes. This means that, once lofted to higher altitudes, polluted air can be transported on intercontinental and global scales. Observations from MLS and other sensors regularly show that pollution emitted from one region of the planet affects the air quality above other regions and countries.
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Aerosols (small particles of dust or soot, or small droplets of sulphate solution or organic liquids) can affect the Earth's radiation budget and climate in two important ways: directly and indirectly. Aerosols can absorb incoming sunlight and also reflect it back to space. This is called the aerosol 'direct' effect. Aerosols also indirectly affect climate by modifying clouds and precipitation when they act as cloud condensation nuclei (i.e., particles provide surfaces on which cloud droplets can form). This is called the aerosol indirect effect. Measuring aerosol indirect effect is challenging, because aerosols cannot be easily detected by satellite sensors when they are inside clouds. One way to solve this problem is to use carbon monoxide (CO) to infer the presence of aerosols since they both are produced by incomplete combustion that occurs, for example, in fires. CO can be measured by MLS inside high clouds, because the microwave wavelengths observed by MLS are larger than the typical cloud particle size. Once MLS measurements identified whether the clouds are clean or dirty by the amount of CO concentration they have, other data from the 'A-train' satellite instruments, such as cloud particle size from Aqua MODIS and precipitation from TRMM, can be used to examine the aerosol indirect effect. Changes of cloud particle size and precipitation due to aerosols are important information for climate studies, and for understanding the response of the climate system to changes in air pollution.
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