Transport and Meteorological Analysis
Research Staff: Leonhard Pfister and Henry Selkirk
The objectives of work performed under this overall task are twofold. First, we provide meteorological guidance to airborne field missions for NASA's Upper Atmosphere Research Program, NASA's Radiation Sciences Program, and NASA's Global Tropospheric Experiment (GTE). This includes providing, in real time, and archiving, extensive meteorological satellite data sets for use by the mission scientist and by the science team. During FY 2001, we did significant meteorological mission preparation work for the upcoming CRYSTAL-FACE mission, a radiation sciences field program aimed at understanding the radiative and microphysical properties of cirrus clouds in the Florida region during summer, 2002. Among the tools available is providing real time information on convectively influenced air for mission planning, essentially by a combination of trajectory modeling and satellite imagery. This technique uses a combination of trajectory modeling and satellite imagery to establish which air masses have been recently influenced by convection. We expect to use it during the upcoming CRYSTAL-FACE mission. For GTE, we have done some preliminary meteorological investigation for the proposed INtercontinental Transport EXperiment (INTEX), to be performed in 2004 and 2005.
The second objective is scientific analysis of the data from the airborne field missions that we support. This divides itself into four basic areas: water vapor and subvisible cirrus clouds in the upper tropical troposphere, water vapor in the winter arctic tropopause region, gravity waves and turbulence, and tropospheric chemistry. The first of these issues is fundamental to the input of water vapor into the stratosphere, which is an important factor in stratospheric gas phase chemistry and for the formation of Polar Stratospheric Clouds (PSCs). The chemistry on PSCs, in turn, is responsible for much of the ozone loss to chlorinated hydrocarbons. The second area, water vapor in the arctic upper troposphere and lower stratosphere, is potentially important to the formation of ice clouds that can have similar chemical effects as the PSCs. Gravity waves are important because: (a) they produce turbulence, which can effect vertical mixing of stratospheric trace constituents; (b) they transport momentum upward, which drives important features of the stratospheric circulation such as the tropical quasi-biennial oscillation; (c) and they produce temperature deviations that can produce subvisible cirrus clouds. The fourth area involves investigating the effects of convection on tropospheric chemical trace constituents, including the distribution of boundary layer trace constituents (either naturally or anthropogenically generated) throughout the troposphere by convection.
To deal with the four areas of scientific analysis, we have developed some important analysis tools. The most novel of these is the "convective influence" calculation, whereby we calculate, using a combination of back trajectories and meteorological satellite data, the amount and age of recent "convective influence" on an air parcel. This technique has been used in connection with the first area of scientific analysis indicated above, namely subvisible cirrus clouds in the upper tropical troposphere. Specifically, we have assessed whether subvisible cirrus clouds observed during the 1995-1996 Tropical Ozone Transport Experiment/Vortex Ozone Transport Experiment (TOTE/VOTE) were produced by local cooling and ice nucleation or were a long-lived outflow from convection. We found good correspondence between the locations of different types of near-tropopause cirrus and the origins of the air, with smooth laminar cirrus clearly the result of local cooling, and lumpier clouds the apparent outflow of convection. Notably, it appears that some of this convective outflow can last several days, based on our calculations. We also noted the presence of inertia-gravity waves and their characteristics. These waves have a very good correspondence to the sloping cloud shapes, indicating that the cooling associated with these waves is probably responsible for the clouds.
This last result is significant, since it lends support to a hypothesis that is the result of modeling work. This hypothesis suggests that long-period waves produce cirrus clouds, which are then heated and lofted into the stratosphere. As the clouds grow, large particles fall out, dehydrating the air. In effect, this mechanism will move air into the stratosphere and dehydrate it at the same time, possibly resolving a crucial question of how very dry air gets into the lower tropical stratosphere.
The convective influence technique has also yielded insight into some of the results from the ACCENT experiment. As a result of this technique, we have been able to trace high values of methyl nitrate in the upper troposphere over the Gulf of Mexico to convection in the eastern Pacific. The significance is that we have the ability to understand one component (convection) of why air masses have the composition that they do. Developing an understanding of the effect of natural processes (e.g. convection) on air masses is certainly a prerequisite for understanding the effect of human processes (e.g., continental scale air pollution).
With the completion of the SOLVE experiment, we have obtained some results on the second area of scientific analysis, namely water vapor in the winter arctic tropopause region. There are three major conclusions. First, troposphere-to-stratosphere exchange extends into the arctic stratosphere to about 13 km, about 3.5 km above the prevailing average tropopause. Second, based on observed water vapor and temperature histories during early Spring, about 20% of air parcels that have ozone values between 300-350 ppbv (in other words, well within the arctic stratosphere) experience ice saturation sometime during a given 10-day period. This is potentially significant, since ice clouds within the lowermost stratosphere can lead to possible chlorine activation. Third, at the tropopause during a given 10-day period during early spring, 5-10% of parcels experience ice saturation even if their water vapor content is at the prevailing stratospheric value of 5 ppmv. This means that the arctic tropopause may act as a major drying mechanism for the upper troposphere during Spring, which is important for the Earth's Radiation Budget.
Point of Contact: Leonhard Pfister, 650/604-3183, firstname.lastname@example.org