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Satellite Data Analyis (SDA) Group
SDA Group Members:
Summary of Activities:
The Satellite Data Analysis Group in ACD focuses on studies of global scale chemical behavior using satellite measurements, meteorological data sets and model simulations. Recent work has focused on understanding the chemical and dynamical behavior of the tropopause region, and its long-term variability, to help quantify processes which contribute to coupling in the upper troposphere – lower stratosphere (UTLS).
William Randel used satellite data to study interannual changes in stratospheric water vapor, focusing on a remarkable step-like decrease observed after 2001 (continuing to present; see Fig. 1). Observations show a contemporaneous cooling in temperatures near the tropical tropopause , and a decrease in tropical ozone at about the same time. The coupled decreases in temperature, water vapor and ozone are consistent with an increase in the stratospheric upwelling circulation in the tropics (the so-called Brewer-Dobson circulation). An increase in the Brewer-Dobson circulation is predicted by models to accompany future climate change, although modeled changes are much smaller than those inferred from the observations after 2001.
Randel also authored a study demonstrating that stratospheric temperature trends derived from historical radiosonde data can have large cooling biases, associated with changes (improvements) in radiosonde instrumentation over time. The biases are demonstrated by direct comparisons between radiosondes and co-located satellite measurements (from the Microwave Sounding Unit, MSU), which often show discontinuities or jumps in the difference time series. The fact that the jumps occur at different times for different radiosonde stations, and not at all for some stations, demonstrates that the biases are a result of radiosonde inhomogeneties . These results allow substantially improved estimates of historical stratospheric temperature trends (by omitting results from radiosonde stations with the most severe problems).
Mijeong Park , a postdoctoral visitor, used satellite and meteorological data sets to study constituent transport near the tropopause within the Asian monsoon anticyclone. This anticyclone dominates the global circulation in the UTLS region during NH summer, and is an important contributor to stratosphere-troposphere coupling. Park used observations of ozone, carbon monoxide and water vapor from the Aura MLS instrument, demonstrating that air with tropospheric chemical characteristics (high CO and low ozone) extends above the tropopause within the anticyclone (the latter acts to confine the constituents within this region). Furthermore, time variability in constituents is closely tied to deep convection in the monsoon region. Ongoing work is aimed at understanding the three-dimensional circulations that contribute to transport in this region.
Andrew Gettelman has focused on understanding the extent of ice supersaturation in the atmosphere and the associated impacts on chemistry and climate. Supersaturation is a common occurrence in the atmosphere, and appears frequently near the tropopause over the globe. A new observational climatology of supersaturation derived from AIRS satellite measurements has been developed, and the satellite data compared in detail with aircraft and balloon measurements. A formulation for supersaturation in CAM has been developed and tested, to examine the impact on climate simulations and chemistry (this parameterization will also be included in future WACCM simulations).
Gettelman has also been a lead coordinator for an international project to evaluate coupled chemistry climate models ( CCMval ). This project is under the auspices of the Stratospheric Processes and their Role in Climate (SPARC) project of the World Climate Research Program (WCRP). Current activities include organizing several international workshops, and contributing to the WMO 2006 Ozone Assessment.
Aerosol indirect effects, i.e. the manner in which changes in aerosol loading influence the reflectivity, lifetime, and precipitation characteristics of clouds, is recognized as an important uncertainty in global climate models. Quantification of these effects as a function of altitude, however, has not been well established. Steven Massie has calculated changes in cloud reflectivity as aerosol loading increases over the Indian Ocean in collaboration with Andy Heymsfield (MMM/NCAR) and Detlef Muller and Patric Seifert of the Leibniz Institute for Tropospheric Research in Leipzig, Germany. Their study utilized satellite observations of aerosol loading and visible and infrared radiances as measured by the Moderate Resolution Imaging Spectroradiometer (MODIS) experiment. Since the MODIS experiment can determine cloud top pressure and temperature, it was possible to quantify aerosol indirect effects as a function of altitude and cloud phase (i.e. liquid droplets and ice crystals). Figure 2 shows cloud reflectivity as a function of aerosol optical depth derived from the MODIS data, demonstrating an increase in reflectivity for higher aerosol optical depth (i.e. a direct measure of the aerosol indirect effect). The measured reflectivity changes are largest in the lower and middle troposphere (corresponding to the region where liquid droplets are present) and very small in the upper troposphere (corresponding to the region where ice crystals are present). Ongoing work is aimed at extending these observations over larger regions of the globe.
Figure 1. (a) Time series of near-global mean (~60 ° N-S) water vapor at 82 hPa derived from HALOE data. The circles show monthly mean values, and error bars denote the monthly standard deviation. (b) Deseasonalized version of the same time series. In both panels the solid lines are running Gaussian-weighted means of the individual points. Note the step-like decrease in the time series of approximately 0.4 ppmv after 2001.
Figure 2. Average 3.75 m m cloud reflectance binned as a function of aerosol optical depth, over the Indian Ocean for February – March 2003-2005. In the lower and middle troposphere, cloud reflectance increases as aerosol optical depth increases, which is a quantitative measurement of the aerosol indirect effect. In contrast, small effects are observed in the upper troposphere.
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