Radiative Processes and Trace Gases in the Middle Atmosphere

K. Minschwaner
Department of Physics
New Mexico Tech
Socorro, NM 87801
phone: 505+835-5226
fax: 505+835-5707
e-mail: krm@kestrel.nmt.edu

Undergraduate Student-Researcher: Robert Carver (carver@kestrel.nmt.edu)

Project Abstract:

Increasing atmospheric burdens of the trace gases CH4, N2O, and chlorofluorocarbons can have a significant impact on the infrared radiation budget and photochemistry of the atmosphere. One element of this project involves application of UARS measurements of these gases to accurately evaluate the infrared radiative forcing at the tropopause level and heating rates in the stratosphere. Variations in the radiative forcing with respect to latitude and season, both related to changes in the trace gas distributions in the lower stratosphere and to variations in temperature and tropospheric cloud fraction, thickness, and height, are being studied using longwave radiation code initialized with complementary satellite climatologies (for example, cloud properties from the International Satellite Cloud Climatology Project).

The second part of the project involves UARS distributions of N2O, CFC11 and CFC12 to determine global destruction rates and instantaneous lifetimes based on rates of stratospheric photolysis. These calculations incorporate solar irradiances measured by UARS instruments and include a line-by-line representation of opacity in the oxygen Schumann-Runge (S-R) system, as well as a precise treatment of UV scattering in the region of S-R/Herzberg overlap near 200 nm.

The new evaluations of radiative forcing and lifetimes will allow for improved estimates for the Global Warming Potentials (GWPs) of these gases. The lifetime for CFC11 has further implications for the Ozone Depletion Potential (ODP) of replacement chemicals for the CFCs.

Examples:

The figure shown below displays the June-July average distribution of CF2Cl2 (CFC-12) computed from daily vertical profiles obtained by the Cryogen Limb Array Etalon Spectrometer (CLAES). The dashed white line is the climatological tropopause; the dashed black line corresponds to the lower boundary of the CLAES observations. The tropospheric distribution is set by surface measurements, and the region between the dashed curves is set by cubic spline interpolation.

The next figure shows the calculated distribution of radiative forcing by CFC-12 on the basis of the observed seasonal and latitudinal distribution (as shown above for June-July). The highest values occur in the summertime subtropical regions, primarily as a result of i) the vertical distribution of CF2Cl2, ii) the high, cold tropopause, and iii) the low fraction of high cloud. These results are still preliminary, but clearly demonstrate the utility of the UARS data to compute global fields of relevance to global change issues.

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This information is being made available by Ken Minschwaner at New Mexico Institute of Mining and Technology's Physics Department.