Anu Dudhia

Abstract:

The infrared emission limb sounder MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) will be operated as an ESA core instrument on the ENVISAT-1 satellite. Near real time retrieval of pressure, temperature (p,T) and volume mixing ratio (VMR) of five key species (O-3, H2O, N2O, CH4, and HNO3) from calibrated spectra will be performed in the Level 2 processor of the ENVISAT Payload Data Segment. In order to develop an optimized (with respect to speed and accuracy) retrieval algorithm suitable for the implementation in MIPAS Level 2 processor, an ESA supported study is being carried out. In the framework of this study, an optimized forward / retrieval code based on the global fit approach was implemented. In this approach all the spectra of a limb-scanning sequence are simultaneously fitted, so that error propagation in the altitude domain is avoided.The attained accuracy performances of the retrieval code are the following:Temperature error < 2 K at all the altitudes covered by the standard MIPAS scan (8-53 km)Tangent pressure error: < 3%Error on the retrieved VMR of the key species: < 5 % at most of the altitudes of scientific interest covered by the standard MIPAS scan.The run-time required to perform p,T and VMR retrieval of the five MIPAS target species from a limb-scanning sequence of 16 limb-views is less than 6 minutes on a SUN SPARCstation 20. The most effective code optimizations were implemented in the radiative transfer model and in the computation of the jacobian of the retrieval.

Abstract:

An analysis of the measurements of the CO2 15 μm radiance emission by the UARS/ISAMS 30W channel and the kinetic temperature from UARS/WINDII taken on July 21, 1992 at northern latitudes around the mesopause is presented. The modeling of the measurements clearly show evidence of non-LTE emissions in the CO2(v2=1) levels of the minor isotopes 636, 628, and 627 in the 70-90 km region. A comparison with non-LTE model predictions by López-Puertas et al. [1992a] shows a good agreement within the errors in the measured quantities. This constitutes the first experimental evidence of non-LTE emissions in these CO2 15 μm weak bands. The measurements also represent indirect evidence of the net radiative heating produced by these bands around the summer mesopause.

Abstract:

Atmospheric temperature measurements from the Wind Imaging Interferometer (WINDII) on the Upper Atmosphere Research Satellite (UARS), determined from the observed Rayleigh scattering are presented. Infrared radiances predicted from these profiles are compared with measurements from the Improved Stratospheric and Mesospheric Sounder on UARS for nine cases of spatial and temporal coincidence with an effective difference of about 5-10 K in the height range 70-85 km. A comparison with monthly averaged SME results and Rayleigh lidar temperature observations at mid-latitudes taken 8 years earlier indicated that WINDII temperatures were cooler on average by 10 K; this is consistent with a temperature decrease of 1.5 K/year inferred from the SME and lidar results. At high latitudes the agreement between falling spheres and WINDII was found to be very good.

Abstract:

Data from the Stratospheric and Mesospheric Sounder (SAMS) on Nimbus 7 have been used as the basis for a model of the abundances of nitrous oxide and methane in the stratosphere. The model is presented in tabular form on seventeen pressure surfaces from 20 to 0.1-mb, in 10° latitude bins from 50°S to 70°N, and for each month of the year. Some details of the acquisition of the data which went into the model, its limitations, and the general behaviour of methane and nitrous oxide in the middle atmosphere are given. Formal errors in the data and other uncertainties, interannual variability, and systematic trends are discussed. As expected, no trends which exceed the estimated error in the data are found in either methane or nitrous oxide over the five-year period of SAMS observations.

Abstract:

Atmospheric temperature measurements from the improved stratospheric and mesospheric sounder (ISAMS) are evaluated. Flown on the Upper Atmosphere Research Satellite (UARS), ISAMS obtained 180 days of science data between September 26, 1991 and July 29, 1992. Typically, over 2600 temperature profiles/day were retrieved, spaced every 200 km along the limb-viewing track and nominally extending from 100 to 0.01 mbar (15-80 km). The latitude coverage ranged from 80°S to 80°N, depending on the particular ISAMS/UARS viewing geometry on any day. UARS is in a near-Sun-synchronous orbit, so that while the 15 orbits/d are spaced approximately every 24° longitude around the equator, the sampled local solar time actually changes by 20 min/d. The ISAMS temperature retrieval process is outlined and the various products are described. A detailed error budget for the retrieval is presented and comparisons are made with temperature measurements from other sources. Finally, a table is provided summarizing the best estimates of ISAMS temperature bias and precision. The results suggest a general cold bias of around 1 K in the stratospheric temperatures, with a superimposed 2-3 K warm bias associated with the densest part of the Pinatubo aerosol cloud. The precision of individual profiles is ±2 K throughout the stratosphere but falls off in the mesosphere to about ±10 K at 80 km. The error bars produced by the retrieval appear to be reasonable (although slightly pessimistic) estimates of the precision. Copyright 1996 by the American Geophysical Union.