Liwen Pan

Retrieval of tropospheric carbon monoxide for the MOPITT experiment

A retrieval method for deriving the tropospheric carbon monoxide (CO) profile and column amount under clear sky conditions has been developed for the Measurements of Pollution In The Troposphere (MOPITT) instrument, scheduled for launch in 1998 onboard the EOS-AM1 satellite. This paper presents a description of the method along with analyses of retrieval information content. These analyses characterize the forward measurement sensitivity, the contribution of a priori information, and the retrieval vertical resolution. Ensembles of tropospheric CO profiles were compiled both from aircraft in situ measurements and from chemical model results and were used in retrieval experiments to characterize the method and to study the sensitivity to different parameters. Linear error analyses were carried out in parallel with the ensemble experiments. Results of these experiments and analyses indicate that MOPITT CO column measurements will have better than 10% precision, and CO profile measurement will have approximately three pieces of independent information that will resolve 3-5 tropospheric layers to approximately 10% precision. These analyses are important for understanding MOPITT data, both for application of data in tropospheric chemistry studies and for comparison with in situ measurements.

Hemispheric asymmetries and seasonal variations of the lowermost stratospheric water vapor and ozone derived from SAGE II data

The distributions of water vapor and ozone in the lowermost stratosphere, the part of the stratosphere between the tropopause and the 380-K potential temperature surface, are examined using the Stratospheric Aerosol and Gas Experiment II measurements for the period of March 1989 to February 1990. The monthly mean values of water vapor and ozone are computed for the stratospheric portions of the 320-K and 350-K potential temperature surfaces for both hemispheres. We find a strong seasonal cycle in the water vapor mixing ratio on the 320-K potential temperature surface for both hemispheres, with maximum values in the summer season and minimum values in early spring. The seasonal cycle at the 350-K isentrope is similar to that at the 320 K but with a reduced amplitude. Comparisons with their distributions on the 420-K isentrope are made in order to explore information on stratosphere-troposphere exchange via the extratropical tropopause. Both water vapor and ozone data suggest that troposphere to stratosphere exchange across the extratropical tropopause has a significant influence on the lowermost stratosphere, especially for the northern hemisphere in the summer season.

Spatial distributions of upper tropospheric water vapor measurements from the UARS Microwave Limb Sounder

We characterize measurements of upper tropospheric (∼300–150 hPa) water vapor obtained from the Microwave Limb Sounder (MLS) instrument on board the Upper Atmosphere Research Satellite (UARS) in terms of their spatial and temporal variability. We present the climatology of water vapor mixing ratio for October 1991-June 1997, which includes the seasonal means, the root-mean-square deviations, and the seasonal differences. The climatology of the divergent wind field is compiled to examine the relationship of upper tropospheric moisture fields and the tropical circulations. The tropical wet and dry patterns are latitudinally distributed in accordance with the Hadley circulation, and longitudinal distributions correspond well to the Walker circulation. We quantify the frequency of dry tropical observations in seasonal maps. The most prominent dry regions are located in the subtropics. On the 316 hPa surface there is some occurrence of low values of humidity throughout the majority of the tropics, while at 215 hPa it is primarily the subsidence regions that show dry values. Subtropical dry observations are more frequent in the Southern Hemisphere winter than in the Northern Hemisphere winter. The seasonal cycle of MLS water vapor is compared with Stratospheric Aerosol and Gas Experiment II (SAGE II) measurements. Good agreement is found between these two data sets for measurements near the extratropical tropopause. Potential vorticity analysis is used to indicate whether extratropical measurements were influenced primarily by stratospheric or tropospheric air. Frequency distributions display distinct characteristics and seasonal dependence for these two regions.

Simultaneous observations of polar stratospheric clouds and HNO3 over Scandinavia in January, 1992

Simultaneous observations of Polar Stratospheric Cloud (PSC) aerosol extinction and HNO 3 mixing ratios over Scandinavia are examined for January 9-10, 1992. Data measured by the Microwave Limb Sounder (MLS), Cryogenic Limb Array Etalon Spectrometer (CLAES), and Improved Stratospheric and Mesospheric Sounder (ISAMS) experiments on the Upper Atmosphere Research Satellite (UARS) are examined at locations adjacent to parcel trajectory positions. Regression coefficients, obtained from Mie calculations, are used to transform aerosol extinctions into aerosol volume densities. Graphs of volume density versus temperature, and importantly, HNO 3 mixing ratio versus temperature, show volume increases and simultaneous loss of HNO 3 as temperatures decrease. The data is consistent with initial PSC growth processes which transform sulfate droplets into temary droplets or nitric acid dihydrate (NAD) particles.

Instrument Sensitivity and Error Analysis for the Remote Sensing of Tropospheric Carbon Monoxide by MOPITT

Abstract Measurement of Pollution in the Troposphere (MOPITT) is an eight-channel gas correlation radiometer selected for the Earth Observing System AM-1 platform to be launched in 1999. Its primary objectives are the measurement of tropospheric carbon monoxide (CO) and methane (CH4). In this paper, the sensitivities of instrument signals and CO retrieval errors to various instrument parameters, especially the gas cell pressure and temperature variations, instrument radiometric noise, and ancillary data errors (such as atmospheric temperature and water vapor profile errors), are presented and discussed. In the MOPITT pressure modulator cell pressure sensitivity study, the instrument calibration process is considered, which leads to the relaxation of previous stringent requirements on the accuracy of in-orbit cell pressure monitoring. The approach of MOPITT CO retrieval error analysis is described, and the error analysis results are compared with retrieval simulation statistics. The error analysis results ...

Retrieval of Tropospheric Carbon Monoxide Profiles from High-Resolution Interferometer Observations: A New Digital Gas Correlation (DGC)Method and Applications

Abstract Global tropospheric carbon monoxide (CO) distributions can be retrieved from observations by spaceborne gas correlation radiometers and high-resolution interferometers. The Measurement of Pollution in the Troposphere (MOPITT) is a gas correlation radiometer designed for tropospheric CO and CH4 remote sensing. It is being developed at the University of Toronto and the National Center for Atmospheric Research for launch on the EOS/AM-1 platform in 1999. Spaceborne high-resolution interferometers with troposphere CO remote sensing capability include the Interferometric Monitor for Greenhouse gases (IMG) instrument and the Troposphere Emission Spectrometer (TES). IMG was developed by the Ministry of International Trade and Industry (MITI) of Japan. It was on the ADEOS-1 spacecraft launched in October 1996. TES is being developed by the Jet Propulsion Laboratory for launch on the EOS/CHEM-1 platform in 2002. For the purpose of testing the MOPITT data processing algorithms before launch, a new digital ...

Analysis and characterization of the retrieval algorithm for measuring tropospheric CO using the MOPITT instrument

We have developed a retrieval algorithm for deriving the tropospheric CO profile and column amount from the radiances measured by the Measurements of Pollution in the troposphere instrument. The main components of the algorithm are a fast radiative transfer model, based on the GENLN2 line-by-line model, and a maximum likelihood inversion method. The retrieval a priori information is derived from the results of several aircraft in situ measurements and a 3D chemical- transport model. This paper discusses the CO retrieval algorithm with an emphasis on the analysis and characterization of the algorithm. Forward model and retrieval sensitivities, along with the a priori information used in the retrieval are discussed in terms of their orthogonal components. Examples of ensemble retrieval experiments are also included.

Model studies and retrieval algorithm development for the MOPITT experiment

The Measurements of Pollution in the Troposphere (MOPJTF) instrument is a spaceborne gas correlation radiometer designed to measure CO and CH4 in the troposphere. This instrument has been selected to be on board of the Earth Observing Systems first platform, EOS-AM, which is scheduled for launch in 1998. A maximum likelihood retrieval algorithm has been selected for the MOPITT CO measurement in clear sky conditions. Performance of the algorithm has been evaluated. This paper describes the algorithm and presents the preliminary results of numerical retrieval experiments.