11th Conference on Atmospheric Radiation and the 11th Conference on Cloud Physics

Monday, 3 June 2002
Evaluation of Terra aerosol and water vapor measurements using ARM SGP data
Richard Ferrare, NASA/LRC, Hampton, VA; and L. Brasseur, M. Clayton, D. Turner, L. Remer, and B. C. Gao
Poster PDF (99.0 kB)
Measurements from the Department of Energy Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site are used to evaluate atmospheric measurements derived from NASA's Moderate-Resolution Imaging Spectroradiometer (MODIS) and Multiangle Imaging Spectroradiometer (MISR) sensors on board the Terra satellite. Both sensors have been used to retrieve aerosol optical thickness over land as well as water. The MODIS aerosol retrieval algorithm operates by comparing radiances measured at several visible and near-infrared wavelengths with tabulated radiances that have been computed for specific aerosol models. Over land, MODIS retrieves aerosol optical thickness (AOT) at two visible wavelengths with a spatial resolution of 10x10 km. MISR measures radiances at four wavelengths at nine discrete view angles. These path radiances are compared with path radiances computed using different aerosol models to derive AOT for 17.6 km x 17.6 km regions.

MODIS and MISR AOT retrievals are evaluated using ARM SGP Cimel Sun photometer and MultiFilter Rotating Shadowband Radiometer (MFRSR) AOT measurements. The generally low range of AOT (0-0.3 at 470 nm) for these comparisons over the ARM SGP site generally has led to large rms differences between the SGP and MODIS measurements. MODIS and SGP AOT (470 nm) were within about 10-20% for AOT>0.1. Larger differences were found between the MODIS and SGP retrievals of AOT at 660 nm where the range of AOT was even smaller than at 470 nm. However, at both wavelengths, differences between the MODIS and SGP AOT measurements were generally within the uncertainty of the MODIS AOT measurements (0.050.20*AOT). Work is underway to perform similar comparisons between the MISR and SGP AOT measurements.

MODIS also has several channels located within and around the 940 nm water vapor absorption region for retrieving precipitable water vapor (PWV) amounts during the day. Comparisons with microwave radiometer (MWR) and Cimel PWV measurements show the MODIS near-infrared water vapor retrievals systematically overestimated PWV by 20-30% for retrievals acquired before November 2000. Recent laboratory measurements indicate that this overestimate is likely due to errors in water vapor line intensities that are listed in the HITRAN-96 database and were initially used in the MODIS near-infrared water vapor retrievals. The MODIS near IR PWV retrievals performed after November 1, 2000, which use updated water vapor line intensities, are only slightly (~10%) higher than the SGP measurements.

Water vapor profiles are obtained using MODIS IR radiances measured between 6.7 and 8.5 mm in a manner similar to that performed for the operational GOES profiling product. PWV values obtained via integration of the derived MODIS water vapor profiles were generally 10-20% higher than the SGP measurements.

The diurnal variations of vertical distributions of aerosol extinction, water vapor mixing ratio, and relative humidity are examined using SGP Raman lidar data acquired since March 1998. These measurements show larger differences exist in these vertical profiles than in the diurnal variations of AOT and PWV.

Supplementary URL: