Cloud detection and clearing for the Earth Observing System Terra satellite Measurements of Pollution in the Troposphere (MOPITT) experiment

Abstract

The Measurements of Pollution in the Troposphere (MOPITT) instrument, which was launched aboard the Earth Observing System (EOS) Terra spacecraft on 18 December 1999, is designed to measure tropospheric CO and CH(4) by use of a nadir-viewing geometry. The measurements are taken at 4.7 mum in the thermal emission and absorption for the CO mixing ratio profile retrieval and at 2.3 and 2.2 mum in the reflected solar region for the total CO column amount and CH(4) column amount retrieval, respectively. To achieve the required measurement accuracy, it is critical to identify and remove cloud contamination in the radiometric signals. We describe an algorithm to detect cloudy pixels, to reconstruct clear column radiance for pixels with partial cloud covers, and to estimate equivalent cloud top height for overcast conditions to allow CO profile retrievals above clouds. The MOPITT channel radiances, as well as the first-guess calculations, are simulated with a fast forward model with input atmospheric profiles from ancillary data sets. The precision of the retrieved CO profiles and total column amounts in cloudy atmospheres is within the expected ?10% range. Validations of the cloud-detecting thresholds with the moderate-resolution imaging spectroradiometer airborne simulator data and MOPITT airborne test radiometer measurements were performed. The validation results showed that the MOPITT cloud detection thresholds work well for scenes covered with more than 5-10% cloud cover if the uncertainties in the model input profiles are less than 2 K for temperature, 10% for water vapor, and 5% for CO and CH(4).

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“…MOPITT retrievals are performed for cloud free scenes using a cloud mask determined by data from Terra-MODIS (Ackerman et al, 1998) and MOPITT (Warner et al, 2001).…”

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“…This instrument orbits the earth in a near-polar sun-synchronous configuration at approximately 700 km. The nadir viewing MOPITT instrument is an eight-channel gas correlation radiometer that measures CO profiles from terrestrial thermal radiation in the spectral region near 4.7 μm and total column densities of CO and CH 4 from reflected solar radiation in the spectral region near 2.3 μm and 2.2 μm, respectively (Warner, 2001). The MOPITT instrument makes use of two methods to modulate the transmittance in the gas cell: the cell pressure through the pressure-modulated cell (PMC) and the cell length through a lengthmodulated cell (LMC) (Drummond and Mand, 1996).…”

“…Total CO densities are obtained by taking ratios of the difference signals to the average signals, which depend only on the absorption by CO. The retrieved total CO densities have the MOPITT pixels, 22×22 km in size, and can anticipate 10% precision (Warner et al, 2001;Wang et al, 1999;Pan et al, 1998). More detail MOPITT instruments are summarized by http://www.atmosp.…”

“…Many standard retrieval techniques from the MOPITT instrument have mainly been introduced to retrieve the CO and CH 4 profiles (Warner et al, 2001;Wang et al, 1999;Pan et al, 1998). Consequently, the standard MOPITT retrieval outputs include CO mixing ratios at surface, 850 hPa, 700 hPa, 500 hPa, 350 hPa, 250 hPa and 150 hPa, surface temperature, surface thermal channel emissivity, and error variance of each of these variables.…”

“…Its main objectives are to retrieve profiles and total column densities of CO and CH 4 (http://www.atmosp.physics.utoronto.ca/ MOPITT/home.html). The retrieved total CO densities can anticipate an accuracy of 10 percent (Warner et al, 2001).…”