Juying Xie Warner

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

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).

Validation of MOPITT retrievals of carbon monoxide

Validation of the MOPITT retrievals of carbon monoxide (CO) has been performed with a varied set of correlative data. These include in situ observations from a regular program of aircraft observations at five sites. Additional in situ profiles are available from several short-term research campaigns. These in situ profiles are critical for the validation of the retrieved CO mixing ratio profiles from MOPITT. Ground-based spectroscopic measurements are compared to MOPITT CO total column densities to validate the observed seasonal cycles. The current validation results indicate good quantitative agreement between MOPITT and in situ profiles, with an average bias less than 20 ppbv. The same seasonal cycles are see in MOPITT and the ground-based spectroscopic data. These validation comparisons provide critical assessments of the retrievals, and continuing improvements to the retrieval algorithms are reducing the validation biases.

EOS MOPITT experiment: extracting the information from the measurements

This paper will serve as an overview of the challenges to the recovery of information on atmospheric CO and CH4 from the measurements made by the MOPITT instrument that has been described by Drummond et al. It will also provide a context and introduction to several of the following papers that go into greater detail on particular topics, and outline plans for the data processing. Here we briefly outline the principles of correlation radiometry as used by MOPITT, and introduce the principles behind the retrievals. After noting plans for data processing, we discuss our approach to data validation, and the ability to see global distributions of CO in the MOPITT data.

Cloud detection and clearing for the MOPITT instrument

The Measurement Of Pollution In The Troposphere (MOPITT) instrument, which will be launched on the Terra spacecraft, is designed to measure the tropospheric CO and CH4 at a nadir-viewing geometry. The measurements are taken at 4.7 micrometer in the thermal region, and 2.3 and 2.2 micrometer in the solar region for CO mixing ratio retrieval, CO total column amount and CH4 column amount retrieval, respectively. To ensure the required measurement accuracy, it is critical to identify and remove any cloud contamination to the channel signals. In this study, we develop an algorithm to detect the cloudy pixels, to reconstruct clear column radiance for pixels with partial cloud covers, and to estimate equivalent cloud top positions under overcast conditions to enable CO profile retrievals above clouds. The MOPITT channel radiances, as well as the first guess calculations, are simulated using 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 plus or minus 10% range. Validations of the cloud detecting thresholds with MODIS Airborne Simulator (MAS) data and MATR (MOPITT Airborne Test Radiometer) measurements are also carried out and will be presented separately.

Retrieval of surface skin temperature from MOPITT measurements: validation and impacts to the retrievals of tropospheric carbon monoxide profiles

The Measurements Of Pollution In The Troposphere (MOPITT) instrument is designed to measure the spatial and temporal variation of the carbon monoxide (CO) profile and total column amount in the troposphere from space. MOPITT channels are sensitive to both thermal emission from the surface and target gas absorption and emission. Surface temperature and emissivity are retrieved simultaneously with the CO profile. To obtain the desired precision for the retrieved CO profiles, it is important to retrieve the surface skin temperature accurately and understand the effects of any errors in retrieved skin temperature on retrieved CO. To demonstrate the impacts of surface skin temperature on the retrieval of the tropospheric CO profile, a simulation study is performed. The collocated Moderate Resolution Imaging Spectroradiometer (MODIS) surface temperatures are used to validate the accuracy of the retrieved MOPITT surface temperatures.

Validation of the retrieval of surface skin temperature and surface emissivity from MOPITT measurements and their impacts on the retrieval of tropospheric carbon monoxide profiles

The Measurements of Pollution In The Troposphere (MOPITT) instrument is designed to measure the spatial and temporal variation of the carbon monoxide (CO) profile and total column amount in the troposphere from the space. MOPITT channels are sensitive to both thermal emission from the surface and target gas absorption and emission. Surface temperature and emissivity are retrieved simultaneously with the CO profile. To obtain the desired 10% precision for the retrieved CO from MOPITT measurements, it is important to understand MOPITT CO channel sensitivity to surface temperature and emissivity and the impacts of the effects of any errors in retrieved skin temperature and emissivity on retrieved CO for various underlying surfaces. To demonstrate the impacts of the surface temperature and emissivity on the retrieval of the tropospheric CO profile, simulation studies are performed. The collocated Moderate Resolution Imaging Spectroradiometer (MODIS) surface products are used to assess the accuracy of the retrieved MOPITT surface temperature and emissivity.

MOPITT cloud detection and its validation

The measurements of Pollution in the Troposphere (MOPITT) instrument aboard the Earth Observing System (EOS) Terra spacecraft measures tropospheric CO and CH4 by use of a nadir-viewing geometry. MOPITT cloud algorithm detects and removes measurements contaminated by clouds before retrieving CO profiles and CO and CH4 total columns. The collocation between MOPITT and MODIS is also established and MODIS cloud mask will be used in the MOPITT cloud algorithm. The cloud detection results in the use of MOPITT data alone agree with MODIS cloud mask for more than 80% of the tested cases.

The EOS MOPITT instrument:validation and early results

Launched on the EOS Terra satellite on December 18, 1999, the MOPITT (Measurements of Pollution in the Troposphere) instrument has been observing tropospheric carbon monoxide (CO) and methane (CH/sub 4/) since March, 2000. The instrument is now fully functional, and the retrieval algorithms are now being validated. Because of the pioneering aspect of this instrument, validation is a particularly important phase of operations and is the main emphasis of this paper.

The measurements of pollution in the troposphere (MOPITT) experiment and some early results

This paper describes the MOPITT experiment and early results. The instrument is an 8 channel nadir-viewing correlation radiometer on the EOS Terra spacecraft. It began collecting data on 4 March 2000, and has operated very stably since then. Processing of data to Level 1 calibrated, geolocated radiances is shaightfomard, but requires that the spectral position of the filters must be adjusted to give agreement with sea surface temperatures. With this done, early retrievals show realistic horizontal and vertical distributions of CO. Validation of these early results is encouraging.

Validating MOPITT cloud detection techniques with MAS images

The Measurements Of Pollution In The Troposphere (MOPITT) experiment will measure the amount of methane and carbon monoxide in the Earths atmosphere utilizing spectroscopy in the near Infrared (IR) (2.2, 2.3, and 4.7 micrometer). In this wavelength region, clouds confound the retrieval of methane and carbon monoxide by shielding both the surface and atmospheric emission below the clouds from MOPITT. A technique has been developed to detect cloudy pixels, and an algorithm has been developed to estimate clear sky radiance from cloud contaminated pixels. This process is validated using images from the MODIS Airborne Simulator (MAS). MAS images are comprised of 50 m pixels in comparison to the larger 22 km MOPITT pixels. We aggregate the higher resolution MAS data to simulate MOPITT pixels. The aggregation is analyzed for clear and cloudy conditions and a cloud fraction is calculated. The aggregate is then averaged to recreate the scene that MOPITT would have seen. The cloud detection algorithms are applied to the degraded MAS image. The results are compared to validate the techniques imbedded in the standard MOPITT processing stream.