Tropospheric methane in the tropics – first year from IASI hyperspectral infrared observations

Simultaneous observations from the Infrared Atmospheric Sounding Interferometer (IASI) and from the Advanced Microwave Sounding Unit (AMSU), launched together onboard the European MetOp platform in October 2006, are used to retrieve a mid-to-upper tropospheric content of methane (CH4) in clear-sky conditions, in the tropics, over sea, for the first 16 months of operation of MetOp (July 2007–October 2008). With its high spectral resolution, IASI provides nine channels in the 7.7 μm band highly sensitive to CH4 with reduced sensitivities to other atmospheric variables. These channels, sensitive to both CH4 and temperature, are used in conjunction with AMSU channels, only sensitive to temperature, to decorrelate both signals through a non-linear inference scheme based on neural networks. A key point of this approach is that no use is made of prior information in terms of methane seasonality, trend, or geographical patterns. The precision of the retrieval is estimated to be about 16 ppbv (~0.9%). Features of the retrieved methane space-time distribution include: (1) a strong seasonal cycle of 30 ppbv in the northern tropics with a maximum in January–March and a minimum in July–September, and a flat seasonal cycle in the southern tropics, in agreement with in-situ measurements; (2) a latitudinal decrease of 30 ppbv from 20° N to 20° S, in boreal spring and summer, lower than what is observed at the surface but in excellent agreement with tropospheric aircraft measurements; (3) geographical patterns in good agreement with simulations from the atmospheric transport and chemistry model MOZART-2, but with a higher variability and a higher concentration in boreal winter; (4) signatures of CH4 emissions transported to the middle troposphere such as a large plume of elevated tropospheric methane south of the Asian continent, which might be due to Asian emissions from rice paddies uplifted by deep convection during the monsoon period and then transported towards Indonesia. In addition to bringing a greatly improved view of methane distribution, these results from IASI should provide a means to observe and understand atmospheric transport pathways of methane from the surface to the upper troposphere

Detection of blue-absorbing aerosols using near infrared and visible (ocean color) remote sensing observations

An algorithm is presented, which is designed to identify blue-absorbing aerosols from near infrared and visible remote-sensing observations, as they are in particular collected by satellite ocean color sensors. The technique basically consists in determining an error budget at one wavelength around 510 nm, based on a first-guess estimation of the atmospheric path reflectance as if the atmosphere was of a maritime type, and on a reasonable hypothesis about the marine signal at this wavelength. The budget also includes the typical calibration uncertainty and the natural variability in the ocean optical properties. Identification of blue-absorbing aerosols is then achieved when the error budget demonstrates a significant over-correction of the atmospheric signal when using non-absorbing maritime aerosols. Implementation of the algorithm is presented, and its application to real observations by the MERIS and SeaWiFS ocean color sensors is discussed. The results demonstrate the skill of the algorithm in various regions of the ocean where absorbing aerosols are present, and for two different sensors. A validation of the results is also performed against in situ data from the AERONET, and further illustrates the skill of the algorithm and its general applicability

Recent increase of the dust load over the Mediterranean Sea, as revealed from 6 years of ocean color satellite (SeaWiFS) observations

International audience[1] An algorithm was previously developed and validated, which is capable of detecting blue-absorbing aerosols from near infrared and visible remote-sensing observations, as they are in particular collected by satellite ocean color sensors. This algorithm has been applied to 7 years ( 1998 - 2004) of Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) observations over the Mediterranean Sea, on the one hand to further illustrate the appropriateness of ocean color observations to provide relevant information about aerosol types and in particular absorbing aerosols, and, on the other hand, to describe the seasonal and interannual variability of Saharan dust over the Mediterranean Sea during the SeaWiFS era. This extensive application allowed the validation of the retrieved aerosol optical thickness to be more thoroughly performed, thanks to data from the Aerosol Robotic Network (AERONET). The results of this validation and the mapping of dust aerosols and of the associated optical thickness demonstrate the skill of the algorithm. These results are in agreement with, and provide a complement to, the results of previous studies based on other remote sensing techniques, which were applied to data from the 1980s and early 1990s. They also show an increase of the dust transport over the Mediterranean over the 1998-to-2004 time period

Simultaneous retrieval of tropospheric CO2 and CH4 in the tropics: almost two years from IASI hyperspectral infrared observations.

International audienceCoupled observations from the Infrared Atmospheric Sounding Interferometer (IASI) and from the Advanced Microwave Sounding Unit (AMSU), launched together onboard the European MetOp platform in October 2006, are used to retrieve mid-to-upper tropospheric contents of carbon dioxide (CO2) and methane (CH4) in clear-sky conditions, in the tropics, since the first month of operation of MetOp (July 2007). In April, 20 months will be available. With its very high spectral resolution, IASI provides a few channels located either in the 15 m band or in the 7.7 m band highly sensitive to, respectively, CO2 and CH4, with reduced sensitivities to other atmospheric variables. These channels, sensitive to both temperature and either CO2 or CH4, are used in conjunction with AMSU channels, only sensitive to temperature, to decorrelate both signals through a non-linear inference scheme based on neural networks. A key point of this approach is that no use is made of prior information in terms of gas seasonality, trend, or geographical patterns. The accuracy of the retrieval is estimated to be about 2 ppmv (less than 1%) for CO2 and 16 pbbv (~0.9%) for CH4. Features of the retrieved methane space-time distribution include: (1) a strong seasonal cycle in the northern tropics, and a lower seasonal cycle in the southern tropics, in agreement with in-situ measurements; (2) a latitudinal decrease from 20 °N to 20 °S lower than what is observed at the surface but in excellent agreement with tropospheric aircraft measurements; (3) geographical patterns in good agreement with simulations from atmospheric transport and chemistry models, but with a higher variability; (4) signatures of CO2/CH4 emissions transported to the troposphere such as a large plume of elevated tropospheric methane south of the Asian continent, which might be due to Asian emissions from rice paddies uplifted by deep convection during the monsoon period and then transported towards Indonesia. In addition to bringing a greatly improved view of CO2 and CH4 distribution, these results from IASI should provide a means to observe and understand atmospheric transport pathways of these two greenhouse gases from the surface to the upper troposphere

Simultaneous retrieval of tropospheric CO2 and CH4 in the tropics: almost two years from IASI hyperspectral infrared observations.

International audienceCoupled observations from the Infrared Atmospheric Sounding Interferometer (IASI) and from the Advanced Microwave Sounding Unit (AMSU), launched together onboard the European MetOp platform in October 2006, are used to retrieve mid-to-upper tropospheric contents of carbon dioxide (CO2) and methane (CH4) in clear-sky conditions, in the tropics, since the first month of operation of MetOp (July 2007). In April, 20 months will be available. With its very high spectral resolution, IASI provides a few channels located either in the 15 m band or in the 7.7 m band highly sensitive to, respectively, CO2 and CH4, with reduced sensitivities to other atmospheric variables. These channels, sensitive to both temperature and either CO2 or CH4, are used in conjunction with AMSU channels, only sensitive to temperature, to decorrelate both signals through a non-linear inference scheme based on neural networks. A key point of this approach is that no use is made of prior information in terms of gas seasonality, trend, or geographical patterns. The accuracy of the retrieval is estimated to be about 2 ppmv (less than 1%) for CO2 and 16 pbbv (~0.9%) for CH4. Features of the retrieved methane space-time distribution include: (1) a strong seasonal cycle in the northern tropics, and a lower seasonal cycle in the southern tropics, in agreement with in-situ measurements; (2) a latitudinal decrease from 20 °N to 20 °S lower than what is observed at the surface but in excellent agreement with tropospheric aircraft measurements; (3) geographical patterns in good agreement with simulations from atmospheric transport and chemistry models, but with a higher variability; (4) signatures of CO2/CH4 emissions transported to the troposphere such as a large plume of elevated tropospheric methane south of the Asian continent, which might be due to Asian emissions from rice paddies uplifted by deep convection during the monsoon period and then transported towards Indonesia. In addition to bringing a greatly improved view of CO2 and CH4 distribution, these results from IASI should provide a means to observe and understand atmospheric transport pathways of these two greenhouse gases from the surface to the upper troposphere

Simultaneous retrieval of tropospheric CO2 and CH4 in the tropics: almost two years from IASI hyperspectral infrared observations.

International audienceCoupled observations from the Infrared Atmospheric Sounding Interferometer (IASI) and from the Advanced Microwave Sounding Unit (AMSU), launched together onboard the European MetOp platform in October 2006, are used to retrieve mid-to-upper tropospheric contents of carbon dioxide (CO2) and methane (CH4) in clear-sky conditions, in the tropics, since the first month of operation of MetOp (July 2007). In April, 20 months will be available. With its very high spectral resolution, IASI provides a few channels located either in the 15 m band or in the 7.7 m band highly sensitive to, respectively, CO2 and CH4, with reduced sensitivities to other atmospheric variables. These channels, sensitive to both temperature and either CO2 or CH4, are used in conjunction with AMSU channels, only sensitive to temperature, to decorrelate both signals through a non-linear inference scheme based on neural networks. A key point of this approach is that no use is made of prior information in terms of gas seasonality, trend, or geographical patterns. The accuracy of the retrieval is estimated to be about 2 ppmv (less than 1%) for CO2 and 16 pbbv (~0.9%) for CH4. Features of the retrieved methane space-time distribution include: (1) a strong seasonal cycle in the northern tropics, and a lower seasonal cycle in the southern tropics, in agreement with in-situ measurements; (2) a latitudinal decrease from 20 °N to 20 °S lower than what is observed at the surface but in excellent agreement with tropospheric aircraft measurements; (3) geographical patterns in good agreement with simulations from atmospheric transport and chemistry models, but with a higher variability; (4) signatures of CO2/CH4 emissions transported to the troposphere such as a large plume of elevated tropospheric methane south of the Asian continent, which might be due to Asian emissions from rice paddies uplifted by deep convection during the monsoon period and then transported towards Indonesia. In addition to bringing a greatly improved view of CO2 and CH4 distribution, these results from IASI should provide a means to observe and understand atmospheric transport pathways of these two greenhouse gases from the surface to the upper troposphere

The 2007-2011 evolution of tropical methane in the mid-troposphere as seen from space by MetOp-A/IASI

International audienceSince July 2007, monthly averages of midtropospheric methane have been retrieved in the tropics over land and sea, by day and night, from IASI onboard MetOp- A, yielding a complete view of the geographical distribution, seasonality and long-term tendency of methane in the mid-troposphere. Retrieved methane displays a clear seasonal cycle of ~25 ppbv in the northern tropics, with a maximum in November and a minimum in April-May, a more complex cycle of ~15 ppbv in the southern tropics, and a south-to-north latitudinal variation of ~30 ppbv - in good agreement with regular aircraft measurements of the CONTRAIL program. Comparisons with CARIBIC aircraft measurements made at ~11 km yield an averaged difference between collocated IASI estimates and CARIBIC measurements of 7.2 ppbv with a standard deviation of 13.1 ppbv. Comparisons with aircraft measurements made above 6 km during five HIPPO campaigns give an averaged difference between collocated IASI estimates and HIPPO measurements of 5.1 ppbv with a standard deviation of 16.3 ppbv. These comparisons show that IASI captures well the evolution of mid-tropospheric methane. In particular, in 2007 and 2008, IASI shows an increase of mid-tropospheric methane in the tropical region of 9.5±2.8 and 6.3±1.7 ppbv yr-1, respectively - in excellent agreement with the rate of increase measured at the surface after almost a decade of nearzero growth. IASI also indicates a slowing down of this increase in the following years to ~2 ppbv yr-1, with the highest increase in 2010. Assuming that the recent evolution of methane is mostly due to an increase in surface emissions, IASI might indicate a decrease in tropical wetland emissions for the period 2009-2011 compared to 2007-2008, in agreement with decreasing tropical precipitation over this period, together with an increase in biomass burning emissions in 2010 in the southern tropics. © Author(s) 2013

CALIPSO IIR Version 2 Level 1b calibrated radiances: analysis and reduction of residual biases in the Northern Hemisphere

Version 2 of the Level 1b calibrated radiances of the Imaging Infrared Radiometer (IIR) on board the Cloud-Aerosol Lidar and Infrared Satellite Observation (CALIPSO) satellite has been released recently. This new version incorporates corrections of small but systematic seasonal calibration biases previously revealed in Version 1 data products mostly north of 30° N. These biases – of different amplitudes in the three IIR channels 8.65 µm (IIR1), 10.6 µm (IIR2), and 12.05 µm (IIR3) – were made apparent by a striping effect in images of IIR inter-channel brightness temperature differences (BTDs) and through seasonal warm biases of nighttime IIR brightness temperatures in the 30–60° N latitude range. The latter were highlighted through observed and simulated comparisons with similar channels of the Moderate Resolution Imaging Spectroradiometer (MODIS) on board the Aqua spacecraft. To characterize the calibration biases affecting Version 1 data, a semi-empirical approach is developed, which is based on the in-depth analysis of the IIR internal calibration procedure in conjunction with observations such as statistical comparisons with similar MODIS/Aqua channels. Two types of calibration biases are revealed: an equalization bias affecting part of the individual IIR images and a global bias affecting the radiometric level of each image. These biases are observed only when the temperature of the instrument increases, and they are found to be functions of elapsed time since night-to-day transition, regardless of the season. Correction coefficients of Version 1 radiances could thus be defined and implemented in the Version 2 code. As a result, the striping effect seen in Version 1 is significantly attenuated in Version 2. Systematic discrepancies between nighttime and daytime IIR–MODIS BTDs in the 30–60° N latitude range in summer are reduced from 0.2 K in Version 1 to 0.1 K in Version 2 for IIR1–MODIS29. For IIR2–MODIS31 and IIR3–MODIS32, they are reduced from 0.4 K to close to zero, except for IIR3–MODIS32 in June, where the night-minus-day difference is around −0.1 K