Tasks : radiative flux, surface temperature and atmospheric temperatures

Task 1 – Atmosphere outgoing radiation derived from IASI data

Radiative flux – Scientific questions to be addressed by IASI-FT:
● What is the temporal/spatial variability for spectrally resolved top-of-atmosphere radiance?
● What are the forcings due to aerosols and clouds on the radiative budget?
● Can anthropogenic contributions and natural phenomena be discriminated?

More that 13 years of global IASI radiance data are now available, with day and night observations at all locations, from which a climatological database for spectrally resolved fluxes can be derived to study regional and temporal variations.

A challenging step for this task is the conversion of spectral radiances (W/cm2/sr/cm−1) to flux values (W/cm2/cm−1), which requires carrying out the angular integration for each frequency. The climatological record for spectrally resolved The-Top-of-Atmosphere (TOA) fluxes will improve our understanding of the interplay between gases and the atmospheric state driving TOA flux variability over the IASI period. Using departures from the mean climatological behavior, we are able to identify and characterize atmospheric and oceanic dynamic phenomena (e.g. El Niño Southern Oscillation, ENSO). Large volcanic eruptions are also seen to affect the transmitted radiation, and their contribution will be quantified. Atlases of the magnitude of the cloud and aerosol radiative forcings will be established in several spectral bands. Reconstructed IASI Outgoing Long-wave Radiation (OLR) are compared with available data from broadband instruments (although the different overpass time might complicate the comparison), as well as with the top net thermal radiation data, available for both all and clear sky from the ECMWF ERA5 reanalysis.

Task 2 – Surface temperature derived from IASI data

Surface temperature – Scientific questions to be addressed by IASI-FT:
● Can time series and maps of surface (land and sea) temperatures be derived from satellite with the required accuracy to analyze natural variability and climate change? 
● How do the seasonal and regional satellite-derived distributions compare with surface local observations and meteorological reanalysis?

Currently long-term temperature time series mostly rely on:

1. Land surface weather stations
2. Ship and buoy observations

However, only satellite data are able to provide systematic global temperature data, at least in cloud-free areas, from pole to pole on a regular basis. Satellites do not measure the near-surface air temperature; instead they measure the skin temperature, a different physical parameter. Skin temperature is derived from IASI upwelling radiances for day-to-day meteorological applications. Emissivity of the surface, which is a function of surface type, viewing angle and wavelength, and temperature, is a primary variable affecting the upwelling radiance. Over the sea, the emissivity is close to one. Above land, accounting for variations in surface emissivity correctly will be essential. For this task, we will carefully select radiative channels in the atmospheric windows, where the atmosphere is relatively transparent for cloud-free scenes. This dataset is used along with appropriate statistical tools to derive accurate skin temperatures locally across the whole globe. We target an accuracy of better than 0.1 K. The new climatology will be compared with the state-of-the-art global in situ land/sea surface temperature records and with the atmospheric reanalysis from ECMWF. The monthly/yearly variations are assessed in terms of the main climate drivers (solar, volcanic eruption, aerosols and greenhouse gases) and modes of variability at inter-annual and longer timescales. Finally, the surface temperature climatology will be compared with climate model (IPSL & NCAR) outputs that take into account forced and natural fluctuations of the coupled Earth system.

Task 3 – Atmospheric temperatures derived from IASI data

Atmospheric temperatures – Scientific questions to be addressed by IASI-FT:
● Can time series and maps of surface (land and sea) temperatures be derived from satellite with the required accuracy to analyze natural variability and climate change?
● How do the seasonal and regional satellite-derived distributions compare with available data?
● How is this vertically-resolved information connected to surface temperature and the top-of-atmosphere outgoing radiation?
● Do the observed ozone/temperature changes match with climate model outputs?

Temperature profiles can be obtained from IASI radiances using CO2 absorption bands at 15 µm (645-800 cm-1) and 4.3 µm (2250-2400 cm-1). Weighting functions from atmospheric radiative transfer models provide information regarding which radiance channel is sensitive to which altitudes. We are investigating the stability for both CO2 absorption bands, and to carefully select the most relevant channels in order to build a long duration homogeneous record for atmospheric temperature at several altitudes. This is achieved by averaging appropriate channels and using statistical tools to extract optimal information. We target an accuracy between 0.5 and 1 K or better.

Once the methodology is established, large-scale processing will be performed for the full IASI period in order to provide a monthly climatology from 2007 onwards. We will also use both the surface and stratospheric temperature monthly climate records, together with ozone distribution maps, to study the seasonal variability and to compare findings with simulation outputs provided by chemistry climate models.

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