UNIVERSITY OF BREMEN (DE)
The ENFORCE project focuses on the investigation and mitigation of an artifact in tropospheric ozone column (TrOC) data obtained by applying the limb-nadir matching (LNM) technique to a combination of limb-scatter and UV nadir-viewing instruments. This artifact is observed as a band of abnormally high values of TrOC over the northern tropical Pacific and Atlantic and is present in various TrOC LNM data sets. Our preliminary investigation shows that the bias originates from the ozone vertical profiles retrieved from the limb-scatter observations and is most probably caused by a gradient along the instrument line-of-sight (LOS) in the surface and tropospheric reflectivity of the scene underlying the limb scatter observation. The artifact is clearly visible near the location of the permanently persisting band of high clouds in the Inter-Tropical Convergence Zone (ITCZ) of the northern tropical Pacific and Atlantic. However, any location around the globe where the radiance upwelling from the troposphere and surface is inhomogeneous along the LOS of a limb measurement is potentially affected.
The study will initially focus on the improvement of ozone vertical profiles and TrOC retrieved from the Nadir Mapper and Limb Profiler instruments of NASA-NOAA OMPS (OMPS-NM and OMPS-LP). It requires an improvement of the radiative transfer model (RTM) SCIATRAN to account for variations in the reflectivity of the nadir scene underlying the limb instrument LOS and includes the development of a method to account for this variation in the retrieval algorithm. The knowledge gained will be used to create a new tropospheric ozone dataset by matching the limb observations from NASA-NOAA OMPS-LP and nadir measurements from ESA TROPOMI. Potentially, the developed method can be used to improve all limb-scatter stratospheric ozone profile retrievals (e.g. SCIAMACHY, ALTIUS) and obtain high quality tropospheric ozone data by combining measurements from past (SCIAMACHY) and upcoming (e.g. ESA ALTIUS, Sentinel-5) limb-scatter and UV nadir viewing instruments.
