Combining ground‐based with satellite‐based measurements in the atmospheric state retrieval: Assessment of the information content

Remote sensing techniques offer the unique possibility to continuously and automatically monitor the atmospheric state from ground and space. Ground‐based microwave radiometers (MWRs), for example, are frequently used for temperature and humidity profiling of the lower troposphere. In order to improve the profiles in the middle and upper troposphere, further information is needed. In this respect, satellite measurements are expected to be very useful. In this study, the synergy benefit in temperature and humidity clear‐sky profiling using different combinations of state‐of‐the‐art microwave and infrared ground‐ and satellite‐based instruments is assessed. The synergy benefit is regarded as the information gain in light of ground‐based MWR observations together with some climatological a priori knowledge. The maximum information content for this kind of synergy is estimated by assuming optimum conditions, e.g., no forward model uncertainties and a horizontal homogeneous atmosphere. For a midlatitude site, the ground‐based MWR gives about 4.4 and 2.4 independent pieces of information on the temperature and humidity profile, respectively. For the temperature profile, the combination with Improved Atmospheric Sounding in the Infrared (IASI) and Atmospheric Microwave Sounding Unit‐A/Microwave Humidity Sounder (AMSU‐A/MHS) increases the information by a factor of about 1.8 and 1.5, respectively, with highest benefit in warm and/or humid conditions. The vertical information on humidity is significantly improved by highly spectrally resolved IR observations from ground or space when the atmosphere is cold and dry; the vertical information is more than tripled. If measurements from AMSU‐A/MHS, IASI, or Spinning Enhanced Visible and Infrared Imager are included, retrieval uncertainties in the middle and upper troposphere are significantly reduced by up to 68%.