DMSP special sensor H (SSH) data rescue and assimilation in reanalysis

The accuracy of climate reanalyses is limited by observation availability, especially satellite soundings prior to 1979, when operational polar-orbiting satellites become more prevalent. This paper reports on successful efforts to locate the data record collected by the pioneering instrument Special Sensor H (SSH) mounted on four Defense Meteorological Satellite Program (DMSP) block-5D1 satellites (F-1 to F-4) from 1976 to 1980. This data record was unavailable in recent decades, until the present investigations. The SSH instrument features 8 Thermal InfraRed (TIR) channels, one sensitive to ozone and the others primarily sensitive to atmospheric temperature and the surface, and 8 Far InfraRed (FIR) water-vapor-sensitive channels. The FIR region of the electromagnetic spectrum, which has not been much monitored since SSH, is under renewed interest with the recent PREFIRE and upcoming ESA FORUM missions. The SSH data record that could be rescued, from March 1977 to February 1980, covers an important time period in the global climate record, coinciding with the acceleration of ozone layer depletion. The paper presents the methodology used to recover the observations from the original tapes, and the derivation of a fast radiative transfer model capable of supporting data assimilation. The observations are assessed using radiance simulations and comparisons to the contemporary instrument High-resolution Infrared Radiation Sounder on NOAA satellites. An Observing System Experiment is conducted, using an early and lower-resolution version of the upcoming ERA6 reanalysis data assimilation system. Several configuration trials are made, to separate the impact of FIR channels from other channels. The results indicate that the assimilation of all available channels sensitive to temperature, ozone, and water vapor, including those in the FIR, yields the largest impact to improve the analyses and forecasts. Radiance simulations of the hyperspectral sounder SI-1 mounted on Soviet satellite Meteor-29 in 1979 demonstrate that DMSP SSH observations’ assimilation leads to a better representation of the SI-1 measurements, confirming the positive impact of SSH to improve reanalysis quality.