Temperature profile retrieval from surface to mesopause by combining GNSS radio occultation and passive microwave limb sounder data

Radio occultation using GNSS (GPS or GLONASS) satellite signals is a fairly new method to probe the atmosphere by the use of refraction. The technique has been demonstrated with the GPS/MET instrument. The main source for refraction is the varying density profile in the dry atmosphere, while contributions from water vapor must be taken into account at lower altitudes for a wet atmosphere. One can derive information about the temperature profile for altitudes of about 0 km to 50 km, and water vapor at altitudes below 5 - 8 km.

Probing the atmosphere by the use of passive microwave emissions is a well developed technique. In general one observes the emission of rotational transition lines of the species of interest and retrieves the profile. Temperature information can be gathered by the observation of oxygen lines, because oxygen has a known number density profile in the atmosphere. Oxygen has a number of lines around 60 GHz, where the strongest ones allow temperature profile retrieval up to altitudes of 90 km. The technique is usually limited to altitudes above the tropopause, owing to uncertainties in the water vapor continuum.

The derivation of the temperature profile from microwave radiance is performed with an a priori constraint. A well suited method for the combination of a measurement and an a priori constraint is the Optimal Estimation Method (OEM), which offers a rigorous error analysis methodology.

The radio occultation allows a direct inversion of the measurement data, but no information about the error is derived by a direct inversion.

We combine the two highly synergistic measurement techniques with the OEM and retrieve one temperature profile, spanning an altitude interval of 0 to 90 km. In addition, information about the water vapor profile below 5 - 8 km is derived. OEM combines in an optimal way the information of the two instruments so that the accuracy of the retrieved profile is maximized over the full range from surface to mesopause. Results of this combination of sensor data will be shown.