MLS Limb Scan Observations

MLS 63 GHz radiometer measures limb radiances at 0-90km tangent heights and resolves the O₂ emissions into 15 spectral channels [Figure 1]. The primary purpose of this radiometer was used to retrieve tangent pressure and temperature [Fishbein et al. 1996]. Radiances can be saturated when the instrument views tangent heights below ~18 km because of strong O₂ absorption. Radiances near the line center (channel 8) saturate at higher altitudes than those near the line wings (channels 1 and 15). The saturated radiances are good measure of air temperature in the saturation layer and depend little on the pointing. Thus, as the satellite moves along, fluctuations in the saturated radiances reflect atmospheric temperature variations in the horizontal direction. Roughly speaking, the saturation layers are defined by the MLS temperature weighting functions [Figure 2].

Figure 1. An example of MLS O₂ radiance profiles near 63 GHz as the instrument step-scans from the mesosphere to the surface. Fluctuations in the saturated radiances are generally large for center channels but small for wing channels due to atmospheric temperature variability and instrument noise.
 

Figure 2. MLS temperature weighting functions for the saturated radiances at the bottom of limb scans.