Temperature minima in the average thermal structure of the middle mesosphere (70–80 km) from analysis of 40‐ to 92‐km SME global temperature profiles

Abstract

Global temperatures have been derived for the upper stratosphere and mesosphere from analysis of Solar Mesosphere Explorer (SME) limb radiance profiles. The analysis of additional SME wavelength radiances at 304, 313, and 442 nm provides for extension of the original SME 60‐ to 90‐km temperature climatology (Clancy and Rusch, 1989a) to a much expanded altitude coverage of 40–92 km. The SME temperatures represent fixed local time observations at 1400–1500 LT, with partial zonal coverage of 3–5 longitudes per day over the 1982–1986 period. These new SME temperatures are compared to the CIRA 86 climatology (Fleming et al., 1990) as well as stratospheric and mesospheric sounder (SAMS); (Barnett and Corney, 1984), National Meteorological Center (NMC); (Gelman et al., 1986), and individual lidar and rocket observations. Significant areas of disagreement between the SME and CIRA 86 mesospheric temperatures are 10 K warmer SME temperatures in the 55‐ to 65‐km altitude region and 10–20 K warmer SME temperatures at altitudes above 80 km. The 1981–1982 SAMS temperatures are in much closer agreement with the SME temperatures between 40 and 75 km. Although much of the SME‐CIRA 86 disagreement probably stems from the poor vertical resolution of the observations comprising the CIRA 86 model, some portion of the differences may reflect 5‐ to 10‐year temporal variations in mesospheric temperatures. The CIRA 86 climatology is based on 1973–1978 measurements. Relatively large (1 K/yr) 5‐ to 10‐year trends in temperatures as functions of longitude, latitude, and altitude have been observed for both the upper stratosphere (Clancy and Rusch, 1989a) and mesosphere (Clancy and Rusch, 1989b; Hauchecorne et al. 1991). The SME temperatures also exhibit enhanced amplitudes for the semiannual oscillation (SAO) of upper mesospheric temperatures at low latitudes, which are not evident in the CIRA 86 climatology. The so‐called mesospheric “temperature inversions” at wintertime midlatitudes, which have been observed by ground‐based lidar (Hauchecorne et al., 1987) and rocket in situ measurements (Schmidlin, 1976), are shown to be a climatological aspect of the mesosphere, based on the SME observations. The winter midlatitude “temperature inversion” is evident in both winter hemispheres and appears more distinctly as a low‐altitude (75 km) temperature minimum at southern midlatitudes. Furthermore, the SME temperatures indicate the related presence of low‐latitude temperature minima at 80‐km altitude during the equinoxes. Both the low‐latitude semiannual minima and the midlatitude annual minima in middle mesospheric temperatures appear to result from the strong SAO in mesospheric temperatures.