Lidar observations of seasonal variation of diurnal mean temperature in the mesopause region over Fort Collins, Colorado (41 degrees N, 105 degrees W)

Two campaigns from February 1997 to January 1999 have resulted in 18 sets o f continuous temperature measurements in the mesopause region over Fort Col lins, Colorado (41 degrees N, 105 degrees W), each covering a complete diur nal cycle, reasonably well distributed throughout the year. It is shown tha t the winter-summer transitions, based on diurnal means, are abrupt and cle ar, further strengthening the concept of two-level mesopause, and demonstra te the robustness of this picture. Similar to data taken at Urbana, Illinoi s, our data at Fort Collins also demonstrate the need to observe over a com plete diurnal cycle to deduce the uncontaminated mean temperature structure and seasonal variations in the mesopause region. The difference between an nual diurnal, nighttime, and daytime means may be ascribed to a temperature wave propagating with a downward phase speed of similar to 0.8 km/h. Our d ata show that the nighttime annual mean is colder than the diurnal annual m ean by no more than 2 K below 88 km and warmer by no more than 3 K above 88 km where the three means are nearly the same. As in the Urbana data, we al so observed the altitude of a secondary temperature minimum in the summer t o be similar to 96 km, which without strong dynamical cooling in the summer would have been the summer mesopause altitude.