NEUTRAL WINDS AND TEMPERATURE IN THE TROPICAL MESOSPHERE AND LOWER THERMOSPHERE DURING JANUARY 1993 - OBSERVATION AND COMPARISON WITH TIME-GCM RESULTS

We present the E region neutral winds and temperature observed by the Arecibo incoherent scatter radar (ISR) and a collocated spectrometer d uring the January 20-30, 1993, period and compare them with the thermo sphere/ionosphere/mesosphere electrodynamics global circulation model (TIME-GCM) predictions. The model is most successful in predicting the meridional wind phase and magnitude throughout the altitude range of 95-145 km. Although the model can also predict the overall pattern of zonal wind and temperature, its daytime eastward wind exceeds the obse rvations by about 35 m/s at 105 km and its temperature deviation from mean is typically 3 times smaller than the observations, The overestim ation of the eastward meanflow in the model is likely associated with an overestimation in gravity wave forcing which prevents the propagati on of the diurnal tide to 100 km, The diurnal tide, which is quite sub stantial at 100 km from the radar measurement, deposits its momentum t o enhance the westward meanflow [Miyahara and Wu, 1989]. One reason fo r the smaller simulated temperature fluctuation is that the current mo del does not include any semidiurnal forcing at the lower boundary (30 km). According to a recent study by Hagan [1996], water vapor absorpt ion of solar near-infrared radiation in the troposphere can have a ver y large effect on the tides in the lower thermosphere. We further note that the observed semidiurnal tide experiences a fast growth rate at 105 km and saturates at slightly above 110 km, while the model predict ion shows a much smaller growth rate in this region despite a much lon ger vertical wavelength, This suggests that the model may overestimate the molecular/eddy viscosity in the turbopause region. During the Jan uary 1993 period, the observed mean mesopause temperature, 160 K, is a bout 30 K colder than the model prediction and current prevailing beli efs. Although such a cold mesopause at a tropical latitude needs to be verified by more experiments, it cannot be precluded theoretically du e to large uncertainties in radiative and dynamical heating/cooling ra tes, most particularly the radiative cooling rate from CO2.