First observations of mesospheric dynamics with a partial reflection radar in Hawaii (22°N, 160°W)

Abstract

We present here an overview of the wind measurements made during the first year of operation of a partial reflection radar system in Hawaii. The system operates at 1.98 MHz and provides wind estimates between ∼60 and 100 km at 2‐km intervals. Our wind observations reveal a zonal mean motion that has several features in common with the mesopause semiannual oscillation (MSAO) seen at more equatorial latitudes, but which is influenced as well by the seasonal variations of the mesospheric jet structure at higher latitudes. Major features of the zonal mean wind structure include a downward progression of strong eastward and westward phases of the MSAO from ∼January to July, coinciding with the period during which the MSAO and the annual cycle of the zonal mean wind at higher latitudes are in phase. This results in an eastward maximum of ∼60 ms−1 near 80 km during January and February which descends rapidly and a westward maximum of ∼50 ms near 85 km during March and April which descends much more slowly. The second MSAO cycle is greatly suppressed relative to the first due to the reversal of the correlation between this and the annual cycle at higher latitudes from ∼July to December. Mean eastward motions occur primarily above ∼85 km early in this cycle and throughout the height range at later times. Planetary wave activity is seen to contribute substantial variability during times of eastward mean flow and to have a smaller role when zonal mean motions are westward. Tidal and 2‐day motions are found to be large and variable, with the maximum 2‐day amplitudes occurring during February and August. The largest diurnal tides were observed during October and November, March and April, and July and August, while the semi‐diurnal amplitudes exhibited a minimum from ∼April to June.