Gravity waves and momentum fluxes in the mesosphere and lower thermosphere using 430 MHz dual-beam measurements at Arecibo: 1. Measurements, methods, and gravity waves

We report here and in a companion paper by Fritts et al. ( 2006a) on a new use of the UHF radar at the Arecibo Observatory in Puerto Rico. We have employed the 430 MHz radar for incoherent scatter measurements of radial wind spectra at altitudes from similar to 71 to 95 km using the Gregorian and line- feed antennas to define beam angles inclined 15 degrees to the east and west of zenith. We find that the two beams define radial velocities with sufficient accuracy to characterize both the gravity waves and the momentum fluxes due to these waves over the majority of the observed altitude range during daylight hours. The characteristics of the gravity waves inferred from these measurements include ( 1) vertical scales ranging from similar to 2 to 20 km, ( 2) downward phase progression of the dominant gravity waves up to similar to 5 ms(-1), and ( 3) vertical wave number spectra having slopes near a value ( - 3) expected for saturated gravity waves. Gravity wave frequency spectra and momentum fluxes are addressed in the companion paper.