Tuesday, 11 February 2003
Lidar observation of mid-latitude water vapor layers near the tropopause
Atmospheric water vapor profiles have been measured up to the mid-latitude tropopause (17 km altitude) using a Raman lidar optimized for high altitude measurements. A series of 10 measurements made from San Nicolas Island from June to November 1998 shows water vapor layering near the tropopause, strongly correlating with layering observed in wind profiles. The average mixing ratio at 16 km was 20 ppmv, which is four times larger than expected from prior climatology. Typical profiles included water vapor layers with 0.2-2 km thickness, at or near saturation vapor pressure, located 1 to 2 km below the tropopause. The moist layers were located above thin dry layers which had mixing ratios as low as 2 ppmv. Layers near the tropopause were uncorrelated with mid-tropospheric water vapor. The latter varied from extremely dry (<10 percent relative humidity) above the planetary boundary layer, to relatively moist up to 12 km. Similar measurements made from Table Mountain Observatory, California, and Barking Sands, Kauai will be described. Satellite microwave imagery combined with MM5 air mass trajectories suggest that the water vapor layers were injected into the tropopause region by energetic convective events.