EVIDENCE FOR ICE CLOUDS CAUSING POLAR MESOSPHERIC SUMMER ECHOES

We compare 22 temperature height profiles derived from falling sphere experiments with simultaneous 50-MHz mesospheric radar echoes obtained during the Middle Atmosphere Co-operation/Summer in Northern Europe c ampaign in the summer of 1987 above Andenes, Norway. Common features o f these observations are as follows: (1) More than 80% of the echoes o ccur at temperatures below 140 K and more than two thirds of the botto mmost echo maxima appear at temperatures that deviate less than 5 degr ees from 140 K. (2) We can find no significant correlation between the echo strength or the echo occurrence probability and the local temper ature once it is below 140 K. (3) The echoes are npt symmetrically dis tributed around the mesospheric temperature minimum, but there are cle arly more echoes below than above the temperature minimum. Equivalentl y, while the local temperature at the height of the bottommost echoes is about 140 K, it is much lower at the height of the topmost echoes. We compare the height range of the observed polar mesospheric summer e cho returns with the height range that can be inferred for an ice clou d from the observed temperature profile by means of a simple stationar y nucleation/sedimentation model. For reasonable values of the height- integrated ice particle nucleation rate of about 3x10(7) m(-2) s(-1) a nd for a water molecule mixing ratio at the bottom of the ice cloud of 0.1 to 1.0 ppmv, we find excellent agreement between the height range within which the ice particles may exist and the height range from wh ich mesospheric echoes are seen. For example, in 17 out of the 22 indi vidual observations, reasonable values for the water vapor mixing rati o can be found such that the height of the strong bottommost echo corr esponds to within the resolution of the radar to the bottom height of our model ice cloud; If our height estimates are correct and the obser vations are representative, our results yield a somewhat smaller water vapor mixing ratio in the summer mesosphere at high latitudes compare d to what has been measured at midlatitudes.