&amp;lt;i&amp;gt;Letter to the Editor:&amp;lt;/i&amp;gt; A strange cloud in the Arctic summer stratosphere 1998 above Esrange (68°N), Sweden

J Siebert,C Timmis,G Vaughan,K H Fricke

doi:10.1007/s00585-000-0505-0

Abstract

Abstract. When the University of Bonn lidar on the Esrange (68°N, 21°E), Sweden, was switched on in the evening of July 18, 1998, a geometrically and optically thin cloud layer was present near 14 km altitude or 400 K potential temperature, where it persisted for two hours. The tropopause altitude was 4 km below the cloud altitude. The cloud particles depolarized the lidar returns, thus must they have been aspherical and hence solid. Atmospheric temperatures near 230 K were approximately 40 K too high to support ice particles at stratospheric water vapour pressures of a few ppmv. The isentropic back trajectory on 400 K showed the air parcels to have stayed clear of active major rocket launch sites. The air parcels at 400 K had traveled from the Aleutians across Canada and the Atlantic Ocean arriving above central Europe and then turned northward to pass over above the lidar station. Parcels at levels at ±25 K from 400 K had come from the pole and joined the 400 K trajectory path above eastern Canada. Apparently the cloud existed in a filament of air with an origin different from those filaments both above and below. Possibly the 400 K level air parcels had carried soot particles from forest wild fires in northern Canada or volcanic ash from the eruption of the Korovin Volcano in the Aleutian Islands.Key words: Atmospheric composition and structure (aerosols and particles; biosphere-atmosphere interactions) · Meteorology and atmospheric dynamics (middle atmospheric dynamics)

Highlights

Clouds in the polar stratosphere (PSC) are usually thought of in terms of the winter season, when temperatures fall suciently low for cloud formation.Correspondence to: K
The temperature pro®les from the midnight radiosondes each show a single tropopause at 10.1 km, 11.3 km, and 11.5 km altitude; the dierence in tropopause altitude shows that a warm front had passed Lule aring; and SodankylaÈ but had not yet arrived at Bodù
These local measurements are supported by the ECMWF analyses for 18 and 0 UT on the 18th and 19th July respectively

Summary

Introduction

Clouds in the polar stratosphere (PSC) are usually thought of in terms of the winter season, when temperatures fall suciently low for cloud formation. The importance of such winter PSCs for the destruction of stratospheric ozone is well established (cf Solomon, 1999). The visually spectacular variants of the PSCs are the mother-of-pearl clouds (MPC). Hesstvedt (1958) investigated the annual variation of MPCs and similar clouds above Norway. Stratospheric clouds in (polar) summer obviously are rare events. In this work we present the observation and explore potential mechanisms for the formation of this cloud

Measurements

Discussion

Conclusion