A case study on the formation and evolution of ice supersaturation in the vicinity of a warm conveyor belt's outflow region

[摘要] A case study is presented on the formation and evolution of anice-supersaturated region (ISSR) that was detected by a radiosondein NE Germany at 06:00 UTC 29 November 2000. The ISSR was situatedin the vicinity of the outflow region of a warm conveyor beltassociated with an intense event of cyclogenesis in the easternNorth Atlantic.Using ECMWF analyses and trajectory calculations itis determined when the air parcels became supersaturated and latersubsaturated again. In the case considered, the state of air parcelsupersaturation can last for longer than 24h.The ISSR wasunusually thick: while the mean vertical extension of ISSRs in NEGermany is about 500m, the one investigated here reached 3km.The ice-supersaturated region investigated was bordered bothvertically and horizontally by strongly subsaturated air.Near thepath of the radiosonde the ISSR was probably cloud free, as inferredfrom METEOSAT infrared images.However, at other locations withinthe ISSR it is probable that there were cirrus clouds.Relativehumidity measurements obtained by the Lindenberg radiosonde are usedto correct the negative bias of the ECMWF humidity and to constructtwo-dimensional maps of ice supersaturation over Europe during theconsidered period. A systematic backward trajectory analysis for theISSRs on these maps shows that the ISSR air masses themselvesexperienced only a moderate upward motion during the previous days,whereas parts of the ISSRs were located just above stronglyascending air masses from the boundary layer. This indicatesqualitatively that warm conveyor belts associated with mid-latitudecyclogenesis are disturbances that can induce the formation of ISSRsin the upper troposphere. The ISSR maps also lead us to a newperception of ISSRs as large dynamic regions of supersaturated airwhere cirrus clouds can be embedded at some locations while there isclear air at others.

[学科分类] 大气科学