Diagnosis of processes controlling water vapour in the tropical tropopause layer by a Lagrangian cirrus model

[摘要] We have developed a Lagrangian air-parcel cirrus model (LACM), to diagnosethe processes controlling water in the tropical tropopause layer (TTL). LACMapplies parameterised microphysics to air parcel trajectories. Theparameterisation includes the homogeneous freezing of aerosol droplets, thegrowth/sublimation of ice particles, and sedimentation of ice particles, socapturing the main dehydration mechanism for air in the TTL. Rehydration isalso considered by resetting the water vapour mixing ratio in an air parcelto the value at the point in the 4-D analysis/forecast data used to generatethe trajectories, but only when certain conditions, indicative of convection,are satisfied. The conditions act to restrict rehydration of the Lagrangianair parcels to regions where convective transport of water vapour from belowis significant, at least to the extent that the analysis/forecast capturesthis process. The inclusion of hydration and dehydration mechanisms in LACMresults in total water fields near tropical convection that have more of the"stripy" character of satellite observations of high cloud, than do eitherthe ECMWF analysis or trajectories without microphysics.

The mixing ratios of total water in the TTL, measured by a high-altitudeaircraft over Brazil (during the TROCCINOX campaign), have been reconstructedby LACM using trajectories generated from ECMWF analysis. Two otherLagrangian reconstructions are also tested: linear interpolation of ECMWFanalysed specific humidity onto the aircraft flight track, and instantaneousdehydration to the saturation vapour pressure over ice along trajectories.The reconstructed total water mixing ratios along aircraft flight tracks arecompared with observations from the FISH total water hygrometer.Process-oriented analysis shows that modelled cirrus cloud events areresponsible for dehydrating the air parcels coming from lower levels,resulting in total water mixing ratios as low as 2 μmol/mol. Withoutadding water back to some of the trajectories, the LACM andinstantaneous-dehydration reconstructions have a dry bias. Theinterpolated-ECMWF reconstruction does not suffer this dry bias, becauseconvection in the ECMWF model moistens air parcels dramatically, by pumpingmoist air upwards. This indicates that the ECMWF model captures the grossfeatures of the rehydration of air in the TTL by convection. Overall, theECMWF models captures well the exponential decrease in total water mixingratio with height above 250 hPa, so that all the reconstruction techniquescapture more than 75% of the standard deviation in the measured total watermixing ratios over the depth of the TTL.By picking up the main contributing processes to dehydration and rehydration in theTTL, LACM reconstructs total water mixing ratiosat the top of the TTL, close to the cold point, that are always insubstantially better agreement with observations thaninstantaneous-dehydration reconstructions, and better than the ECMWF analysisfor regions of high relative humidity and cloud.

[学科分类] 大气科学