Mountain Leewave Cloud Forecasting and Analysis


 

Theory Investigation:Forecasting of mountain wave occurrence and analysis of associated PSCs
Principal Investigator:Ken Carslaw
Organization:School of the Environment
University of Leeds
Leeds LS2 9JT, U.K.
Co-Investigators:Thomas Peter (ETH-Zurich), Andreas Dornbrack (DLR Oberpfaffenhofen), Simon Vosper (University of Leeds)

Investigation Description: There are two aspects to our contribution to SOLVE:

  1. Forecasts of mesoscale cooling over mountains and
  2. detailed optical/microphysical/chemical modelling of PSCs observed in mountain wave clouds and elsewhere.
We will produce 6-hourly forecasts of mountain wave cooling over Scandinavia and other selected mountain ranges for the duration of the SOLVE mission. These forecasts will provide a detailed picture of the 3-dimensional structure of mesoscale cooling over the mountains as an aid to flight planning. This technique was used with great success during the European APE-POLECAT campaign over Scandinavia in 1997. Observations of the clouds using lidar and in situ particle instrumentation will be analysed using optical and microphysical models to determine the evolution of particle size distributions and the factors controlling aerosol phase transitions. We will also couple the microphysical models to chemical box models that will enable the activation of chlorine species in the wave clouds to be calculated. Details of our planned contribution can be found in a more detailed document

Relevance to SOLVE Objectives: Two central objectives of the SOLVE campaign are to quantify the chlorine activation process on particles and to understand how PSC particles form. Our contribution to SOLVE will enable detailed and reliable planning of flights into and around the "natural laboratory" of mesoscale polar stratospheric clouds. In our previous work we have been able to extract detailed information about particle microphysics from lidar observations of mountain wave clouds alone (see, for example Carslaw et al. (1998a)). Mesoscale clouds offer several unique opportunities to study aerosol microphysics and chemistry in much greater detail than by studying synoptic scale clouds. For example,

  1. the clouds are small enough to enable upwind and downwind cloud edges to be located and probed several times in a single flight;
  2. the clouds usually contain several different PSC types in close proximity;
  3. the clouds are often in a quasi-stationary state, which enables the complete lifecycle of cloud elements to be studied.
The SOLVE mission offers the opportunity, for the first time ever, to probe not only cloud microphysics but also chlorine activation. Our calculations have shown that passage of air through a single wave cloud can lead to significant and potentially observable chlorine activation (Carslaw et al., 1998b). A well planned mesoscale cloud flight would be close to a PSC closure experiment and could enable the rates of chlorine activation reactions on different PSC surfaces to be verified. Our previous studies suggest that the greatest scientific returns can be obtained if the flights are quasi-Lagrangian.

We define an observation to be quasi-Lagrangian when the air parcels that are probed move along the same streamline through spatial fields of physical and chemical quantities (temperature, photolysis rates, deposition rates etc.), which are stationary on the timescale of the process studied (quasi-stationarity). (Read an article on quasi-Lagrangian flying here).

The mesoscale model forecasts will enable flight paths to be defined that closely follow the air flow at cloud level.

 

References:

Carslaw, K. S., M. Wirth, A. Tsias, B. P. Luo, A. Doernbrack, M. Leutbecher, H. Volkert, W. Renger, J. T. Bacmeister, and T. Peter, Particle microphysics and chemistry in remotely observed mountain polar stratospheric clouds, J. Geophys. Res., 103, 5785-5796, 1998a.
Carslaw, K. S., et al., Increased stratospheric ozone depletion due to mountain-induced atmospheric waves, Nature, 391, 674-678, 1998b.