Influence of mountain waves and NAT nucleation mechanisms on polar stratospheric cloud formation at local and synoptic scales during the 1999-2000 Arctic winter S. H. Svendsen1, N. Larsen1, B. Knudsen1, S. D. Eckermann2, and E. V. Browell3 1Danish Meteorological Institute, Copenhagen, Denmark 2Naval Research Laboratory, Washington DC, USA 3NASA Langley Research Center, Hampton, Virginia, USA
Abstract. A scheme for introducing mountain wave-induced temperature pertubations in a microphysical PSC model has been developed. A data set
of temperature fluctuations attributable to mountain waves as computed by the Mountain Wave Forecast Model (MWFM-2) has been used
for the study. The PSC model has variable microphysics, enabling different nucleation mechanisms for nitric acid trihydrate, NAT,
to be employed. In particular, the difference between the formation of NAT and ice particles in a scenario where NAT formation is
not dependent on preexisting ice particles, allowing NAT to form at temperatures above the ice frost point, Tice, and a
scenario, where NAT nucleation is dependent on preexisting ice particles, is examined. The performance of the microphysical model
in the different microphysical scenarios and a number of temperature scenarios with and without the influence of mountain waves is
tested through comparisons with lidar measurements of PSCs made from the NASA DC-8 on 23 and 25 January during the
SOLVE/THESEO 2000 campaign in the 1999-2000 winter and the effect of mountain waves on local PSC production is evaluated in the
different microphysical scenarios. Mountain waves are seen to have a pronounced effect on the amount of ice particles formed in the
simulations. Quantitative comparisons of the amount of solids seen in the observations and the amount of solids produced in the
simulations show the best correspondence when NAT formation is allowed to take place at temperatures above Tice. Mountain
wave-induced temperature fluctuations are introduced in vortex-covering model runs, extending the full 1999-2000 winter season, and
the effect of mountain waves on large-scale PSC production is estimated in the different microphysical scenarios. It is seen that
regardless of the choice of microphysics ice particles only form as a consequence of mountain waves whereas NAT particles form
readily as a consequence of the synoptic conditions alone if NAT nucleation above Tice is included in the simulations.
Regardless of the choice of microphysics, the inclusion of mountain waves increases the amount of NAT particles by as much as 10%.
For a given temperature scenario the choice of NAT nucleation mechanism may alter the amount of NAT substantially; three-fold
increases are easily found when switching from the scenario which requires pre-existing ice particles in order for NAT to form to
the scenario where NAT forms independently of ice.
Citation: Svendsen, S. H., Larsen, N., Knudsen, B., Eckermann, S. D., and Browell, E. V.: Influence of mountain waves and NAT nucleation mechanisms on polar stratospheric cloud formation at local and synoptic scales during the 1999-2000 Arctic winter, Atmos. Chem. Phys., 5, 739-753, doi:10.5194/acp-5-739-2005, 2005.