Representation of gravity-wave-induced stratospheric temperature fluctuations over the Antarctic Peninsula Andrew Orr, Scott Hosking, Howard Roscoe (British Antarctic Survey) Lars Hoffman (Forschungszentrum Juelich, Germany) Sam Dean (Nat. Inst. Water and Atmos. Res., New Zealand) Luke Abraham, James Keeble (University of Cambridge) Simon Vosper (UK MetOffice)

A type II PSC showing iridesence A type I (white cloud above the orange tropospheric clouds) Extreme cold of the polar winter results in polar stratospheric clouds (PSCs) Type Ia clouds condense at or below 195K [nitric acid trihydrate (NAT)] Type Ib clouds condense at or below 191 K [supercooled ternary solutions (STS)] Type II clouds condense at or below 188 K (water ice) Such low temperatures relatively common in the Antarctic but rarer in the Arctic In both cases the generation of gravity waves by mountains can locally cool the lower stratosphere and lead to the formation of PSCs PSCs give rise to ozone loss (conversion of inactive to reactive chlorine compounds) Durran, 2003

Detection of PSCs with optical depth > 0.3 in CALIOP profiles over Antarctica during 2006 and 2007 austral winters (Noel et al., 2009) Antarctic Peninsula ~1000 km ~100s km

Temperature fluctuations (obtained by removing a fourth order polynomial fit) at 21 km in K simulated by the MetUM at 4 km resolution for three case studies. CS UTC 7 Aug 2011, T+36h CS UTC 2 Aug 2010, T+40h CS UTC 13 July 2010, T+41h

CS1: Temperature (top) and temperature fluctuations (bottom) at 21 km in K as measured by AIRS (left) and simulated by the MetUM at 4 km resolution (middle) and N96 at 1.25°×1.875° (right). AIRS (0300UTC 7 Aug 2011) MetUM at 4 km (T+36h) MetUM N96 at 1.25°×1.875° (T+39h)

4-km model UK MetOffice Unified Model Version 7.8 Forced by N512 model (~25 km) 85 vertical levels Model top ~85 km Validation: compare simulated brightness temperatures obtained from radiative transfer calculations using model output to real AIRS data. Validate 4 km simulations using AIRS observations Assess parameterization against 4 km simulations Atmospheric Infra-red Sounder (AIRS) Measures radiances (~3% accuracy) Horizontal resolution ~14 km Limited vertical resolution (~9 km) cm -1 channel selected (most sensitive to atmospheric temperature at 22 km).

CS1 CS2 CS3

CS1 CS2 CS3

Description of Dean et al. (2007) orographic gravity wave scheme Dean, Flowerdew, Lawrence, and Eckermann, 'Parameterisation of orographic cloud dynamics in a GCM', Clim. Dyn., 28, , Evolution of the vertical displacement amplitude is governed by (McFarlane 1987) A “saturation amplitude” is given by (McFarlane 1987) The wave phase given by Following Queney (1948), vertical displacement is given by Computes displacement derived from analytical mountain wave solutions based on steady, stably stratified flow over a two-dimensional bell shaped ridge. Assumes the waves are hydrostatic, rotational effects not important, and that U and N vary slowly in the vertical (WKBJ generalisation).

The maximum and minimum wave-induced displacement is computed Temperature perturbations can be calculated from the local potential temperature gradient Launches mountain waves at every model grid box over land at every time step. Vertical displacement of the gravity waves launched by the mountains is equal to the height of the unresolved orography (n σ σ) minus the depth of the blocked layer Representation of Antarctic Peninsula orography in the MetUM. Resolved orography height at 4 km resolution (A), and N96 (1.25°×1.875°) resolved (B) and sub-grid (C) orography height σ.

Temperature fluctuations at 4 km (left) and gw scheme (right). CS1 at 70S Vertical profile averaged over longitudinal transect of temperature fluctuations (left) and temperature (right).

CS2 at 65S Temperature fluctuations at 4 km (left) and gw scheme (right). CS2 at 70S

CS3 at 65S Temperature fluctuations at 4 km (left) and gw scheme (right). CS3 at 70S

Summary Satellite observations show temperature perturbations of ~10 K occurring in the lower stratosphere above the Antarctic Peninsula for the selected case study This is realistically simulated by the MetUM at high-resolution, consistent with assumption that the model is resolving most of the gravity wave spectrum The N96 climate model does not simulate any gravity-wave-induced temperature disturbances Initial encouraging results using a a parameterization of orographic gravity-wave- induced temperature perturbations (e.g. Dean et al., 2007) Implement the scheme in the UKCA (UK Chemistry-Aerosol ) model to realistically simulate PSCs and associated impacts on ozone etc