Abstract. We have used the SLIMCAT 3-D off-line chemical transport model (CTM) to quantify the Arctic chemical ozone loss in the year 2002/2003 and compare it with similar calculations for the winters 1999/2000 and 2003/2004. Recent changes to the CTM have improved the model's ability to reproduce polar chemical and dynamical processes. The updated CTM uses σ-θ as a vertical coordinate which allows it to extend down to the surface. The CTM has a detailed stratospheric chemistry scheme and now includes a simple NAT-based denitrification scheme in the stratosphere. In the model runs presented here the model was forced by ECMWF ERA40 and operational analyses. The model used 24 levels extending from the surface to ~55 km and a horizontal resolution of either 7.5°×7.5° or 2.8°×2.8°. Two different radiation schemes, MIDRAD and the CCM scheme, were used to diagnose the vertical motion in the stratosphere. Based on tracer observations from balloons and aircraft, the more sophisticated CCM scheme gives a better representation of the vertical transport in this model which includes the troposphere. The higher resolution model generally produces larger chemical O3 depletion, which agrees better with observations. The CTM results show that very early chemical ozone loss occurred in December 2002 due to extremely low temperatures and early chlorine activation in the lower stratosphere. Thus, chemical loss in this winter started earlier than in the other two winters studied here. In 2002/2003 the local polar ozone loss in the lower stratosphere was ~40% before the stratospheric final warming. Larger ozone loss occurred in the cold year 1999/2000 which had a persistently cold and stable vortex during most of the winter. For this winter the current model, at a resolution of 2.8°×2.8°, can reproduce the observed loss of over 70% locally. In the warm and more disturbed winter 2003/2004 the chemical O3 loss was generally much smaller, except above 620 K where large losses occurred due to a period of very low minimum temperatures at these altitudes.
Abstract. Simulations of Arctic denitrification using a 3-D chemistry-microphysics transport model are compared with observations for the winters 1994/1995, 1996/1997 and 1999/2000. The model of Denitrification by Lagrangian Particle Sedimentation (DLAPSE) couples the full chemical scheme of the 3-D chemical transport model, SLIMCAT, with a nitric acid trihydrate (NAT) growth and sedimentation scheme. We use observations from the Microwave Limb Sounder (MLS) and Improved Limb Atmospheric Sounder (ILAS) satellite instruments, the balloon-borne Michelsen Interferometer for Passive Atmospheric Sounding (MIPAS-B), and the in situ NOy instrument on-board the ER-2. As well as directly comparing model results with observations, we also assess the extent to which these observations are able to validate the modelling approach taken. For instance, in 1999/2000 the model captures the temporal development of denitrification observed by the ER-2 from late January into March. However, in this winter the vortex was already highly denitrified by late January so the observations do not provide a strong constraint on the modelled rate of denitrification. The model also reproduces the MLS observations of denitrification in early February 2000. In 1996/1997 the model captures the timing and magnitude of denitrification as observed by ILAS, although the lack of observations north of ~67° N make it difficult to constrain the actual timing of onset. The comparison for this winter does not support previous conclusions that denitrification must be caused by an ice-mediated process. In 1994/1995 the model notably underestimates the magnitude of denitrification observed during a single balloon flight of the MIPAS-B instrument. Agreement between model and MLS HNO3 at 68 hPa in mid-February 1995 was significantly better. Sensitivity tests show that a 1.5 K overall decrease in vortex temperatures or a factor 4 increase in assumed NAT nucleation rates produce the best statistical fit to MLS observations. Both adjustments would be required to bring the model into agreement with the MIPAS-B observations. The agreement between the model and observations suggests that a NAT-only denitrification scheme (without ice), which was discounted by previous studies, must now be considered as one mechanism for the observed Arctic denitrification. The timing of onset and the rate of denitrification remain poorly constrained by the available observations.
Abstract. Denitrification of the Arctic winter stratosphere has been calculated using a 3-D microphysical model for the winters 1994/95, 1995/96, 1996/97 and 1999/2000. Denitrification is assumed to occur through the sedimentation of low number concentrations of large nitric acid trihydrate (NAT) particles, as observed extensively in 1999/2000. We examine whether the meteorological conditions that allowed NAT particles to grow to the very large sizes observed in 1999/2000 also occurred in the other cold winters. The results show that winter 1999/2000 had conditions that were optimum for denitrification by large NAT particles, which are a deep concentric cold pool and vortex. Under these conditions, NAT particles can circulate in the cold pool for several days, reaching several micrometres in radius and leading to a high downward flux of nitric acid. The other winters had shorter periods with optimum conditions for denitrification. However, we find that NAT particles could have grown to large sizes in all of these winters and could have caused significant denitrification. We define the quantity "closed flow area'' (the fraction of the cold pool in which air parcel trajectories can form closed loops) and show that it is a very useful indicator of possible denitrification. We find that even with a constant NAT nucleation rate throughout the cold pool, the average NAT number concentration and size can vary by up to a factor of 10 in response to this meteorological quantity. These changes in particle properties account for a high degree of variability in denitrification between the different winters. This large meteorologically induced variability in denitrification rate needs to be compared with that which could arise from a variable nucleation rate of NAT particles, which remains an uncertain quantity in models.
Abstract. Observations of gas-phase HNO3 and N2O in the polar stratosphere from the Michelson Interferometer for Passive Atmospheric Sounding aboard the ENVISAT satellite (MIPAS-E) were made during the cold Arctic winter of 2002/3. Vortex temperatures were unusually low in early winter and remained favourable for polar stratospheric cloud formation and denitrification until mid-January. MIPAS-E observations provide the first dataset with sufficient coverage of the polar vortex in mid-winter which enables a reasonable estimate of the timing of onset and spatial distribution of denitrification of the Arctic lower stratosphere to be performed. We use the observations from MIPAS-E to test the evolution of denitrification in the DLAPSE (Denitrification by Lagrangian Particle Sedimentation) microphysical denitrification model coupled to the SLIMCAT chemical transport model. In addition, the predicted denitrification from a simple equilibrium nitric acid trihydrate-based scheme is also compared with MIPAS-E. Modelled denitrification is compared with in-vortex NOy and N2O observations from the balloon-borne MarkIV interferometer in mid-December. Denitrification was clearly observed by MIPAS-E in mid-December 2002 and reached 80% in the core of the vortex by early January 2003. The DLAPSE model is broadly able to capture both the timing of onset and the spatial distribution of the observed denitrification. A simple thermodynamic equilibrium scheme is able to reproduce the observed denitrification in the core of the vortex but overestimates denitrification closer to the vortex edge. This study also suggests that the onset of denitrification in simple thermodynamic schemes may be earlier than in the MIPAS-E observations.
Abstract. The SLIMCAT three-dimensional chemical transport model (CTM) is used to infer chemical ozone loss from Polar Ozone and Aerosol Measurement (POAM) III observations of stratospheric ozone during the Arctic winter of 2002–2003. Inferring chemical ozone loss from satellite data requires quantifying ozone variations due to dynamical processes. To accomplish this, the SLIMCAT model was run in a "passive" mode from early December until the middle of March. In these runs, ozone is treated as an inert, dynamical tracer. Chemical ozone loss is inferred by subtracting the model passive ozone, evaluated at the time and location of the POAM observations, from the POAM measurements themselves. This "CTM Passive Subtraction" technique relies on accurate initialization of the CTM and a realistic description of vertical/horizontal transport, both of which are explored in this work. The analysis suggests that chemical ozone loss during the 2002–2003 winter began in late December. This loss followed a prolonged period in which many polar stratospheric clouds were detected, and during which vortex air had been transported to sunlit latitudes. A series of stratospheric warming events starting in January hindered chemical ozone loss later in the winter of 2003. Nevertheless, by 15 March, the final date of the analysis, ozone loss maximized at 425 K at a value of about 1.2 ppmv, a moderate amount of loss compared to loss during the unusually cold winters in the late-1990s. SLIMCAT was also run with a detailed stratospheric chemistry scheme to obtain the model-predicted loss. The SLIMCAT model simulation also shows a maximum ozone loss of 1.2 ppmv at 425 K, and the morphology of the loss calculated by SLIMCAT was similar to that inferred from the POAM data. These results from the recently updated version of SLIMCAT therefore give a much better quantitative description of polar chemical ozone loss than older versions of the same model. Both the inferred and modeled loss calculations show the early destruction in late December and the region of maximum loss descending in altitude through the remainder of the winter and early spring.
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