R. L. Collins

A climatology of planetary wave‐driven mesospheric inversion layers in the extratropical winter

Mesospheric inversion layers (MILs) are a useful diagnostic to simultaneously investigate middle atmosphere radiation, chemistry, and dynamics in high-top general circulation models. Climatologies of long-lived extratropical winter MILs observed by the Microwave Limb Sounder (MLS) and the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) satellite instruments are compared to MILs in the Whole Atmosphere Community Climate Model (WACCM). In general, MIL location, amplitude, and thickness statistics in WACCM are in good agreement with the observations, though WACCM middle- and high-latitude winter MILs occur 30%–50% more often than in MLS and SABER. This work suggests that planetary wave-driven MILs may form as high as 90 km. In the winter, MILs display a wave-1 pattern in both hemispheres, forming most often over the region where the climatological winter stratospheric anticyclones occur. These MILs are driven by the decay of vertically propagating planetary waves in the mesospheric surf zone in both observations and in the model. At the base of polar inversions there is climatological local ascent and cooling situated atop the stratospheric anticyclones, which enhances the cold base of the MILs near 60 km and 120°E longitude.

First lidar observation of the mesospheric nickel layer

Ground-based resonance lidar has been used to detect the mesospheric nickel (Ni) layer. The Ni layer is detected through absorption in the 3d8(3F)4s2 → 3d9(2D)4p transition at 337.1 nm and reemission in both the 3d9(2D)4p → 3d8(3F)4s2 and 3d9(2D)4p → 3d9(2D)4s transitions at 337.1 and 339.4 nm, respectively. Results from wintertime lidar observations on two nights (27–28 November and 20–21 December 2012) at Chatanika, Alaska (65°N, 147°W), are presented. The wintertime Ni layer has a peak concentration of 1.6 × 104 cm−3 at 87 km, with a column abundance of 2.7 × 1010 cm−2, centroid height of 88 km, and root-mean-square width of 6.4 km. The midwinter abundance of mesospheric iron (Fe) at Chatanika is 3.4 × 1010 cm−2, indicating a ratio of Fe to Ni of 1.2 in the upper atmosphere. This is significantly lower than the value of 18 for the ratio of Fe to Ni in chondrite meteorites.

Chemical definition of the mesospheric polar vortex

We present a simple chemical definition to demark the edge of the mesospheric polar vortices. Because this vortex definition does not rely on the wind field, it is useful in the mesosphere where wind observations are sparse and reanalysis winds are unreliable. The chemical definition is also insensitive to double jets that complicate vortex identification in the mesosphere. The algorithm is based on horizontal gradients of carbon monoxide (CO) and mirrors the widely used vortex edge definition in the stratosphere based on potential vorticity (PV) gradients. Here the approach is used to identify the Arctic vortex in the mesosphere during a 10 year (2004–2014) record of Microwave Limb Sounder data. Vortex size and shape comparisons are made where the CO and PV methods overlap in the upper stratosphere. A case study is presented during the NH 2008–2009 winter that demonstrates the fidelity of the CO gradient method on individual days and emphasizes the impact of double jets on methods to identify the polar vortex. We recommend transitioning from a PV or stream function-based vortex definition in the stratosphere to using a CO gradient definition above 0.1 hPa (~60 km). The CO gradient method identifies a coherent region of high CO at 80 km that is confined to mid-to-high latitudes 99.8% of the time during Arctic winter. Taking advantage of the CO gradient method to identify the polar vortex adds ~20 km of reliable vortex information (from 60 to 80 km) in a region of the atmosphere where reanalyses are most suspect.