Low-Level Jet and Boundary Layer Development during Cold-Air Outbreaks in the Arctic: A Simple Model

A quasi-analytical mixed-layer model is formulated describing the evolution of the convective atmospheric boundary layer (ABL) during cold-air outbreaks (CAO) over polar oceans downstream of the marginal sea-ice zones. The new model is superior to previous ones since it predicts not only temperature and mixed-layer height but also the height-averaged horizontal wind components. In particular, the model accounts for the low-level baroclinicity due to the ABL heating and sloping inversion, as well as for the surface friction.

Results of the mixed-layer model are compared with dropsonde and aircraft observations carried out during several CAOs over the Fram Strait and also with results of a 3D non-hydrostatic (NH3D) model. It is shown that the mixed-layer model reproduces well the observed ABL height, temperature, low-level baroclinicity and its influence on the ABL wind speed. The mixed-layer model underestimates the observed ABL temperature only by about 10 %, most likely due to the neglect of condensation and subsidence. The comparison of the mixed-layer and NH3D model results shows good agreement with respect to wind speed including the formation of a low-level jet downwind the ice edge. The latter is important since it further increases the surface heat flux which is already extremely large during CAOs. It is concluded that baroclinicity within the ABL governs the structure of the wind field while the sloping inversion is important for reproducing the actual values of wind speed. It is shown that the baroclinicity in the ABL is strongest close to the ice edge and slowly decays further downwind. Analytical solutions demonstrate that the e" id="MathJax-Element-1-Frame" class="MathJax" role="presentation">e$\mathrm{}$-folding distance of this decay is the same as for the decay of the difference between the surface temperature of open water and of the mixed-layer temperature. This distance characterizing cold-air mass transformation ranges from 400 to 900 km for high-latitude CAOs.

Although analytical solutions qualitatively explain the presence or absence of low-level jets in the observations, we further show that the orography of Svalbard has a significant impact on wind speed over the Fram Strait. Idealized three-dimensional simulations demonstrate that the orographic tip jet to the north-west of Svalbard forms during CAOs. It is superimposed over the baroclinic jet related to the ABL heating and can intensify the latter.