ATSC Colloquium: 1 March, 3:00 pm, EN6085A

Boundary-layer Processes and Dryline Formation

Singular radar fine-lines are sometimes observed in the convective boundary-layer (BL) over land. If convective initiation is imminent, then it generally occurs along such lines.  This, and the concentration of clear-air scatterers itself, suggest that these lines are BL convergence belts. Several radar fine-lines were sampled in the Great Plains of North America during IHOP (the International H2O Project).  They were all marked by a sharp humidity contrast, thus we call them dryline boundaries, separating different airmasses. The question is, what drives this convergence, i.e. what causes the boundary?

The IHOP cases suggest that the convergence is the result of baroclinicity in the BL.  That is, the airmass contrast itself, in particular the horizontal density (virtual potential temperature) difference, is responsible for the sharp definition of the boundary and the sustained convergence, leading to a radar fine-line.  In the presence of large-scale [O(100 km)] baroclinicity, one or more frontogenetic BL solenoids form, along which the background gradients tighten by at least an order of magnitude. The question then becomes, what causes this baroclinicity?

Several obvious mechanisms are known, such as BL cooling by a convective system, or surface heating over land adjacent to an ocean.  In the southern high plains in spring, a unique combination of terrain slope and land-surface contrasts leads to the recurrent formation of an inland sea breeze front, known as the dryline.  We hypothesize that dryline formation is directly or indirectly the result of gradients in surface heat fluxes on the scale of O(100 km) or more. The indirect route relates to differences in BL depth, and thus in westerly momentum transfer into the BL.

This seminar thus concludes with a rationale for the SONDE-08 (Simultaneous Observation of the Dryline Environment) experiment.