Post on 30-Jan-2016
Inversion Effects on Lee-wave Rotors
Simon Vosper, Stephen Mobbs, Ralph Burton
Institute for Atmospheric ScienceUniversity of Leeds, UK
UK Met Office BLASIUS model Dry Boussinesq equations of motion using a first-order
(mixing-length) turbulence closure scheme Free-slip and no-slip (via a log-law formulation) lower-
boundary conditions 2 dimensional bell-shaped ridge Upstream wind independent of height, apart from within the
boundary layer in the no-slip case Upstream stratification neutral in a layer immediately above
the ground, capped by a sharp temperature inversion Above inversion buoyancy frequency independent of height
(N=0.01 s-1) Range of inversion strengths (measured by the difference
in potential temperature across the inversion Δθ) and inversion heights, z
i
Numerical Model
No slip case. Horizontal flow speed shaded, potential temperature contoured at 1K intervals. F
i=0.6, z
i=800 m, H=400 m
Closed Rotors
No slip case. Horizontal flow speed shaded, potential temperature contoured at 1K intervals. F
i=0.4, z
i=800 m, H=400 m
Stationary Hydraulic Jump
No Slip Case Closed rotors Stationary hydraulic jumps
Free Slip Case No closed rotors Stationary hydraulic jumps
Regime Diagram – No slip Case
Solid line – critical Fi trapped lee waves (linear theory)
10 min wind vectors, 9 February 2001, East Falkland
High degree of spatial variability during rotor streaming Suggests the use of wind variances in rotor diagnostics Calculate instantaneous spatial standard deviation of
wind at stations downwind of orography σ given by
Rotor diagnostics σ and σU Energy argument suggests that closed rotors can occur if
Analysis of Observations of Rotor Streaming
Time series of rotor diagnostics
Regime Diagram – Observations
Solid line - R=1
Mean Speed-up and Rotors
Uup
is wind speed upstream of mountains U is mean wind speed over 8 stations downwind of
mountains
Regime Diagram – Observations
Idealised modelling demonstrates connection between rotor streaming and trapped lee waves on inversion
Needs no-slip boundary condition
Correlation between inversions observed downwind of mountains and rotors is low
Rotor activity (spatial variability of wind) directly proportional to mean speed-up