A numerical modeling study of the marine boundary layer over the Gulf Stream during cold air advection

A two-dimensional mesoscale model has been developed to simulate the air flow over the Gulf Stream area where typically large gradients in surface temperature exist in the winter. Numerical simulations show that the magnitude and the maximum height of the mesoscale circulation that develops downwind of the Gulf Stream depends on both the initial geostrophic wind and the large-scale moisture. As expected, a highly convective Planetary Boundary Layer (PBL) develops over this area and it was found that the Gulf Stream plays an important role in generating the strong upward heat fluxes causing a farther seaward penetration as cold air advection takes place. Numerical results agree well with the observed surface fluxes of momentum and heat and the mesoscale variation of vertical velocities obtained using Doppler Radars for a typical cold air outbreak. Precipitation pattern predicted by the numerical model is also in agreement with the observations during the Genesis of Atlantic Lows Experiment (GALE).List of Symbols u east-west velocity m s^–1] - v north-south velocity m s^–1] - vertical velocity in coordinate m s^–1] - w vertical velocity inz coordinate m s^–1] - gq potential temperature K] - q moisture kg kg^–1] - scaled pressure J kg^–1 K^–1] - U _g the east-south component of geostrophic wind m s^–1] - V _g the north-south component of geostrophic wind m s^–1] - vertical coordinate following terrain - x east-west spatial coordinate m] - y north-south spatial coordinate m] - z vertical spatial coordinate m] - t time coordinate s] - g gravity m² s^–1] - E terrain height m] - H total height considered in the model m] - q _s saturated moisture kg kg^–1] - p pressure mb] - p ₀₀ reference pressure mb] - P precipitation kg m^–2] - vertical lapse rate for potential temperature K km^–1] - L latent heat of condensation J kg^–1] - C _p specific heat at constant pressure J kg^–1 K^–1] - R gas constant for dry air J kg^–1 K^–1] - R _v gas constant for water vapor J kg^–1 K^–1] - f Coriolis parameter (2 sin ) s^–1] - angular velocity of the earth s^–1] - latitude ^o] - K _H horizontal eddy exchange coefficient m² s^–1] - t integration time interval s] - x grid interval distance inx coordinate m] - y grid interval distance iny coordinate m] - adjustable coefficient inK _H - subgrid momentum flux m² s^–2] - subgrid potential temperature flux m K s^–1] - subgrid moisture flux m kg kg^–1 s^–1] - u _* friction velocity m s^–1] - _* subgrid flux temperature K] - q _* subgrid flux moisture kg kg^–1] - w _* subgrid convective velocity m s^–1] - z ₀ surface roughness m] - L Monin stability length m] - _s surface potential temperature K] - k von Karman's constant (0.4) - v air kinematic viscosity coefficient m² s^–1] - K _M subgrid vertical eddy exchange coefficient for momentum m² s^–1] - K subgrid vertical eddy exchange coefficient for heat m² s^–1] - K _q subgrid vertical eddy exchange coefficient for moisture m² s^–1] - z _i the height of PBL m] - h _s the height of surface layer m]