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M. Rouault, S. A. White, C. J. C. Reason, J. R. E. Lutjeharms, and I. Jobard

1. Introduction The Agulhas Current is considered to be the most intense western boundary current in the Southern Hemisphere ( Stramma and Lutjeharms 1997 ). Like the Gulf Stream or the Kuroshio Current, a marked sea surface temperature (SST) gradient of order 10°C (100 km) −1 can be found between the current and the surrounding ocean ( Fig. 1 ). This results in high latent heat fluxes of several hundred watts per square meter ( Rouault et al. 2000 ) throughout the year. More is known on the

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Rodger A. Brown, Bradley A. Flickinger, Eddie Forren, David M. Schultz, Dale Sirmans, Phillip L. Spencer, Vincent T. Wood, and Conrad L. Ziegler

. Doppler velocity differences across tornado and tornadic vortex signatures showed similar improvements. On fine-resolution displays, low-altitude boundaries were more readily apparent. At ranges greater than 100 km, fine-resolution reflectivity displays revealed severe storm signatures, such as bounded weak echo regions and hook echoes, that were not readily apparent on current-resolution displays. An advantage of fine-resolution measurements over current-resolution measurements is the ability to

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Kay Sušelj, Timothy F. Hogan, and João Teixeira

prediction models, the turbulence parameterization in NAVGEM is split between the boundary layer, shallow convection, and deep convection. The boundary layer parameterization is modeled with the eddy-diffusivity parameterization from Louis (1979) . In the current implementation, this parameterization assumes that the turbulent mixing approaches zero as the wind shear production of turbulence is small. The convection parameterization in NAVGEM is modeled with a simplified Arakawa–Schubert (SAS) mass

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Jongil Han, Marcin L. Witek, Joao Teixeira, Ruiyu Sun, Hua-Lu Pan, Jennifer K. Fletcher, and Christopher S. Bretherton

heating is parameterized and included in the hybrid EDMF PBL scheme. In addition, the current local scheme in the stable boundary layer (SBL) is modified to improve vertical turbulent mixing for weakly and moderately stable conditions. The implementation procedure associated with model physics changes in the NCEP operational numerical weather prediction (NWP) system is as follows: 1) identify model forecast biases or problems (e.g., the present study was motivated by the problem of underestimated PBL

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Jongil Han and Christopher S. Bretherton

the current hybrid EDMF PBL scheme in GFS (EDMF-CTL), we conduct the single-column model (SCM) simulations for three diverse boundary layer cases that have been used in past intercomparison studies. These include a convective boundary layer (CBL) with no mean wind, a marine stratocumulus-topped boundary layer, and a moderately stable boundary layer (SBL). These cases were also adopted in the study by Bretherton and Park (2009) for tests of their moist TKE parameterization. a. Convective boundary

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Barry E. Schwartz, Charles F. Chappell, William E. Togstad, and Xiao-Ping Zhong

, areemployed to identify the large-scale forcing that set the stage for the development of mesoscale convection.Surface mesoanalysis identified a significant outflow boundary and mesohigh-pressure system produced fromafternoon thunderstorms over east-central Minnesota and western Wisconsin. This outflow boundary becamestationary over Minneapolis-St. Paul (the Twin Cities), providing a convergence zone that acted to focus thunderstorm development. Satellite imagery shows that the thunderstorms associated

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Robert H. Johns and William D. Hirt

of currentnomenclature. A comprehensive dataset consisting of 70 derecho cases has been developed from the warmseason months of May through August for the 4-year pedod 1980-1983. Analyses of this dataset reveal thatthe warm season derecho typically emanates from a mesoscale convective system (MCS) moving along a quasistationary, low-level thermal boundary in an environment characterized by high potential instability and relativelystrong midtropospheric winds. In the continental United States

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Waylon G. Collins, Charles H. Paxton, and Joseph H. Golden

vorticity is stretched ( Wakimoto and Wilson 1989, hereafter WW89 ; Brady and Szoke 1989, hereafter BS89 ) or when local near-surface horizontal streamwise vorticity is tilted and stretched ( Wilzak et al. 1992 ). Further, Lee and Wilhelmson (1996, hereafter LW96) and Roberts and Wilson (1995, hereafter RW95) have demonstrated that convergence associated with outflow boundaries can help determine the timing and/or intensity of nonsupercell tornadoes. These foregoing mechanisms are not exhaustive

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T. Keenan, R. Potts, and T. Stevenson

-radius circles for periods up to 3 h in the tropics near Darwin, Australia ( 12-S, 131 -E),are described. The forecasts, based primarily on the detection of mesoscale convergent boundaries by Dopplerradar data, are evaluated in terms of probabitistic and categorical products. Brier skill scores of the forecastsindicate significantly increased accuracy relative to those based on persistence and climatology for lead timesless than 90 rain, as well as significantly better accuracy than those based on

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S. B. Trier, M. A. LeMone, F. Chen, and K. W. Manning

and its contrasting precipitation events along with a description of the model and experiment design. The sensitivity of the simulated surface fluxes, planetary boundary layer (PBL), and precipitation to the strength of the parameterized surface exchange is examined and compared with observations in section 4 . We emphasize how the strength of the surface exchange can influence convection initiation and precipitation forecasts in selected individual cases with different synoptic situations, and

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