The Experts below are selected from a list of 306 Experts worldwide ranked by ideXlab platform

Peter Sullivan - One of the best experts on this subject based on the ideXlab platform.

  • Effects of Mesoscale Surface Thermal Heterogeneity on Low-Level Horizontal Wind Speeds
    Boundary-Layer Meteorology, 2012
    Co-Authors: Song-lak Kang, Donald Lenschow, Peter Sullivan
    Abstract:

    Using large-eddy simulation, we investigate characteristics of Horizontal wind speed at 100 m above the ground, with surface heat-flux variations that are sinusoidal with amplitudes of 0, 50, and 200 W m^−2 and wavelengths of 16, 32, and 128 km, and no background flow. When the amplitude is 200 W m^−2, wind speeds induced by the surface-flux variations on scales of 16 and/or 32 km have multiple temporal oscillations from 0600 to 1800 local standard time. The positive peaks first appear before noon. In contrast, for wind speeds induced by the 128-km surface heterogeneity, a single oscillation occurs in the late afternoon, which is much larger than those generated by the 16- and 32-km surface heterogeneity. In addition, at the oscillation onset the kurtosis of the velocity increment over a distance of 1 km significantly increases, which implies intermittency in the generation of 1-km scale eddies. The spatially intermittent energy cascade generated by surface heterogeneity scaled down to 1-km eddies is analogous to the well-known intermittent energy cascade in the inertial subrange. The kurtosis of the 1-km eddies is much larger with the 128-km surface heterogeneity than with the 16- and 32-km heterogeneities. Thus we conclude that localized rapid changes of low-Level Horizontal wind speed may be caused by significant local surface heterogeneity on scales between a few tens and a few hundreds of kilometres.

Lewis D. Grasso - One of the best experts on this subject based on the ideXlab platform.

  • A numerical simulation of dryline sensitivity to soil moisture
    Monthly Weather Review, 2000
    Co-Authors: Lewis D. Grasso
    Abstract:

    Abstract Previous studies have explained dryline movement to be a result of vertical turbulent mixing. Such mixing was shown to efficiently erode the western edge of the shallow moist layer above sloping terrain. Two- and three-dimensional simulations have been used to demonstrate the impact of surface physiography on dryline evolution. Those simulations included changes in vegetation type, vegetation coverage, and soil moisture. In particular, dryline morphology has been shown to be dependent on the Horizontal distribution of soil moisture. Modeling studies have also suggested that increases in the low-Level Horizontal water vapor gradient, associated with a dryline, are a result of frontogenetic forcing. The current study will extend past results by including more sensitivity experiments showing the dependence of dryline morphology on soil moisture. In this paper, the Regional Atmospheric Modeling System was used to simulate the 26 April 1991 central plains dryline. Five simulations were conducted in wh...

Song-lak Kang - One of the best experts on this subject based on the ideXlab platform.

  • Effects of Mesoscale Surface Thermal Heterogeneity on Low-Level Horizontal Wind Speeds
    Boundary-Layer Meteorology, 2012
    Co-Authors: Song-lak Kang, Donald Lenschow, Peter Sullivan
    Abstract:

    Using large-eddy simulation, we investigate characteristics of Horizontal wind speed at 100 m above the ground, with surface heat-flux variations that are sinusoidal with amplitudes of 0, 50, and 200 W m^−2 and wavelengths of 16, 32, and 128 km, and no background flow. When the amplitude is 200 W m^−2, wind speeds induced by the surface-flux variations on scales of 16 and/or 32 km have multiple temporal oscillations from 0600 to 1800 local standard time. The positive peaks first appear before noon. In contrast, for wind speeds induced by the 128-km surface heterogeneity, a single oscillation occurs in the late afternoon, which is much larger than those generated by the 16- and 32-km surface heterogeneity. In addition, at the oscillation onset the kurtosis of the velocity increment over a distance of 1 km significantly increases, which implies intermittency in the generation of 1-km scale eddies. The spatially intermittent energy cascade generated by surface heterogeneity scaled down to 1-km eddies is analogous to the well-known intermittent energy cascade in the inertial subrange. The kurtosis of the 1-km eddies is much larger with the 128-km surface heterogeneity than with the 16- and 32-km heterogeneities. Thus we conclude that localized rapid changes of low-Level Horizontal wind speed may be caused by significant local surface heterogeneity on scales between a few tens and a few hundreds of kilometres.

René D. Garreaud - One of the best experts on this subject based on the ideXlab platform.

V. Krishnamurthy - One of the best experts on this subject based on the ideXlab platform.

  • Seasonality and mechanisms of tropical intraseasonal oscillations
    Climate Dynamics, 2017
    Co-Authors: Abheera Hazra, V. Krishnamurthy
    Abstract:

    This study has compared the monsoon intraseasonal oscillation (MISO) during the boreal summer and Madden Julian Oscillation (MJO) during the boreal winter. Based on MISO and MJO in high-resolution three-dimensional diabatic heating, the possible mechanisms are discussed through observational analyses of dynamical and thermodynamical variables. The MISO and MJO are extracted as nonlinear oscillations during boreal summer and winter, respectively, by applying multi-channel singular spectrum analysis on daily anomalies of diabatic heating over the Indo-Pacific region. Lead and lag relations among moisture, temperature and surface fields relative to diabatic heating are analyzed to compare the mechanisms of MISO and MJO. While both the oscillations show eastward propagation, MISO has a strong northward propagation and MJO has a weak southward propagation as well. The analysis shows that MJO and MISO are essentially driven by the same mechanisms but with some difference in the meridional propagation. The westerly shear leads the diabatic heating, while the vorticity has weak correlation. Large-scale circulation creates positive moisture preconditioning before convection and negative moisture preconditioning before suppressed conditions. A positive lower Level Horizontal advection of temperature and upper Level temperature tendencies lead the convective state while a negative lower Level Horizontal advection of temperature and upper Level temperature tendencies lead the suppressed state. There is positive feedback from the SST to atmosphere. The difference in the meridional propagation of MISO and MJO is hypothesized to be because of the different differential heating meridionally during the two seasons.

  • Simulation of the South Asian Monsoon in a Coupled Model with an Embedded Cloud-Resolving Model
    Journal of Climate, 2014
    Co-Authors: V. Krishnamurthy, Cristiana Stan, David A. Randall, Ravi P. Shukla, James L. Kinter
    Abstract:

    AbstractThe simulation of the South Asian monsoon by a coupled ocean–atmosphere model with an embedded cloud-resolving model is analyzed on intraseasonal and interannual time scales. The daily modes of variability in the superparameterized Community Climate System Model, version 3 (SP-CCSM), are compared with those in observation, the superparameterized Community Atmospheric Model, version 3 (SP-CAM3), and the control simulation of CCSM (CT-CCSM) with conventional parameterization of convection. The CT-CCSM fails to simulate the observed intraseasonal oscillations but is able to generate the atmospheric El Nino–Southern Oscillation (ENSO) mode, although with regular biennial variability. The dominant modes of variability extracted from daily anomalies of outgoing longwave radiation, precipitation, and low-Level Horizontal wind in SP-CCSM consist of two intraseasonal oscillations and two seasonally persisting modes, in good agreement with observation. The most significant observed features of the intraseas...

  • Sources of Subseasonal Predictability over CONUS during Boreal Summer
    Journal of Climate, 1
    Co-Authors: V. Krishnamurthy, Jessica Meixner, Lydia Stefanova, Jiande Wang, Denise Worthen, Shrinivas Moorthi, Travis Sluka, Cristiana Stan
    Abstract:

    AbstractThe predictability of the Unified Forecast System (UFS) Coupled Model Prototype 2 developed by the National Centers for Environmental Prediction is assessed for the boreal summer over the continental United States (CONUS). The retrospective forecasts of low-Level Horizontal wind, precipitation and 2-m temperature for 2011–17 are examined to determine the predictability at subseasonal time scale. Using a data-adaptive method, the leading modes of variability are obtained and identified to be related to El Niño–Southern Oscillation (ENSO), intraseasonal oscillation (ISO), and warming trend. In a new approach, the sources of enhanced predictability are identified by examining the forecast errors and correlations in the weekly averages of the leading modes of variability. During the boreal summer, the ISO followed by the trend in UFS are found to provide better predictability in weeks 1–4 compared to the ENSO mode and the total anomaly. The western CONUS seems to have better predictability on weekly time scale in all three modes.