Shouping Wang

Controls on precipitation and cloudiness in simulations of trade-wind cumulus as observed during RICO

Twelve large-eddy simulations, with a wide range of microphysical representations, are compared to each other and to independent measurements. The measurements and the initial and forcing data for the simulations are taken from the undisturbed period of the Rain in Cumulus over the Ocean (RICO) field study. A regional downscaling of meteorological analyses is performed so as to provide forcing data consistent with the measurements. The ensemble average of the simulations plausibly reproduces many features of the observed clouds, including the vertical structure of cloud fraction, profiles of cloud and rain water, and to a lesser degree the population density of rain drops. The simulations do show considerable departures from one another in the representation of the cloud microphysical structure and the ensuant surface precipitation rates, increasingly so for the more simplified microphysical models. There is a robust tendency for simulations that develop rain to produce a shallower, somewhat more stable cloud layer. Relations between cloud cover and precipitation are ambiguous.

The Coupled Boundary Layers and Air–Sea Transfer Experiment in Low Winds

The Office of Naval Researchs Coupled Boundary Layers and Air–Sea Transfer (CBLAST) program is being conducted to investigate the processes that couple the marine boundary layers and govern the exchange of heat, mass, and momentum across the air–sea interface. CBLAST-LOW was designed to investigate these processes at the low-wind extreme where the processes are often driven or strongly modulated by buoyant forcing. The focus was on conditions ranging from negligible wind stress, where buoyant forcing dominates, up to wind speeds where wave breaking and Langmuir circulations play a significant role in the exchange processes. The field program provided observations from a suite of platforms deployed in the coastal ocean south of Marthas Vineyard. Highlights from the measurement campaigns include direct measurement of the momentum and heat fluxes on both sides of the air–sea interface using a specially constructed Air–Sea Interaction Tower (ASIT), and quantification of regional oceanic variability over sca...

Turbulence, Condensation, and Liquid Water Transport in Numerically Simulated Nonprecipitating Stratocumulus Clouds

Abstract Condensation and turbulent liquid water transport in stratocumulus clouds involve complicated interactions between turbulence dynamics and cloud microphysical processes, and play essential roles in defining the cloud structure. This work aims at understanding this dynamical–microphysical interaction and providing information necessary for parameterizations of the ensemble mean condensation rate and turbulent fluxes of liquid water variables in a coupled turbulence–microphysics model. The approach is to simulate nonprecipitating stratocumulus clouds with a coupled large eddy simulation and an explicit bin-microphysical model, and then perform a budget analysis for four liquid water variables: mean liquid water content, turbulent liquid water flux, mean cloud droplet number concentration, and the number density flux. The results show that the turbulence contribution to the mean condensation rate comes from covariance of the integral cloud droplet radius and supersaturation, which enhances condensat...

On Boundary Layer Separation in the Lee of Mesoscale Topography

Abstract The onset of boundary layer separation (BLS) forced by gravity waves in the lee of mesoscale topography is investigated based on a series of numerical simulations and analytical formulations. It is demonstrated that BLS forced by trapped waves is governed by a normalized ratio of the vertical velocity maximum to the surface wind speed; other factors such as the mountain height, mountain slope, or the leeside speedup factor are less relevant. The onset of BLS is sensitive to the surface sensible heat flux—a positive heat flux tends to increase the surface wind speed through enhancing the vertical momentum mixing and accordingly inhibits the occurrence of BLS, and a negative heat flux does the opposite. The wave forcing required to cause BLS decreases with an increase of the aerodynamical roughness zo; a larger zo generates larger surface stress and weaker surface winds and therefore promotes BLS. In addition, BLS shows some sensitivity to the terrain geometry, which modulates the wave characterist...

The Impact of Ice Phase Cloud Parameterizations on Tropical Cyclone Prediction

AbstractThe impact of ice phase cloud microphysical processes on prediction of tropical cyclone environment is examined for two microphysical parameterizations using the Coupled Ocean–Atmosphere Mesoscale Prediction System–Tropical Cyclone (COAMPS-TC) model. An older version of microphysical parameterization is a relatively typical single-moment scheme with five hydrometeor species: cloud water and ice, rain, snow, and graupel. An alternative newer method uses a hybrid approach of double moment in cloud ice and rain and single moment in the other three species. Basin-scale synoptic flow simulations point to important differences between these two schemes. The upper-level cloud ice concentrations produced by the older scheme are up to two orders of magnitude greater than the newer scheme, primarily due to differing assumptions concerning the ice nucleation parameterization. Significant (1°–2°C) warm biases near the 300-hPa level in the control experiments are not present using the newer scheme. The warm bi...

Investigating the Summertime Low-Level Jet Over the East Coast of the U.S.A.: A Case Study

The vertical structure of the marine atmospheric boundary layer (MABL) and the summertime low-level jet (LLJ) along the east coast of the U.S.A. was studied in the framework of a joint expedition (CBLAST-Low), carried out during the summer of 2003 (31 July–27 August) at Nantucket island, Massachusetts, U.S.A. Analyzing measurements from radiosondes, in-situ and remote sensing systems, it was found that within the lower part of the stable MABL, intermittent and persistent LLJ events were frequently observed. Evidence is presented implying that the analyzed jet case was induced by the interaction of a slow-moving cyclone over north-eastern U.S.A. and the stationary high pressure system over the Atlantic Ocean, as well as by the sloping inversion of the MABL. Focused on a 5-day period of persistent south-westerly (marine) flow, the analysis of sodar and radiosonde data revealed the presence of a stable layer associated with increased static stability just before the emergence of low-level flow acceleration. As indicated by the Richardson number profiles, the increased stability of the lower MABL suppressed turbulence, allowed the decoupling of LLJ from friction, providing a favourable environment for the development of inertial oscillations. Significant amplitudes of inertial motions, which were confirmed by the application of a Hilbert–Huang transform, are associated with the acceleration at the LLJ’s core, due to the frontal events and the subsequent frictional decoupling, both leading to a modification of the large-scale flow structure.

Turbulent and thermodynamic structure of the autumnal arctic boundary layer due to embedded clouds

We have studied the role of low-level clouds in modifying the thermodynamic and turbulence properties of the Arctic boundary layer during autumn. This was achieved through detailed analyses of boundary-layer properties in two regions, one with low-level cloud cover and the other free of clouds, using measurements from a research aircraft during the Beaufort and Arctic Storms Experiment (BASE). Both regions were measured on the same day under similar synoptic forcing. The cloudy region was characterized by strong horizontal inhomogeneity in low-level temperature and moisture that varied with the cloud-top height. The clear region was relatively homogeneous in temperature and specific humidity with a strong temperature inversion extending between heights of 100 m and 3 km. From measurements at the lowest levels, we also identified a shallow mixed layer below the deep stable layer in the clear region.Our spectral analyses revealed significant modifications of boundary-layer properties due to the presence of low-level clouds. In the cloudy region, turbulent perturbations dominated the boundary-layer flow and made large contributions to the scalar variances. In the clear boundary-layer, wave motion contributed significantly to the observed variances, while turbulent flow was relatively weak. The clear region was saturated, although no detectable clouds were measured.

Impact of Swell on Air–Sea Momentum Flux and Marine Boundary Layer under Low-Wind Conditions

AbstractThe impact of fast-propagating swell on the air–sea momentum exchange and the marine boundary layer is examined based on multiple large-eddy simulations over a range of wind speed and swell parameters in the light-wind–fast-wave regime. A wave-driven supergeostrophic jet forms near the top of the wave boundary layer when the forwarding-pointing (i.e., negative) form drag associated with fast wind-following swell overpowers the positive surface shear stress. The magnitude of the form drag increases with the wavelength and slope and decreases with increasing wind speed, and the jet intensity in general increases with the magnitude of the surface form drag. The resulting negative vertical wind shear above the jet in turn enhances the turbulence aloft. The level of the wind maximum is found to be largely determined by the wavenumber and the ratio of the surface shear stress and form drag: the larger the magnitude of this ratio, the higher the altitude of the wind maximum.Although the simulated wind pr...

Large-Eddy Simulation Study of Log Laws in a Neutral Ekman Boundary Layer

Abstract The characteristics of wind profiles in a neutral atmospheric boundary layer and their dependence on the geostrophic wind speed Ug, Coriolis parameter f, and surface roughness length z0 are examined utilizing large-eddy simulations. These simulations produce a constant momentum flux layer and a log-law layer above the surface characterized by a logarithmic increase of wind speed with height. The von Karman constant derived from the mean wind profile is around 0.4 over a wide range of control parameters. The depths of the simulated boundary layer, constant-flux layer, and surface log-law layer tend to increase with the wind speed and decrease with an increasing Coriolis parameter. Immediately above the surface log-law layer, a second log-law layer has been identified from these simulations. The depth of this upper log-law layer is comparable to its counterpart in the surface layer, and the wind speed can be scaled as , as opposed to just in the surface log-law layer, implying that in addition to s...