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H-L. Liu

Abstract

The vertical wavenumber and frequency spectra of horizontal wind and temperature in stochastically driven systems with diffusion, either due to uniform background eddy and molecular transport, or due to adjustment processes associated with shear or convective instability, are studied. Because of the dominating role of vertical transport in a stratified fluid, one-dimensional Langevin-type equations could be ascribed to such systems in the vertical direction. The linear equation with uniform diffusion is solved explicitly, and the spectra follow power-law distributions if the stochastic force is Gaussian. The nonlinear equations with gradient (either shear or lapse rate) dependent diffusion coefficients are shown to support scale invariance, and the power-law indices of the spectra are determined from dynamic renormalization group (DRG) analysis under rather general conditions. The exact power-law indices vary with the spectrum of the stochastic force and the nonlinearity of the systems. If the wavenumber spectrum of the force is moderately red (between k 0 and k −2), the spectral indices of horizontal wind and temperature and the range of their variability are in general agreement with those inferred from wind and temperature measurements. The indices in both linear and nonlinear cases are confirmed by numerical simulations. This theory may suggest an alternative explanation to the universal vertical wavenumber and frequency spectra and their variability. By relating the universal spectra to systems characterized by stochastic forcing and background diffusion or diffusive adjustment due to shear or convective instability, which are ubiquitous in a stratified fluid, the difficulty to associate the time- and location-independent spectral features directly with the highly time- and location-dependent gravity waves or wave-breaking events is avoided. If such systems are suggestive of the real atmosphere, there is a need to be cautious in making assumptions regarding gravity waves solely based on the universal spectra when analyzing and interpreting wind and temperature observations.

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H. Liu and X. Zou

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During the North Pacific Experiment (NORPEX), both the Navy Operational Global Atmospheric Prediction System and the National Centers for Environmental Prediction (NCEP) operational forecast systems found a 48-h forecast degradation over the NORPEX forecast verification region due to the inclusion of a set of NORPEX targeted dropsondes deployed north of Hawaii during 29–30 January 1998. The NCEP three- and four-dimensional varitional data assimilation (3DVAR and 4DVAR) systems are used here to reassess the impact of these dropsonde observations on model predictions. The assimilation of these targeted dropsondes excluding the conventional observations improved the 48-h forecast over the NORPEX forecast verification region. However, the addition of the dropsonde data to an analysis that already contained various conventional observations degraded the 48-h forecast over the NORPEX forecast verification region. In the later case, the dropsonde data still improved and had its largest impact on the forecast over the northeast Pacific (outside of the forecast verification region). In this region, errors in the forecast using only conventional observations were largest. Furthermore, assimilation of the targeted dropsonde data using the 4DVAR approach produced greater improvements in the 1–3-day forecasts over the Pacific Ocean than the 3DVAR approach did in both cases, with and without conventional observations.

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C. Liu, Y. Liu, and H. Xu

Abstract

In this work, the forecast accuracy of a numerical weather prediction model is improved by emulating physical dissipation as suggested by the second law of thermodynamics, which controls the irreversible evolutionary direction of a many-body system like the atmosphere. The ability of the new physics-based scheme to improve model accuracy is demonstrated via the case of the one-dimensional viscous Burgers equation and the one-dimensional diffusion equation, as well as a series of numerical simulations of the well-known 1998 successive torrential rains along the Yangtze River valley and 365 continuous 24-h simulations during 2005–06 with decreased root-mean-square errors and improved forecasts in all of the simulations.

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Yangang Liu and Peter H. Daum
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Abraham H. Oort and Huanzhu Liu

Abstract

New time series of the hemispheric and global mean temperature anomalies in the troposphere and lower stratosphere are presented for the period May 1958 through December 1989. The statistics are based on objective monthly analyses of all available daily soundings from the global rawinsonde network (∼700–800 stations). The results are compared with Angell's earlier statistics based on a subset of 63 stations. Excellent agreement is found with these earlier results as well as with an 11-year set of satellite-derived microwave sounding unit data. These detailed comparisons support the conclusion that the rawinsonde network can provide reliable estimates of the actual interseasonal hemispheric-scale temperature changes that have occurred between the earth's surface and about 20 km (50 mb) height since the 1950s.

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Zhong Liu and David H. Bromwich

Abstract

The surface wind pattern over the ice sheets of Antarctica is irregular with marked areas of airflow confluence near the coastal margins. Where cold air from a large interior area of the ice sheet converges (a confluence zone), an anomalously large supply of air is available to feed the coastal katabatic winds, which, as a result, are intensified and more persistent. The confluence zone inland of Siple Coast, West Antarctica, differs from its East Antarctic counterparts in that the terrain slopes become gentler rather than steeper as the coast is approached. In addition, synoptic processes exert substantially more impact on the behavior of the surface winds.

A month-long field program to study the dynamics of the springtime katabatic wind confluence zone has been carried out near Siple Coast. Two sites, Upstream B (83.5°S, 136.1°W) and South Camp (84.5°S, 134.3°W), were established roughly perpendicular to the downslope direction. The field program involved the use of the ground-based remote sensing equipment (sodar and RASS) along with conventional surface and balloon observations. Previous analyses revealed the cross-sectional structure of the confluence zone as consisting of a more buoyant West Antarctic katabatic airflow overlying a less buoyant katabatic airflow originating from East Antarctica.

The force balances inside the confluence zone are here investigated for three situations: mean (all available wind profiles from balloon launches), and two extreme cases (light and strong winds). A linear regression method is used to estimate the mean vertical wind shears and horizontal temperature gradients. The vertical wind shears are used to examine whether or not the airflows are in geostrophic balance. The results are 1) the airflow above the surface at both sites is in geostrophic balance for the three situations; 2) inside the West Antarctic katabatic wind zone, there are three forces in the north–south direction—the restoring pressure gradient force associated with blocking of the katabatic and synoptic winds, the downslope buoyancy force, and the synoptic pressure gradient force associated with the time-averaged low in the South Pacific Ocean; 3) above the West Antarctic katabatic wind layer, the observed easterly wind is due to the synoptic pressure gradient force associated with the low; 4) inside the East Antarctic katabatic wind zone, in addition to the above three forces, there is the downslope buoyancy force associated with the inversion; and 5) large-scale transient synoptic systems strongly influence the downslope wind speed and the boundary layer depth, resulting in the light and strong wind cases.

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David H. Bromwich and Zhong Liu

Abstract

A month-long field program to study the springtime katabatic wind confluence zone (where katabatic winds converge) has been carried out near Siple Coast, West Antarctica. Based on previous observations and numerical studies, two surface camps, Upstream B (83.5°8, 136.1°W) and South Camp (84.5°S, 134.3°W), were established. Ground-based remote sensing equipment (sodar and RASS), along with conventional observations, were used. Combining the analyses of surface observations and wind and temperature profiles at the above camps, the following picture for the cross-sectional structure of the confluence zone emerges. A relatively cold katabatic airflow, which probably comes from Fast Antarctica, occupies the layer between the surface and roughly 500 m ACL. Low-level jets are present below 200 m AGL and are stronger near the Transantarctic Mountains. Diurnal variation is present in this cold drainage flow and decreases toward the Transantarctic Mountains. Weak-inversion-layer tops are found near 500 m AGL, which is roughly equal to the depth of the cold katabatic flow. The warmer West Antarctic katabatic airflow overlies the cold drainage flow from East Antarctica and has a depth of approximately 1000 m at Upstream B and more than 1500 m at South Camp; this is caused by blocking of the converging West Antarctic airflow by the Transantarctic Mountains. This warm flow originates near the surface far upslope in the vicinity of Byrd Station (80°S, 120°W). A baroclinic zone, formed where the two drainage flows are horizontally adjacent, appears to become unstable with sonar frequency to generate mesoscale cyclones.

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Yonggang Liu and Robert H. Weisberg

Abstract

The across-shelf structures of the ocean circulation and the associated sea surface height (SSH) variability are examined on the west Florida shelf (WFS) for the 3-yr interval from September 1998 to December 2001. Five sets of characteristic circulation patterns are extracted from 2-day, low-pass-filtered data using the self-organizing map: extreme upwelling and downwelling structures with strong currents, asymmetric upwelling and downwelling structures with moderate currents, and a set of transitional structures with weak currents. The temporal variations of these structures are coherent with the local winds on synoptic weather time scales. On seasonal time scales they are related to both the local winds and the water density variations. The circulation is predominantly upwelling during autumn to spring months (October–April) and downwelling during summer months (June–September). Coastal sea level fluctuations are related to both the dynamical responses of the inner shelf circulation to meteorological forcing and the offshore SSH. On long time scales, the offshore SSH variations appear to dominate, whereas on synoptic weather time scales, the inner shelf wind-driven circulation responses are largest. The across-shelf distribution of SSH is estimated from the velocity, hydrography, wind, and coastal sea level data, and the results are compared with satellite altimetry data, thereby providing a means for calibrating satellite altimetry on the shelf.

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Yangang Liu and Peter H. Daum

Abstract

Various commonly used Kessler-type parameterizations of the autoconversion of cloud droplets to embryonic raindrops are theoretically derived from the same formalism by applying the generalized mean value theorem for integrals to the general collection equation. The new formalism clearly reveals the approximations and assumptions that are implicitly embedded in these different parameterizations. A new Kessler-type parameterization is further derived by eliminating the incorrect and/or unnecessary assumptions inherent in the existing Kessler-type parameterizations. The new parameterization exhibits a different dependence on liquid water content and droplet concentration, and provides theoretical explanations for the multitude of values assigned to the tunable coefficients associated with the commonly used parameterizations. Relative dispersion of the cloud droplet size distribution (defined as the ratio of the standard deviation to the mean radius of the cloud droplet size distribution) is explicitly included in the new parameterization, allowing for investigation of the influences of the relative dispersion on the autoconversion rate and, hence, on the second indirect aerosol effect. The new analytical parameterization compares favorably with those parameterizations empirically obtained by curve-fitting results from simulations of detailed microphysical models.

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Zhong Liu and David H. Bromwich

Abstract

The blocking effect of Ross Island and Hut Point peninsula, Antarctica, has been investigated since the early part of this century. Due to lack of continuous measurements of boundary-layer winds, the investigations were limited to an overall description of the blocking effect with no information on the diurnal variation or the detailed vertical structure of the approaching airflow.

An acoustic sounder (sodar) was deployed during the 1990/91 austral summer season at Williams Field in the upwind area south of Ross Island, Antarctica. Such equipment can continuously measure three-dimensional winds from a few tens of meters above the surface up to an altitude of several hundred meters, thus providing a new opportunity to study the dynamics of the stably stratified planetary boundary layer.

In addition to confirming earlier work, the sodar winds show a significant diurnal variation of the blocking effect, which amplifies with height. Such variation is dominated by the changes in the upstream air mass in which katabatic airflow from Byrd, Mulock, and Skelton glaciers plays a central role.

Through case studies, the breakdown of the prevailing wind regime in the Ross Island area was associated with the influence of meso- and synoptic-scale pressure gradients on the katabatic airflow approaching from the south and with very localized geostrophic winds deflected around the topography of Ross Island.

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