			Home About us Contact

Height Fields (height + field)

Distribution by Scientific Domains

Earth and Environmental Science	62%

Selected Abstracts

Fast Global Illumination on Dynamic Height Fields

COMPUTER GRAPHICS FORUM, Issue 4 2009
Derek Nowrouzezahrai
Abstract We present a real-time method for rendering global illumination effects from large area and environmental lights on dynamic height fields. In contrast to previous work, our method handles inter-reflections (indirect lighting) and non-diffuse surfaces. To reduce sampling, we construct one multi-resolution pyramid for height variation to compute direct shadows, and another pyramid for each indirect bounce of incident radiance to compute inter-reflections. The basic principle is to sample the points blocking direct light, or shedding indirect light, from coarser levels of the pyramid the farther away they are from a given receiver point. We unify the representation of visibility and indirect radiance at discrete azimuthal directions (i.e., as a function of a single elevation angle) using the concept of a "casting set" of visible points along this direction whose contributions are collected in the basis of normalized Legendre polynomials. This analytic representation is compact, requires no precomputation, and allows efficient integration to produce the spherical visibility and indirect radiance signals. Sub-sampling visibility and indirect radiance, while shading with full-resolution surface normals, further increases performance without introducing noticeable artifacts. Our method renders 512×512 height fields (> 500K triangles) at 36Hz. [source]

Recalibration of general circulation model output to austral summer rainfall over southern Africa

INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 12 2003
A. G. Bartman
Abstract Empirical techniques are developed to adjust dynamic model forecasts on the seasonal time scale for southern African summer rainfall. The techniques, perfect prognosis and model output statistics (MOS), are utilized to ,recalibrate' the CSIRO 9 general circulation model (GCM) large-scale fields statistically to three equi-probable rainfall categories for December to February. The recalibration is applied to a GCM experiment where simultaneously observed sea-surface temperature fields serve as the lower boundary forcing. An optimal canonical correlation analysis model is designed for MOS and perfect prognosis and the 700 hPa geopotential height field is selected as the single predictor field in the two sets of statistical equations that are subsequently used to produce recalibrated rainfall simulations over a 10 year independent test period. MOS produced the higher forecast skill for southern Africa over the independent test period. Copyright © 2003 Royal Meteorological Society [source]

A critical comparison of stratosphere,troposphere coupling indices

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 644 2009
Mark P. Baldwin
Abstract Assessing stratosphere,troposphere coupling in observational data or model output requires a multi-level index with high time resolution. Ideally, such an index would (1) represent spatial patterns in the troposphere that are most strongly coupled with stratospheric variability and (2) be robust and computationally feasible in both observations and standard model output. Several of the indices used to diagnose extratropical stratosphere,troposphere coupling are based on the Northern and Southern Hemisphere annular modes. The annular mode indices are commonly defined as the leading empirical orthogonal functions (EOFs) of monthly-mean, hemispheric geopotential height. In the lowermost troposphere, the structure of the annular modes is defined as the leading EOF of the near-surface geopotential height field, and these patterns correspond well to the patterns of variability induced by stratospheric circulation changes. At pressure levels above the surface, the structure of the annular modes is typically found by either calculating the local EOF or regressing geopotential height data onto the leading principal component time series of near-surface geopotential height. Here we make a critical comparison of the existing methodologies used to diagnose stratosphere,troposphere coupling, including EOF-based indices as well as measures based on zonal-mean wind at a fixed latitude and geopotential height over the polar cap. We argue in favour of an alternative methodology based on EOFs of daily zonally-averaged geopotential. We find that (1) the daily evolution of stratosphere,troposphere coupling events is seen most clearly with this methodology, and (2) the methodology is robust and requires few subjective choices, making it readily applicable to climate model output available only in zonal-mean form. Copyright © 2009 Royal Meteorological Society [source]

Wavelet analysis and the governing dynamics of a large-amplitude mesoscale gravity-wave event along the East Coast of the United States

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 577 2001
Fuqing Zhang
Abstract Detailed diagnostic analyses are performed upon a mesoscale numerical simulation of a well-observed gravity-wave event that occurred on 4 January 1994 along the East Coast of the United States. The value of using wavelet analysis to investigate the evolving gravity-wave structure and of using potential vorticity (PV) inversion to study the nature of the flow imbalance in the wave generation region is demonstrated. The cross-stream Lagrangian Rossby number, the residual in the nonlinear balance equation, and the unbalanced geopotential-height field obtained from PV inversion are each evaluated for their usefulness in diagnosing the flow imbalance. All of these fields showed clear evidence of strong imbalance associated with a middle-to-upper tropospheric jet streak, and tropopause fold upstream of the large-amplitude gravity wave several hours before the wave became apparent at the surface. Analysis indicates that a train of gravity waves was continuously generated by geostrophic adjustment in the exit region of the unbalanced upper-level jet streak as it approached the inflection axis in the height field immediately downstream of the maximum imbalance associated with the tropopause fold. A split front in the middle troposphere, characterized by the advance of the dry conveyor belt above the warm front, was overtaken by one of these propagating waves. During this merger process, a resonant interaction resulted, which promoted the rapid amplification and scale contraction of both the incipient wave (nonlinear wave development) and the split front (frontogenesis). The gravity wave and front aloft became inseparable following this merger. The situation became even more complex within a few hours as the vertical motion enhanced by this front-wave interaction acted upon a saturated, potentially unstable layer to produce elevated moist convection. An analysis of the temporal changes in the vertical profile of wave energy flux suggests that moist convective downdraughts efficiently transported the wave energy from the midlevels downward beneath the warm-front surface, where the wave became ducted. However, pure ducting was not sufficient for maintaining and amplifying the waves; rather, wave-CISK (Conditional Instability of the Second Kind) was crucial. This complex sequence of nonlinear interactions produced a long-lived, large-amplitude gravity wave that created hazardous winter weather and disrupted society over a broad and highly populated area. Although gravity waves with similar appearance to this large-amplitude wave of depression occasionally have been seen in other strong cyclogenesis cases involving a jet streak ahead of the upper-level trough axis, it is unknown whether other such events share this same sequence of interactions. [source]

Fast Global Illumination on Dynamic Height Fields

Characteristics of wintertime daily and extreme minimum temperature over South Korea

INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 2 2004
Sang-Boom Ryoo
Abstract In South Korea, consecutive positive temperature anomalies have been observed since the mid-1980s. The objective of this study is to assess the recent trends in, and variability of, daily minimum temperature over South Korea with particular emphasis on its extremes. Temporal characteristics of wintertime daily and extreme minimum temperature-related variables were analysed on a seasonal basis for the period of 1958,59 to 2000,01. The results show continually fewer days with extreme low minimum temperature since the mid-1980s. However, no significant change in the 1 day temperature difference was observed during the same period, indicating little change in the frequency of cold surges. Also, during the period analysed, there is a significant positive trend in the seasonal mean temperature, a negative trend in the frequency of the extreme cold days, and no significant trend in the seasonal occurrence of cold surges. Northern Hemisphere geopotential height fields before and after 1986,87, i.e. the start of successive positive anomalies in the winter surface air temperature over South Korea, showed a substantial decrease throughout the troposphere over the polar region. In the upper levels the overall pattern becomes more wavelike, with eddies embedded between meanders. The differences in the lower troposphere are remarkably similar to the Arctic oscillation, although the centre in the North Atlantic is shifted toward western Europe and differences in the North Pacific are relatively weaker than those in the polar region. The recent positive phase of the Arctic oscillation may contribute to these abrupt changes in wintertime daily minimum temperatures over South Korea. El Niño,southern oscillation phenomena appear to contribute to the interannual variation of cold surge days in South Korea. Years with no cold surges were experienced during La Niña episodes. On the other hand, all years with more than four cases of cold surges were during El Niño episodes. Copyright © 2004 Royal Meteorological Society [source]

Patterns of convection in the tropical pacific and their influence on New Zealand weather

INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 2 2002
John W. Kidson
Abstract Characteristic patterns of convection in the tropical Pacific Ocean have previously been inferred from analysis of outgoing longwave radiation (OLR), and associated with year-to-year variations in El Niño (EN),Southern Oscillation events. This study examines both the effects of these convection patterns on the New Zealand climate, and the more general influence of tropical convection on the New Zealand sector of the Southern Hemisphere. The Southern Hemisphere circulation, as a whole, is found to be most strongly influenced by equatorial convection near the Philippines, and in a broad band over the central Pacific. Where increased convection occurs west of 160°E, La Niña-like (LN) conditions prevail. When the anomalous convective activity is located near the dateline, in ,moderate' EN conditions, SW flow prevails over New Zealand. This gives way to stronger WSW anomalies as the centre of convection is displaced further eastwards and a second centre of reduced convection becomes prominent west of the dateline in strong EN (EN+) events. The changes in wind regimes over the New Zealand region implied by the hemispheric 1000 hPa height fields are supported by mean sea-level pressure differences between a number of New Zealand and adjacent island stations. Indices of the zonal flow show a weak reduction in strength of the westerlies for LN OLR composites, and no apparent effects for EN composites, whereas EN+ conditions strongly favour above-normal westerlies. The meridional flow over New Zealand is skewed towards more frequent southerlies in both the EN and EN+ composites, whereas LN conditions favour northerly flow anomalies. A change is also observed in the frequency of New Zealand-area ,weather regimes'. Enhanced convection centred on 5°S and east of the dateline, as found in the EN+ composites, leads to an increase in zonal regimes and a corresponding decrease in blocking regimes. The direct influence of tropical OLR variations on New Zealand temperature and precipitation has also been assessed. These indicate that the response is not simply one of degree. Different spatial anomaly patterns in the climatic elements result from the varying regional circulation patterns, and these need to be considered if present climate-forecasting schemes are to be improved. Copyright © 2002 Royal Meteorological Society. [source]