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Curtis N. James and Robert A. Houze Jr.

Abstract

This study compiles and interprets three-dimensional Weather Surveillance Radar-1988 Doppler (WSR-88D) data during a 2.5-yr period and examines the typical orographic effects on precipitation mainly associated with winter storms passing over coastal northern California.

The three-dimensional mean reflectivity patterns show echo structure that was generally stratiform from over the ocean to inland over the mountains. The flow above the 1-km level was strong enough to be unblocked by the terrain, and the mean echo pattern over land had certain characteristics normally associated with an unblocked cross-barrier flow, both on the broad scale of the windward slopes of the coastal mountains and on the scale of individual peaks of the terrain on the windward side. Upward-sloping echo contours on the scale of the overall region of coastal mountains indicated broadscale upslope orographic enhancement. On a smaller scale, the mean stratiform echo pattern over the mountains contained a strong embedded core of maximum reflectivity over the first major peak of terrain encountered by the unblocked flow and a secondary echo core over the second major rise of the coastal mountain terrain.

Offshore, upstream of the coastal mountains, the reflectivity pattern showed a region of enhanced mainly stratiform echo within ∼100 km of the coast, with an embedded echo core, similar to those over the inland mountain peaks, along its leading edge. It is suggested that the offshore enhancement is caused by intensified frontogenesis in the offshore coastal zone and/or by the onshore directed low-level flow rising over a thin layer of cool, stable air dammed against the coastal mountains.

The orographically enhanced precipitation offshore and over the coastal mountains was present to some degree in all the landfalling storms. However, the degree to which each feature was present varied. All the features were more pronounced when the 500–700-hPa flow was strong, the midlevel humidity was high, and the low-level cross-barrier wind component was strong. When the low-level stability was greater, the offshore enhancement of precipitation was proportionately increased, and the general broadscale enhancement inland was reduced.

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Curtis N. James and Robert A. Houze Jr.

Abstract

A new dealiasing scheme uses the full four-dimensionality available in an operational Doppler radar data stream. It examines one tilt angle at a time, beginning at the highest elevation where clutter is minimal and gate-to-gate shear is typically low compared to the Nyquist velocity. It then dealiases each tilt in descending order until the entire radial velocity volume is corrected.

In each tilt, the algorithm performs six simple steps. In the first two steps, a reflectivity threshold and filter are applied to the radial velocity field to remove unwanted noise. The third step initializes dealiasing by attempting to adjust the value of each gate by Nyquist intervals such that it agrees with both the nearest gate in the next higher tilt and the nearest gate in the previous volume. The gates that pass the third step at a high confidence level become the initial values for step four, which consist of correcting the neighboring gates within the scan, while preserving environmental shear as much as possible. In step five, remaining gates are compared to an average of neighboring corrected gates, and anomalous gates are deleted. As a last resort, step six uses a velocity azimuth display (VAD) analysis of the wind field to interpret and correct any remaining isolated echoes.

This scheme uses all available data dimensions to interpret and dealias each tilt and is efficient enough to operate on a continuous data stream. It performs reliably even in difficult dealiasing situations and at low Nyquist velocity. During two complex events observed by low-Nyquist Doppler radar in the European Alps, 93% of 4300 tilts were dealiased without error. When errors did occur, they were usually confined to small regions and most frequently resulted from the occurrence of gate-to-gate shear that was impossible to resolve by the Nyquist velocity.

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Curtis N. James, Stacy R. Brodzik, Harry Edmon, Robert A. Houze Jr., and Sandra E. Yuter

Abstract

MountainZebra is a data flow configuration that processes and displays radar data over complex terrain. The system combines three elements: the data stream from an operational radar, 3D topographical information, and the NCAR Zebra data visualization and integration software. MountainZebra operates routinely on a 3D data stream from the National Weather Service Weather Surveillance Radar-1988 Doppler (WSR-88D) at Camano Island, Washington (near Seattle). The WSR-88D data are continuously acquired, archived, formatted, and interpolated for multidimensional display. The three-dimensional topographical information in MountainZebra can be automatically underlaid on any horizontal or vertical display of the radar data. This system allows radar data and other geophysical fields to be analyzed in precise relation to the underlying terrain.

Terrain-based visualization facilitates radar data analysis by identifying terrain clutter and shadowing and by identifying orographic precipitation mechanisms. The utility of MountainZebra is illustrated in the investigation of stable orographic enhancement over the windward slopes of the Cascade Mountains of the Pacific Northwest and an orographically enhanced squall line to the lee of the European Alps.

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Monika Feldmann, Curtis N. James, Marco Boscacci, Daniel Leuenberger, Marco Gabella, Urs Germann, Daniel Wolfensberger, and Alexis Berne

Abstract

Region-based Recursive Doppler Dealiasing (R2D2) is a novel dealiasing algorithm to unfold Doppler velocity fields obtained by operational radar measurements. It specializes in resolving issues when the magnitude of the gate-to-gate velocity shear approaches or exceeds the Nyquist velocity. This occurs either in highly sheared situations, or when the Nyquist velocity is low. Highly sheared situations, such as convergence lines or mesocyclones, are of particular interest for nowcasting and warnings. R2D2 masks high-shear areas and adds a spatial buffer around them. The areas between the buffers are then identified as continuous regions that lie within the same Nyquist interval. Each region subsequently is assigned its most likely Nyquist interval by applying vertical and temporal continuity constraints, as well as supplemental wind information from an operational mesoscale model. The shear zones are then resolved using 2D continuity in azimuth and range. This 4D procedure is repeated until no further improvement can be achieved. Each iteration with fewer folds identifies fewer but larger continuous regions and less shear zones until an optimum is reached. Residual errors, often related to shear greater than the Nyquist velocity, are contained to small areas within the buffer zones. This approach maximizes operational performance in high-shear situations and restricts errors to minimal areas to mitigate error propagation.

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Curtis R. Wood, Samantha J. Arnold, Ahmed A. Balogun, Janet F. Barlow, Stephen E. Belcher, Rex E. Britter, Hong Cheng, Adrian Dobre, Justin J. N. Lingard, Damien Martin, Marina K. Neophytou, Fredrik K. Petersson, Alan G. Robins, Dudley E. Shallcross, Robert J. Smalley, James E. Tate, Alison S. Tomlin, and Iain R. White

In the event of a release of toxic gas in the center of London, emergency services personnel would need to determine quickly the extent of the area contaminated. The transport of pollutants by turbulent flow within the complex streets and building architecture of London, United Kingdom, is not straightforward, and we might wonder whether it is at all possible to make a scientifically reasoned decision. Here, we describe recent progress from a major U.K. project, Dispersion of Air Pollution and its Penetration into the Local Environment (DAPPLE; information online at www.dapple.org.uk). In DAPPLE, we focus on the movement of airborne pollutants in cities by developing a greater understanding of atmospheric flow and dispersion within urban street networks. In particular, we carried out full-scale dispersion experiments in central London from 2003 through 2008 to address the extent of the dispersion of tracers following their release at street level. These measurements complemented previous studies because 1) our focus was on dispersion within the first kilometer from the source, when most of the material was expected to remain within the street network rather than being mixed into the boundary layer aloft; 2) measurements were made under a wide variety of meteorological conditions; and 3) central London represents a European, rather than North American, city geometry. Interpretation of the results from the full-scale experiments was supported by extensive numerical and wind tunnel modeling, which allowed more detailed analysis under idealized and controlled conditions. In this article, we review the full-scale DAPPLE methodologies and show early results from the analysis of the 2007 field campaign data.

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