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Ryan D. Torn and David Cook

Abstract

An ensemble of Weather Research and Forecasting Model (WRF) forecasts initialized from a cycling ensemble Kalman filter (EnKF) system is used to evaluate the sensitivity of Hurricanes Danielle and Karl’s (2010) genesis forecasts to vortex and environmental initial conditions via ensemble sensitivity analysis. Both the Danielle and Karl forecasts are sensitive to the 0-h circulation associated with the pregenesis system over a deep layer and to the temperature and water vapor mixing ratio within the vortex over a comparatively shallow layer. Empirical orthogonal functions (EOFs) of the 0-h ensemble kinematic and thermodynamic fields within the vortex indicate that the 0-h circulation and moisture fields covary with one another, such that a stronger vortex is associated with higher moisture through the column. Forecasts of the pregenesis system intensity are only sensitive to the leading mode of variability in the vortex fields, suggesting that only specific initial condition perturbations associated with the vortex will amplify with time. Multivariate regressions of the vortex EOFs and environmental parameters believed to impact genesis suggest that the Karl forecast is most sensitive to the vortex structure, with smaller sensitivity to the upwind integrated water vapor and 200–850-hPa vertical wind shear magnitude. By contrast, the Danielle forecast is most sensitive to the vortex structure during the first 24 h, but is more sensitive to the 200-hPa divergence and vertical wind shear magnitude at longer forecast hours.

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Garry D. Cook and Michael J. Nicholls

Abstract

A reexamination of the wind hazard from tropical cyclones for the city of Darwin (Northern Territory), Australia, by Cook and Nicholls concluded that its wind hazard is substantially underestimated by its allocation to region C in the Australian wind code. This conclusion was dismissed by Harper et al. on the basis of interpretation of anemometer records and Dvorak central pressure estimates as well as criticism of the simple technique and data used to interpret historic records. Of the 44 years of historical anemometer records presented by Harper et al. for Darwin, however, only one record was for a direct hit by an intense tropical cyclone. The other records derive from distant and/or weak tropical cyclones, which are not applicable to understanding the wind hazard at long return periods. The Dvorak central pressure estimates from which Harper et al. conclude that Port Hedland (Western Australia), Australia, has a greater wind hazard than Darwin does, when back transformed to Dvorak current-intensity values and gust speeds, indicate the converse. The simple technique used to derive wind hazard from historical cyclone occurrence is defended in detail and shown to produce estimates of wind hazard that are close to those accepted for five locations on the hurricane-affected coastline of the U.S. mainland. Thus the criticisms by Harper et al. of Cook and Nicholl’s work are shown to be invalid and the original conclusion that Darwin’s wind hazard is substantially underestimated in the current Australian wind code is supported.

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J. D. Lenters and K. H. Cook

Abstract

The regional precipitation climatology of South America during austral summer is studied by means of an R30 general circulation model (GCM). Results from perpetual January experiments, which differ only in their distributions of topography and sea surface temperature (SST), are presented. The precipitation field of the most realistic experiment compares well with the observed January precipitation climatology of South America. reproducing, in particular, five regions of maximum precipitation.

To understand how structure in the surface conditions is mapped onto the precipitation field. the results of the three GCM experiments are compared. Continentality, through the generation of a thermal low, is responsible for much of the structure in the modeled precipitation field of South America. Topography introduces orographic precipitation maxima in the Central and Southern Andes and modifies precipitation rates elsewhere. Longitudinal structure in SSTs, which is also largely an expression of continentality, is not a dominant source of structure for the South American precipitation field. However, the positions and magnitudes of some of the precipitation maxima (especially those in the cast) are moderately affected by SSTs.

Analysis of the atmospheric water vapor budget associates structure in the precipitation field with structure in the moisture flux convergence field. Connections with the large-scale circulation are explored to explain the convergence of moisture flux in each region of enhanced precipitation. Comparisons with observed low-level wind and moisture fields suggest that the mechanisms responsible for the modeled precipitation maxima are, for the most part, reflective of those in the real world.

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Todd D. Ringler and Kerry H. Cook

Abstract

Idealized simulations of the atmosphere’s stationary response to the Rockies, Tibetan Plateau, and the Greenland Ice Sheet are made using a nonlinear, quasigeostrophic model and are compared to observations. Observational data indicate low-level heating (cooling) occurs above the Rockies and Tibet in the summer (winter). Low-level cooling is found above Greenland in both seasons. The atmosphere responds to both diabatic heating (termed thermal forcing) and low-level flow being obstructed by the mountain’s presence (termed mechanical forcing).

The response to thermal and mechanical forcing together can be very different from the response to either forcing individually. The presence of modest low-level heating or cooling (±1.5 K day−1) causes significant changes to the mechanical forcing and, thereby, to the stationary wave response. For example, while the nonlinear response to mechanical forcing and low-level heating is characterized by a cyclone over the orography, the response to mechanical forcing and low-level cooling consists of an anticyclone over the orography. These differences cannot be fully explained using linear theory. The presence of heating (cooling) tends to reduce (amplify) both the mechanical forcing and the far-field stationary wave response. In addition, the presence of low-level heating or cooling lowers the critical mountain height below which the response is essentially linear;including nonlinear temperature advection at the surface is especially important for obtaining an accurate response.

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Todd D. Ringler and Kerry H. Cook

Abstract

The forcing of stationary waves by the earth’s large-scale orography is studied using a nonlinear stationary wave model based on the quasigeostrophic equations. The manner in which wind speed, meridional temperature gradient, Ekman pumping parameter, linear damping, orographic shape, and meridional wind structure affect the validity of the linearized equations is examined and the nonlinear response is investigated.

A critical mountain height that separates the linear from the nonlinear regime is defined based on the linear quasigeostrophic potential temperature equation applied at the surface. The largest critical heights (those responses in which nonlinearity is least important) are obtained when the surface damping is weak or nonexistent. Also, relative maximums in mountain critical heights are obtained when the ratio of surface wind to surface wind shear does not vary in the meridional direction. These critical height results are validated using the fully nonlinear stationary wave model.

The nonlinearly balanced response to imposed orography is diagnosed at the surface and aloft. The nonlinear effects of eddy wind/orography interaction and nonlinear advection are found to be important only in the vicinity of the orography. The structure of the nonlinear response at the surface is found to be robust and is characterized (in the Northern Hemisphere) by a high and low situated to the northwest and southeast, respectively, of the mountain center. This orientation of the surface response leads to a stationary wave train that propagates preferentially toward the equator.

The system is sensitive enough to both the surface wind and meridional temperature gradient that the observed seasonal variations in the zonal mean circulation will significantly alter the character of the response. As the meridional temperature gradient decreases, the relative importance of nonlinearity increases while the amplitude of the response at the upper levels decreases. Therefore, this model indicates that summertime mechanically forced stationary waves should be weaker, but more nonlinear, than their wintertime counterparts.

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T. D. Ringler and K. H. Cook

Abstract

A general circulation model (GCM) with idealized boundary conditions is used to study the effects of a mountain's latitudinal position on the stationary wave response. In each of a series of experiments the only asymmetry in the boundary conditions is a Gaussian-shaped mountain with an e-folding width of 15° latitude placed at 0°, 15°, 30°, 45°, and 60° latitude in separate integrations. The stationary wave response in the GCM is analyzed using a linearized primitive equation model, a 3D wave-flux vector, and a barotropic ray-tracing technique.

Stationary waves in the GCM are generated by modifications to the diabatic heating field, termed thermal forcing, and by obstructing the surface winds, termed mechanical forcing. With a mountain at 0° latitude, latent heating anomalies provide the forcing mechanism. In the 15° mountain experiment, forcing by anomalous latent heating is also found, but mechanical forcing (which occurs within the easterlies) seems to dominate. In the 30°, 45°, and 60° mountain experiment, the stationary wave response results from mechanical forcing in westerly flow. As the mountain's latitudinal position is moved poleward, two distinct regions of stationary wave forcing appear, as indicated by a 3D wave-flux vector. This results in two wave trains emanating from the mountain placed at 60° latitude. One region of forcing occurs in the region where the westerlies are perturbed, while the other region occurs on the east and poleward flank of the mountain. In this idealized setting, the propagation patterns of the stationary waves can, for the most part, be understood through quasigeostrophic theory. The dispersion of stationary wave energy throughout the atmosphere is largely dependent upon the upper-level flow.

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J. D. Lenters and K. H. Cook

Abstract

The climatological structure in the upper-tropospheric summertime circulation over South America is diagnosed using a GCM (with and without South American topography), a linear model, and observational data. Emphasis is placed on understanding the origin of observed features such as the Bolivian high and the accompanying “Nordeste low” to the east. Results from the linear model indicate that these two features are generated in response to precipitation over the Amazon basin, central Andes, and South Atlantic convergence zone, with African precipitation also playing a crucial role in the formation of the Nordeste low. The direct mechanical and sensible heating effects of the Andes are minimal, acting only to induce a weak lee trough in midlatitudes and a shallow monsoonal circulation over the central Andes. In the GCM, the effects of the Andes include a strengthening of the Bolivian high and northward shift of the Nordeste low, primarily through changes in the precipitation field. The position of the Bolivian high is primarily determined by Amazonian precipitation and is little affected by the removal of the Andes. Strong subsidence to the west of the high is found to be important for the maintenance of the high’s warm core, while large-scale convective overshooting to the east is responsible for a layer of cold air above the high.

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Garry D. Cook and Michael J. Nicholls

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The passage of three Australian Category 5 cyclones within 350 km of Darwin (Northern Territory), Australia, during the last decade indicates that that city should have a high wind hazard. In this paper, the wind hazard for Darwin was compared with that for Port Hedland (Western Australia) and Townsville (Queensland) using data from a coupled ocean–atmosphere simulation model and from historical and satellite-era records of tropical cyclones. According to the authoritative statement on wind hazard in Australia, Darwin’s wind hazard is the same as Townsville’s but both locations’ hazards are much less than that of Port Hedland. However, three different estimates in this study indicate that Darwin’s wind hazard at the long return periods relevant to engineering requirements is higher than for both Port Hedland and Townsville. The discrepancy with previous studies may result from the inadequate cyclone records in the low-latitude north of Australia, from accumulated errors from estimates of wind speeds from wind fields and wind–pressure relationships, and from inappropriate extrapolations of short-period records based on assumed probability distributions. It is concluded that the current wind-hazard zoning of northern Australia seriously underestimates the hazard near Darwin and that coupled ocean–atmosphere simulation models could contribute to its revision.

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J. D. Lenters and K. H. Cook

Abstract

The observed large-scale circulation mechanisms associated with summertime precipitation variability over South America are investigated. Particular attention is paid to the Altiplano where a close relationship has been observed between rainfall and the position and intensity of the Bolivian high. Empirical orthogonal function (EOF), correlation, and composite analyses suggest that on intraseasonal timescales (typically 5–20 days), rainy periods on the Altiplano are associated with at least three types of circulation anomalies, involving either extratropical cyclones, cold-core lows, or the westward enhancement of the South Atlantic high. In each instance, the primary support for high rainfall rates is a moist, poleward flow at low levels along the eastern flank of the central Andes in association with the South Atlantic convergence zone (SACZ). The warm, low-level flow along the SACZ also inflates the overlying atmospheric column, resulting in an intensification and southward shift of the Bolivian high. Thus, the position of the SACZ (and associated frontal activity) plays a crucial role in the variability of both the Bolivian high and Altiplano rainfall.

On longer timescales, the Bolivian high also shifts southward and intensifies during wet periods on the Altiplano. The seasonal transition from December to the wetter month of January is accompanied by a westward enhancement of the SACZ and northwesterly flow along the central Andes. The transition to the drier month of February is accompanied by weakening northwesterly flow and cooler, drier conditions along the Altiplano. Interannual precipitation variability on the Altiplano is strongly correlated with the amplitude of the fifth EOF of the 200-mb height field, the “dry phase”, which is much like the anomalous conditions during February. A case study of the dry conditions during the 1987 El Niño associates reduced convection on the Altiplano with the presence of a strong cold front over eastern South America and cold, dry air to the west. The characteristic eastward shift of the South Pacific convergence zone during El Niño may be responsible for this enhanced frontal activity in the SACZ (through teleconnections) and, therefore, the cool, dry, convectively unfavorable conditions in the central Andes.

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J. D. Lenters, K. H. Cook, and T. D. Ringler

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