Monthly Weather Review

Marquis, James; Richardson, Yvette; Markowski, Paul; Dowell, David; Wurman, Joshua

doi: 10.1175/MWR-D-11-00025.1pmid: N/A

Dual-Doppler wind synthesis and ensemble Kalman filter analyses produced by assimilating Doppler-on-Wheels velocity data collected in four tornadic supercells are examined in order to further understand the maintenance of tornadoes. Although tornado-scale features are not resolved in these analyses, larger-scale processes involved with tornado maintenance are well represented. The longest-lived tornado is maintained underneath the midlevel updraft within a zone of low-level horizontal convergence along a rear-flank gust front for a considerable time, and dissipates when horizontally displaced from the midlevel updraft. The shortest-lived tornado resides in a similar zone of low-level convergence briefly, but dissipates underneath the location of the midlevel updraft when the updraft becomes tilted and low-level convergence is displaced several kilometers from the tornado. This suggests that a location beneath the midlevel updraft is not always a sufficient condition for tornado maintenance, particularly in the presence of strongly surging outflow. Tornadoes in two other storms persist within a band of low-level convergence in the outflow air (a possible secondary rear-flank gust front), suggesting that tornado maintenance can occur away from the main boundary separating the outflow air and the ambient environment. In at least one case, tilting of horizontal vorticity occurs near the tornado along the secondary gust front, as evidenced by three-dimensional vortex line arching. This observation suggests that a relatively cold secondary rear-flank downdraft may assist with tornado maintenance through the baroclinic generation and tilting of horizontal vorticity, despite the fact that parcels composing them would be more negatively buoyant than the preceding outflow air.

Pook, Michael J.; Risbey, James S.; McIntosh, Peter C.

doi: 10.1175/MWR-D-11-00048.1pmid: N/A

Synoptic weather systems form an important part of the physical link between remote large-scale climate drivers and regional rainfall. A synoptic climatology of daily rainfall events is developed for the Central Wheatbelt of southwestern Australia over the April–October growing season for the years 1965–2009. The climatology reveals that frontal systems contribute approximately one-half of the rainfall in the growing season while cutoff lows contribute about a third. The ratio of frontal rainfall to cutoff rainfall varies throughout the growing season. Cutoff lows contribute over 40%% of rainfall in the austral autumn and spring, but this falls to about 20%% in August when frontal rainfall climbs to more than 60%%. The number of cutoff lows varies markedly from one growing season to another, but does not exhibit a significant long-term trend. The mean rainfall per cutoff system is also highly variable, but has gradually declined over the analysis period, particularly in the past decade. The decline in rainfall per frontal system is less significant. Cutoff low rainfall has contributed more strongly in percentage terms to the recent decline in rainfall in the Central Wheatbelt than the frontal component and accounts for more than half of the overall trend. Atmospheric blocking is highly correlated with rainfall in the region where cutoff low rainfall makes its highest proportional contribution. Hence, the decline in rain from cutoff low systems is likely to have been associated with changes in blocking and the factors controlling blocking in the region.

Villarini, Gabriele; Vecchi, Gabriel A.; Smith, James A.

doi: 10.1175/MWR-D-11-00063.1pmid: N/A

Time series of U.S. landfalling and North Atlantic hurricane counts and their ratios over the period 1878–2008 are modeled using tropical Atlantic sea surface temperature (SST), tropical mean SST, the North Atlantic Oscillation (NAO), and the Southern Oscillation index (SOI). Two SST input datasets are employed to examine the uncertainties in the reconstructed SST data on the modeling results. Because of the likely undercount of recorded hurricanes in the earliest part of the record, both the uncorrected hurricane dataset (HURDAT) and a time series with a recently proposed undercount correction are considered. Modeling of the count data is performed using a conditional Poisson regression model, in which the rate of occurrence can depend linearly or nonlinearly on the climate indexes. Model selection is performed following a stepwise approach and using two penalty criteria. These results do not allow one to identify a single “best” model because of the different model configurations (different SST data, corrected versus uncorrected datasets, and penalty criteria). Despite the lack of an objectively identified unique final model, the authors recommend a set of models in which the parameter of the Poisson distribution depends linearly on tropical Atlantic and tropical mean SSTs. Modeling of the fractions of North Atlantic hurricanes making U.S. landfall is performed using a binomial regression model. Similar to the count data, it is not possible to identify a single best model, but different model configurations are obtained depending on the SST data, undercount correction, and penalty criteria These results suggest that these fractions are controlled by local (related to the NAO) and remote (SOI and tropical mean SST) effects.

Wu, Liang; Wen, Zhiping; Huang, Ronghui; Wu, Renguang

doi: 10.1175/MWR-D-11-00078.1pmid: N/A

The present study investigates the influence of the monsoon trough (MT) on the interannual variability of tropical cyclone (TC) activity over the western North Pacific during July–November for the period 1979–2007. It is shown that the TC activity is closely related to the MT location. During the years when the MT extends eastward (retreats westward), more (less) TCs form within the southeastern quadrant of the western North Pacific. Such a relationship can be explained by the changes in large-scale environmental factors associated with the movement of the MT. An eastward extension of the MT coincides with warmed ocean surface, enhanced convection, increased relative humidity in the lower and midtroposphere, reduced vertical shear of zonal wind, intensified upper-level divergence, and low-level anomalous cyclonic vorticity over the southeast quadrant of the western North Pacific. These conditions associated with the eastern extension of the MT are favorable for TC genesis, while those associated with the westward retreat of the MT are not. Diagnosis of the barotropic energy conversion indicates that synoptic-scale disturbances moving westward from tropical eastern Pacific will gain the energy from the mean flow when they meet with the eastward-extending MT. This is an important reason for the linkage between MT variability and TC genesis over the western North Pacific.

Liu, Hui; Anderson, Jeffrey; Kuo, Ying-Hwa

doi: 10.1175/MWR-D-11-00024.1pmid: N/A

Radio occultation (RO) refractivity observations provide information about tropospheric water vapor and temperature in all weather conditions. The impact of using RO refractivity observations on analyses and forecasts of Hurricane Ernesto’s genesis (2006) using an ensemble Kaman filter data assimilation system is investigated. Assimilating RO refractivity profiles in the vicinity of the storm locally moistens the analysis of the lower troposphere and also adjusts the wind analysis in both the lower and upper troposphere through forecast multivariate correlations of RO refractivity and wind. The model forecasts propagate and enhance the added water vapor and the wind adjustments leading to more accurate analyses of the later stages of the genesis of the storm. The root-mean-square errors of water vapor and wind forecasts compared to dropsonde and radiosonde observations are reduced consistently. As a result, assimilating RO refractivity data in addition to traditional observations leads to a stronger initial vortex of the storm and improved forecasts of the storm’s intensification. The benefits of the RO data are much reduced when the RO data in the lower troposphere (below 6 km) are ignored.

Zhang, Man; Zhang, Da-Lin

doi: 10.1175/MWR-D-11-00029.1pmid: N/A

A nocturnal torrential-rain-producing mesoscale convective system (MCS) occurring during the mei-yu season of July 2003 in east China is studied using conventional observations, surface mesoanalysis, satellite and radar data, and a 24-h multinested model simulation with the finest grid spacing of 444 m. Observational analyses reveal the presence of several larger-scale conditions that were favorable for the development of the MCS, including mei-yu frontal lifting, moderate cold advection aloft and a moist monsoonal flow below, and an elongated old cold dome left behind by a previously dissipated MCS. Results show that the model could reproduce the evolution of the dissipating MCS and its associated cold outflows, the triggering of three separate convective storms over the remnant cold dome and the subsequent organization into a large MCS, and the convective generation of an intense surface meso-high and meso- β -scale radar reflectivity morphologies. In particular, the model reproduces the passage of several heavy-rain-producing convective bands at the leading convective line and the trailing stratiform region, leading to the torrential rainfall at nearly the right location. However, many of the above features are poorly simulated or missed when the finest model grid uses either 1.33- or 4-km grid spacing. Results indicate the important roles of isentropic lifting of moist monsoonal air over the cold dome in triggering deep convection, a low-level jet within an elevated moist layer in maintaining conditional instability, and the repeated formation and movement of convective cells along the same path in producing the torrential rainfall.

Bryan, George H.; Morrison, Hugh

doi: 10.1175/MWR-D-11-00046.1pmid: N/A

Idealized simulations of the 15 May 2009 squall line from the Second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2) are evaluated in this study. Four different microphysical setups are used, with either single-moment (1M) or double-moment (2M) microphysics, and either hail or graupel as the dense (rimed) ice species. Three different horizontal grid spacings are used: Δ x == 4, 1, or 0.25 km (with identical vertical grids). Overall, results show that simulated squall lines are sensitive to both microphysical setup and horizontal resolution, although some quantities (i.e., surface rainfall) are more sensitive to Δ x in this study. Simulations with larger Δ x are slower to develop, produce more precipitation, and have higher cloud tops, all of which are attributable to larger convective cells that do not entrain midlevel air. The highest-resolution simulations have substantially more cloud water evaporation, which is partly attributable to the development of resolved turbulence. For a given Δ x , the 1M simulations produce less rain, more intense cold pools, and do not have trailing stratiform precipitation at the surface, owing to excessive rainwater evaporation. The simulations with graupel as the dense ice species have unrealistically wide convective regions. Comparison against analyses from VORTEX2 data shows that the 2M setup with hail and Δ x == 0.25 km produces the most realistic simulation because (i) this simulation produces realistic distributions of reflectivity associated with convective, transition, and trailing stratiform regions, (ii) the cold pool properties are reasonably close to analyses from VORTEX2, and (iii) relative humidity in the cold pool is closest to observations.

Kirkil, Gokhan; Mirocha, Jeff; Bou-Zeid, Elie; Chow, Fotini Katopodes; Kosovićć, Branko

doi: 10.1175/MWR-D-11-00037.1pmid: N/A

The performance of a range of simple to moderately-complex subfilter-scale (SFS) stress models implemented in the Weather Research and Forecasting (WRF) model is evaluated in large-eddy simulations of neutral atmospheric boundary layer flow over both a flat terrain and a two-dimensional symmetrical transverse ridge. Two recently developed dynamic SFS stress models, the Lagrangian-averaged scale-dependent (LASD) dynamic model and the dynamic reconstruction model (DRM), are compared with the WRF model’s existing constant-coefficient linear eddy-viscosity and (as of version 3.2) nonlinear SFS stress models to evaluate the benefits of more sophisticated and accurate, but also more computationally expensive approaches. Simulation results using the different SFS stress models are compared among each other, as well as against the Monin–Obukhov similarity theory. For the flat terrain case, vertical profiles of mean wind speed from the newly implemented dynamic models show the best agreement with the similarity solution, improving even upon the nonlinear model, which likewise yields a significant improvement compared to the Smagorinsky model. The more sophisticated SFS stress models more successfully predict the expected production and inertial range scaling of power spectra, especially near the surface, with the dynamic models achieving the best scaling overall. For the transverse ridge case, the nonlinear model predicts the greatest amount of reverse flow in the lee of the ridge, and also demonstrates the greatest ability to duplicate qualitative features of the highest-resolution simulations at coarser resolutions. The dynamic models’ flow distributions in the lee of the ridge did not differ significantly from the constant-coefficient Smagorinsky model.