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Gan Zhang, Zhuo Wang, Melinda S. Peng, and Gudrun Magnusdottir

Abstract

This study investigates the characteristics of extratropical Rossby wave breaking (RWB) during the Atlantic hurricane season and its impacts on Atlantic tropical cyclone (TC) activity. It was found that RWB perturbs the wind and moisture fields throughout the troposphere in the vicinity of a breaking wave. When RWB occurs more frequently over the North Atlantic, the Atlantic main development region (MDR) is subject to stronger vertical wind shear and reduced tropospheric moisture; the basinwide TC counts are reduced, and TCs are generally less intense, have a shorter lifetime, and are less likely to make landfalls. A significant negative correlation was found between Atlantic TC activity and RWB occurrence during 1979–2013. The correlation is comparable to that with the MDR SST index and stronger than that with the Niño-3.4 index. Further analyses suggest that the variability of RWB occurrence in the western Atlantic is largely independent of that in the eastern Atlantic. The RWB occurrence in the western basin is more closely tied to the environmental variability of the tropical North Atlantic and is more likely to hinder TC intensification or reduce the TC lifetime because of its proximity to the central portion of TC tracks. Consequently, the basinwide TC counts and the accumulated cyclone energy have a strong correlation with western-basin RWB occurrence but only a moderate correlation with eastern-basin RWB occurrence. The results highlight the extratropical impacts on Atlantic TC activity and regional climate via RWB and provide new insights into the variability and predictability of TC activity.

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Peter S. Ray, David P. Jorgensen, and Sue-Lee Wang

Abstract

Airborne Doppler radar can collect data on target storms that are quite widely dispersed. However, the relatively long time required to sample an individual storm in detail, particularly with a single aircraft, and the amplification of the statistical uncertainty in the radial velocity estimates when Cartesian wind components are derived, suggests that errors in wind fields derived from airborne Doppler radar measurements would exceed those from a ground based radar network which was better located to observe the same storm. Error distributions for two analysis methods (termed Overdetermined and Direct methods) are given and discussed for various flight configurations. Both methods are applied to data collected on a sea breeze induced storm that occurred in western Florida on 28 July 1982. Application of the direct solution, which does not use the continuity equation, and the overdetermined dual-Doppler method, which requires the use of the continuity equation, resulted in similar fields. Since the magnitude of all errors are unknown and the response of each method to errors is different, it is difficult to assess overall which analysis performs better; indeed each might be expected to perform best in different parts of the analysis domain. A flexible collection strategy can be followed with different analysis methods to optimize the quality of resulting synthesized wind fields.

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S-K. Lee, D. B. Enfield, and C. Wang

Abstract

The annual heat budget of the Western Hemisphere warm pool (WHWP) is explored using the output of an ocean general circulation model (OGCM) simulation. According to the analysis, the WHWP cannot be considered as a monolithic whole with a single set of dominating processes that explain its behavior. The three regions considered, namely the eastern north Pacific (ENP), the Gulf of Mexico (GoM), and the Caribbean Sea (CBN), are each unique in terms of the atmospheric and oceanic processes that dominate the corresponding heat budgets. In the ENP region, clear-sky shortwave radiation flux is responsible for the growth of the warm pool in boreal spring, while increased cloud cover in boreal summer and associated reduction in solar radiation play a crucial role for the ENP warm pool’s demise. Ocean upwelling in the Costa Rica Dome connected to surrounding areas by horizontal advection offers a persistent yearlong cooling mechanism. Over the Atlantic, the clear-sky radiation flux that increases monotonically from December to May and decreases later is largely responsible for the onset and decay of the Atlantic-side warm pool in boreal summer and fall. The CBN region is affected by upwelling and horizontal advective cooling within and away from the coastal upwelling zone off northern South America during the onset and peak phases, thus slowing down the warm pool’s development, but no evidence was found that advective heat flux divergence is important in the GoM region. Turbulent mixing is also an important cooling mechanism in the annual cycle of the WHWP, and the vertical shear at the warm pool base helps to sustain the turbulent mixing. Common to all three WHWP regions is the reduction of wind speed at the peak phase, suggestive of a convection–evaporation feedback known to be important in the Indo-Pacific warm pool dynamics.

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S. J. Martin, P. K. Wang, and H. R. Pruppacher

Abstract

Two theoretical models are presented which allow computing the efficiency with which aerosol particles of radius 0.001 ≤ r ≤ 10 μm are collected by simple ice crystal plates of radius 50 ≤ ac ≤ 640 μm, in air of various relative humidity, temperature and pressure. Particle capture due to Brownian diffusion, thermophoresis, diffusiophoresis and inertial impaction is considered. It is shown that, analogous to water drops, ice crystal plates exhibit a minimum collection efficiency within a specific size interval of aerosol particles. This minimum is strongly affected by the relative humidity of the ambient air. The collection efficiency of particles with r > 1 μm is controlled by the flow field around the ice crystal, while the collection efficiency of particles with r < 0.01 μm is controlled by convective Brownian diffusion. Trajectory analysis predicts that aerosol particles are preferentially captured by the ice crystal rim. Our theoretical results are found to agree satisfactorily with laboratory studies presently available.

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H. Wang, R. J. Barthelmie, A. Clifton, and S. C. Pryor

Abstract

Defining optimal scanning geometries for scanning lidars for wind energy applications remains an active field of research. This paper evaluates uncertainties associated with arc scan geometries and presents recommendations regarding optimal configurations in the atmospheric boundary layer. The analysis is based on arc scan data from a Doppler wind lidar with one elevation angle and seven azimuth angles spanning 30° and focuses on an estimation of 10-min mean wind speed and direction. When flow is horizontally uniform, this approach can provide accurate wind measurements required for wind resource assessments in part because of its high resampling rate. Retrieved wind velocities at a single range gate exhibit good correlation to data from a sonic anemometer on a nearby meteorological tower, and vertical profiles of horizontal wind speed, though derived from range gates located on a conical surface, match those measured by mast-mounted cup anemometers. Uncertainties in the retrieved wind velocity are related to high turbulent wind fluctuation and an inhomogeneous horizontal wind field. The radial velocity variance is found to be a robust measure of the uncertainty of the retrieved wind speed because of its relationship to turbulence properties. It is further shown that the standard error of wind speed estimates can be minimized by increasing the azimuthal range beyond 30° and using five to seven azimuth angles.

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J. M. Toole, R. C. Millard, Z. Wang, and S. Pu

Abstract

Hydrographic surveys were conducted off the Philippine coast in September 1987 and April 1988 as part of the United States/People's Republic of China cooperative research program. These cruises sampled the western Pacific Ocean where the North Equatorial Current (NEC) meets the western boundary and divides into the Kuroshio and Mindanao Currents. The requirement for mass conservation within a region enclosed by stations is utilized here to obtain absolute circulation fields for the two surveys. In both realizations, the surface flow of the NEC was observed to bifurcate near latitude 13°N; NEC flow poleward of this latitude turned north as the Kuroshio while flow to the south fed the Mindanao Current. Most striking was a twofold increase in the strength of the current system in spring 1988 as compared with fall 1987. We note that the observations in fall 1987 were obtained during the height of the 1986/87 El Niño, while those in spring 1988 were during a cold phase of the El Niño/Southern Oscillation. It is not clear how the observed current changes relate to the evolution of this event. The potential vorticity (Q) distributions of the surface waters were examined to explore the dynamics of the bifurcation. Within the NEC, Q was nearly constant (layer thickness change balanced meridional planetary vorticity variation). Within the Kuroshio and Mindanao currents, near constant Q (with magnitude comparable to that in the NEC) was also found with a balance between relative vorticity variation and layer depth change as would be expected for inertia] boundary currents.

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Chester F. Ropelewski, Michael S. Halpert, and Xueliang Wang

Abstract

Tropospheric biennial variability in several components of the Southern Oscillation (SO) is defined and described through analysis of observational data from the Comprehensive Ocean-Atmosphere Data Set (COADS), as well as through investigation of several SO index time series. The analysis suggests that the temporal behavior of the SO can be described in terms of three components: 1) a pervasive biennial pulse, which appears to be strong in both the Indian Ocean and the west Pacific surface zonal winds as well as in several SO indices, 2) the annual cycle, which tends to set the phase of biennial variability for the major SO excursions, and 3) a low-frequency, or residual, variability, which may be associated with temporal scales between large SO episodes. This study also supports recent papers in suggesting that complete models of the SO must include the Indian Ocean basin.

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Boniface O. Fosu, S.-Y. Simon Wang, and Jin-Ho Yoon
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P. K. Wang, S. N. Grover, and H. R. Pruppacher

Abstract

A theoretical model is described which determines the efficiency E with which aerosol particles of radius r are collected by water drops of radius a due to the combined action of Brownian diffusion, thermo- and diffusiophoresis and electric forces, in the absence of inertial impaction effects. The results of this model are combined with the results of our earlier model which determines the collection efficiency of drops for particles due to the combined action of inertial impaction, thermo- and diffusiophoresis and electric forces, in the absence of effects due to Brownian diffusion. Both models combined quantitatively determine the variation of E vs r for 0.001≤r≤10 μm, and 42≤a≤310 μm, for relative humidities up to and including 100%, and for electric charges on drops and aerosol particles ranging in magnitude up to that found under thunder-storm conditions. In particular, a combination of both models allows a quantitative description of the particle size range where the collection efficiency of drops is minimum.

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Shih-Yu Wang, Tsing-Chang Chen, and S. Elwynn Taylor

Abstract

In the U.S. northern plains, summer progressive convective storms that occur in weakly forced environments are often coupled with short-wave perturbations that are embedded in the midlevel northwesterly flow. These midtropospheric perturbations (MPs) are capable of inducing propagating convection that contributes to a majority of the rainfall over the northern plains during July and August. There is a possibility that the difficulties of numerical weather prediction models in forecasting summer convective rainfall over the northern plains are partly attributed to their deficiency in forecasting MPs. The present study tests this possibility through examining operational forecasts by the North American Mesoscale (NAM) model during the summers of 2005 and 2006.

Forecasted MPs exhibit slower propagation speeds and weaker relative vorticity than the observations leading to systematic position errors. Underpredicted vorticity magnitudes weaken horizontal vorticity advection that influences the vorticity tendency throughout the MP life cycle and, in turn, slows the propagation speed of MPs. Moreover, biases of weak ambient flow speed and vortex stretching contribute to the magnitude and propagation speed errors of MPs. Skill scores of precipitation forecasts associated with MPs are low, but can be considerably improved after removing the MP position error that displaces the rainfall pattern. The NAM also tends to underpredict precipitation amounts. A modified water vapor budget analysis reveals that the NAM insufficiently generates atmospheric humidity over the central United States. The shortage of moisture in the forecast reduces the water vapor flux convergence that is part of the precipitation process. The precipitation bias may feed back to affect the MP growth through the bias in heating, thus further slowing the perturbation.

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