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Hui Wang and Yuqing Wang

Abstract

Typhoon Megi (2010) experienced drastic eyewall structure changes when it crossed the Luzon Island and entered the South China Sea (SCS), including the contraction and breakdown of the eyewall after landfall over the Luzon Island, the formation of a new large outer eyewall accompanied by reintensification of the storm after it entered the SCS, and the appearance of a short-lived small inner eyewall. These features were reproduced reasonably well in a control simulation using the Advanced Weather Research and Forecasting (ARW-WRF) Model. In this study, the eyewall processes of the simulated Megi during and after landfall have been analyzed. Results show that the presence of the landmass of the Luzon Island increased surface friction and reduced surface enthalpy flux, causing the original eyewall to contract and break down and the storm to weaken. The formation of the new large eyewall results mainly from the axisymmetrization of outer spiral rainbands after the storm core moved across the Luzon Island and entered the SCS. The appearance of the small inner eyewall over the SCS was due to the increased surface enthalpy flux and the revival of convection in the central region of the storm core. In a sensitivity experiment with the mesoscale mountain replaced by flat surface over the Luzon Island, a new large outer eyewall formed over the western Luzon Island with its size about one-third smaller after the storm entered the SCS than that in the control experiment with the terrain over the Luzon Island unchanged.

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Hui Wang and Yuqing Wang

Abstract

Typhoon Megi (15W) was the most powerful and longest-lived tropical cyclone (TC) over the western North Pacific during 2010. While it shared many common features of TCs that crossed Luzon Island in the northern Philippines, Megi experienced unique intensity and structural changes, which were reproduced reasonably well in a simulation using the Advanced Research Weather Research and Forecasting Model (ARW-WRF) with both dynamical initialization and large-scale spectral nudging. In this paper processes responsible for the rapid intensification (RI) of the modeled Megi before it made landfall over Luzon Island were analyzed. The results show that Megi experienced RI over the warm ocean with high ocean heat content and decreasing environmental vertical shear. The onset of RI was triggered by convective bursts (CBs), which penetrate into the upper troposphere, leading to the upper-tropospheric warming and the formation of the upper-level warm core. In turn, CBs with their roots inside of the eyewall in the boundary layer were buoyantly triggered/supported by slantwise convective available potential energy (SCAPE) accumulated in the eye region. During RI, convective area coverage in the inner-core region was increasing while the updraft velocity in the upper troposphere and the number of CBs were both decreasing. Different from the majority of TCs that experience RI with a significant eyewall contraction, the simulated Megi, as the observed, rapidly intensified without an eyewall contraction. This is attributed to diabatic heating in active spiral rainbands, a process previously proposed to explain the inner-core size increase, enhanced by the interaction of the typhoon vortex with a low-level synoptic depression in which Megi was embedded.

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Hui Wang and Rong Fu

Abstract

The relationship between South American precipitation and cross-equatorial flow over the western Amazon is examined using the 15-yr (1979–93) European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis dataset. A meridional wind index, the V index, is constructed to represent the variability of the cross-equatorial flow, based on area-averaged (5°S–5°N, 65°–75°W) daily 925-hPa meridional winds. The V index displays large submonthly, seasonal, and interannual variabilities, and correlates well with precipitation over South America. Two circulation regimes are identified, that is, a southerly regime and a northerly regime. Linear regression shows that when the V index is southerly, precipitation is mainly located to the north of the equator. When the V index is northerly, precipitation shifts toward the Amazon basin and subtropical South America. The V index is predominately southerly in austral winter and northerly in summer. The onset (demise) of the Amazon rainy season is led by an increase in the frequency of the northerly (southerly) V index. The relation between the V index and upper-level circulation is consistent with the seasonal cycle of the South American monsoon circulation. Hence, the V index is a good indicator for precipitation change over tropical and subtropical South America.

The singular value decomposition (SVD) analysis suggests that the V-index-related variation represents 92% of the total covariance between the low-level meridional wind and precipitation over South America. It also represents 37% of the seasonal variance of precipitation. On the seasonal timescale, the V index appears to correlate well with the meridional migration of the Hadley cell globally. On submonthly timescales, the change of V index is not correlated with the meridional wind over the adjacent oceans except in the South Atlantic convergence zone, suggesting a control by more localized and higher-frequency dynamic processes. The SVD analysis also suggests that during spring and fall precipitation changes over the equatorial eastern Amazon are associated with the seasonal variations of sea surface temperature in the Pacific and the Atlantic Oceans.

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Hui Wang and Rong Fu

Abstract

The characteristics of winter monthly mean extratropical circulation associated with El Niño, including precipitation and surface temperature over the United States, are examined for nine El Niño events during 1950–94. Precipitation and surface temperature over the United States, also the 500-mb geopotential height and sea level pressure over the North Pacific and North America, are significantly different between early winter (November and December) and late winter (January to March). The typical El Niño-related U.S. precipitation and surface temperatures identified in many previous studies, as well as the Pacific–North American (PNA) circulation pattern, emerge in January and persist through February and March. The PNA patterns during these late winter months are coupled both with the tropical El Niño sea surface temperature (SST) variation and with the North Pacific SST variation. In contrast, the PNA patterns in the early winter months correlate only with the North Pacific SST. The tendency for the PNA pattern to occur during El Niño years is much less in early winter months than in late winter months. An ensemble analysis of 12 45-yr (1950–94) integrations of the National Center for Atmospheric Research Community Climate Model forced by the observed time-varying SST shows that the model 500-mb heights display a PNA-like pattern in both early and late winters of El Niño. The ensemble model response to the El Niño SST is thus unable to reproduce the observed differences in the extratropical atmospheric circulation between early and late winter months.

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Hui Wang and Rong Fu

Abstract

Using outgoing longwave radiation (OLR) and Tropical Rainfall Measuring Mission (TRMM) daily rain-rate data, systematic changes in intensity and location of the Atlantic intertropical convergence zone (ITCZ) were detected along the equator during boreal spring. It is found that the changes in convection over the tropical Atlantic may be induced by deep convection in equatorial South America. Lagged regression analyses demonstrate that the anomalies of convection developed over the land propagate eastward across the Atlantic and then into Africa. The eastward-propagating disturbances appear to be convectively coupled Kelvin waves with a period of 6–7.5 days and a phase speed of around 15 m s−1. These waves modulate the intensity and location of the convection in the tropical Atlantic and result in a zonal variation of the Atlantic ITCZ on synoptic time scales. The convectively coupled Kelvin wave has substantial signals in both the lower and upper troposphere. Both a reanalysis dataset and the Quick Scatterometer (QuikSCAT) ocean surface wind are used to characterize the Kelvin wave. This study suggests that synoptic-scale variation of the Atlantic ITCZ may be linked to precipitation anomalies in South America through the convectively coupled Kelvin wave. The results imply that the changes of Amazon convection could contribute to the large variability of the tropical Atlantic ITCZ observed during boreal spring.

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Hui Wang and Rong Fu

Abstract

This study examines the interannual variability of winter upper-troposphere water vapor over the Northern Hemisphere using the National Aeronautics and Space Administration Water Vapor Project, the International Satellite Cloud Climatology Project data, and the European Centre for Medium-Range Weather Forecasting reanalysis. The El Niño–Southern Oscillation related tropical sea surface temperature (SST) anomalies dominate the upper-troposphere water vapor anomalies south of the climatological jet. The anomalies of baroclinic instability in the storm track regions, which relate to the Pacific–North American and the North Atlantic oscillation patterns, dominate those north of the climatological jet. The upper-troposphere water vapor increases in the eastern tropical Pacific, the Gulf of Mexico, and some areas of the North Atlantic with warmer tropical SST. It decreases in the subtropical and extratropical northeastern Pacific. Deep convection and vertical moisture fluxes dominate these changes. To the north of the climatological jet, stronger upper-level cyclonic flow dries the upper troposphere when the baroclinicity of the storm tracks is enhanced. Both vertical and meridional moisture transport contribute to these water vapor anomalies in the midlatitudes. High clouds, as a possible source/sink of water vapor, respond to the tropical SST anomalies and extratropical circulation in a pattern similar to the upper-troposphere water vapor, and they consequently positively correlate to the latter. In the Tropics and extratropics where high clouds are relatively abundant, water vapor concentration increases with temperature. Thus, the increase of evaporation or sublimation of high clouds probably contributes to the observed moistening of the upper troposphere, in addition to enhanced vapor transport. Conversely, in the subtropics where high clouds appear infrequently, water vapor concentration decreases with temperature, suggesting that the downward advection of drier air associated with subsidence dominates the drying of the upper troposphere.

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Hui Wang and Mingfang Ting

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The variability of winter average U.S. precipitation displays strong geographical dependence with large variability in the southeastern and northwestern United States. The covariance of the U.S. winter mean precipitation with Pacific sea surface temperature (SST) is examined in this study using the singular value decomposition (SVD) method. The first SVD mode indicates the U.S. precipitation pattern that is associated with the tropical El Niño/La Niña SST variation, while the second and third SVD modes relate the precipitation variability in the Pacific Northwest and southeast that is associated with the North Pacific SST variation. About 45% of the U.S. precipitation variabilities is related to the Pacific SST anomalies, among which, 35% is related to the North Pacific SST and 10% is related to the tropical Pacific SST. Each SVD precipitation pattern is associated with well-organized 500-mb height and zonal mean zonal wind anomalies. It is shown that the North Pacific SST anomalies associated with the U.S. precipitation are primarily driven by extratropical atmospheric circulation anomalies.

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Mingfang Ting and Hui Wang

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The year-to-year fluctuations in summertime precipitation over the U.S. Great Plains are examined in this study using data from 1950 to 1990. There are large interannual variabilities in precipitation amounts over the Great Plains during the period considered. A long-term trend in Great Plains precipitation from relatively wet conditions in the 1950s to relatively dry conditions in the 1980s is also identified. The spatial scale of the anomalous precipitation covers a large portion of the United States on seasonal mean timescales.

It is shown that the Great Plains precipitation fluctuations are significantly correlated with the tropical, as well as North Pacific, sea surface temperature (SST) variations. Two leading modes of covariation between Pacific SST and the U.S. precipitation are identified, with the first mode having spatial and temporal characteristics of the El Niño–La Niña SST variation, while the second mode is confined to the North Pacific and contains the decadal trend. The relationship of both the SST and the precipitation variation with the atmospheric circulation is established through 500-mb height, as well as the sea level pressure fields. A well-defined wave train over the Pacific and North American region is found to be associated with the two leading modes. A southward-shifted jet stream over the central United States brings more synoptic storms into the region and causes excessive precipitation during wet events. The tropical SST and the U.S. precipitation may be connected through the anomalous tropical convection and its effects on the circulation. The relation between North Pacific SST and the U.S. precipitation is consistent with a strong atmospheric forcing on the North Pacific SST at a 1-month lead. It is also hypothesized that North Pacific SST feeds back onto the circulation through an enhanced (reduced) Pacific jet due to the increase (decrease) of the meridional SST gradient during dry (wet) summers. This appears to be consistent with the enhanced convection along the Pacific storm track and the intensified Pacific jet stream in the two recent dry summers (1983 and 1988).

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Hui Wang and Rong Fu

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By analyzing the 15-yr (1979–93) reanalysis data of the European Centre for Medium-Range Weather Forecasts (ECMWF), it has been found that the seasonal and synoptic time-scale variations of the South American low-level jets (LLJs) are largely controlled by an upper-level trough and associated low-level zonal flow, rather than by horizontal temperature gradients along the slope of the Andes. The northerly LLJs are maintained by strong zonal pressure gradients caused by the upstream trough and westerly flow crossing the Andes through lee cyclogenesis. The process involves both baroclinic development of the upper-level trough and mechanical deflection of the westerly flow by the Andes. When an anticyclonic circulation replaces the trough and westerly flow over the eastern South Pacific, the northerly LLJs tend to diminish or reverse into southerly LLJs. The dependence of the LLJs upon the upstream wind pattern helps to explain how the seasonal variation of the South American LLJs is related to the seasonal changes of the large-scale circulation pattern over the eastern South Pacific. On synoptic time scales, the relation between LLJs and cross-Andes zonal flow is strong in austral winter, spring, and fall. This relation weakens somewhat in summer, when Amazon convection is strongest. The analysis also demonstrated strong connections of the LLJs with South American precipitation, intensity of the South Atlantic convergence zone (SACZ), and low-level cross-equatorial flow. A method for up to 5-day forecasts of the LLJs based on 700-hPa zonal winds over the subtropical eastern South Pacific was also introduced. A cross validation indicates a certain degree of predictability for South American LLJs. The results further suggest that the upstream flow pattern over the South Pacific should be closely monitored to determine the variability of the South American LLJs.

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Yi-Hui Wang and Gudrun Magnusdottir

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An objective analysis of tropospheric anticyclonic- and cyclonic-breaking Rossby waves is performed for the Southern Hemisphere in austral summer (December–February) of 1979–2009. The climatology of both anticyclonic and cyclonic Rossby wave breaking (RWB) frequency is presented. The frequency of anticyclonic RWB is highest in an extended region of the Eastern Hemisphere on the anticyclonic side of the jet, while that of cyclonic RWB is highest on the cyclonic side of the jet. A composite analysis of anticyclonic and cyclonic RWB shows how they contribute to a positive and negative southern annual mode (SAM) index, respectively. The time series of austral summer anticyclonic RWB occurrence has a trend that closely matches the trend in the SAM index.

Regions of RWB that are significantly correlated with the SAM index are objectively determined. Even though several such regions are identified, only two regions (anticyclonic and cyclonic) covering 17% of the area of the hemisphere are required in a linear regression model of the SAM index. The anticyclonic RWB region is zonally extended at 45°S and explains 78% of the variability of the summer-mean SAM index. The cyclonic region is located at high latitudes somewhat decoupled from the jet, in the longitudinal sector of the Indian Ocean. On synoptic time scales, transitions of the SAM index respond to RWB without time lag.

ENSO cycles present an interesting zonal asymmetry to the distribution of Southern Hemispheric RWB in the central Pacific. Anticyclonic RWB is increased in the tropical/subtropical central Pacific during La Niña compared to El Niño. This increase is related to the strong local decrease in zonal wind. At the same time, anticyclonic RWB outside the central Pacific is increased in frequency poleward and decreased in frequency equatorward of 42°S, corresponding to a positive SAM index.

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