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Qian Wang, Qingqing Li, and Gang Fu

Abstract

Four methods for determining the extratropical transition (ET) onset and completion times of Typhoons Mindulle (2004) and Yagi (2006) were compared using four numerically analyzed datasets. The open-wave and scalar frontogenesis parameter methods failed to smoothly and consistently determine the ET completion from the four data sources, because some dependent factors associated with these two methods significantly impacted the results. Although the cyclone phase space technique succeeded in determining the ET onset and completion times, the ET onset and completion times of Yagi identified by this method exhibited a large distinction across the datasets, agreeing with prior studies. The isentropic potential vorticity method was also able to identify the ET onset times of both Mindulle and Yagi using all the datasets, whereas the ET onset time of Yagi determined by such a method differed markedly from that by the cyclone phase space technique, which may create forecast uncertainty.

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Gang Li, Chongyin Li, Yanke Tan, and Xin Wang

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The present study investigates the relationships between the December–February (DJF) South Pacific tripole (SPT) sea surface temperature anomaly (SSTA) pattern and the following March–May (MAM) rainfall over eastern China based on multiple datasets. It is found that the relationships between the DJF SPT and the following MAM rainfall over eastern China are modulated by the El Niño–Southern Oscillation (ENSO). When the ENSO signal is removed, the positive DJF SPT is significantly associated with more rainfall over eastern China during the following boreal spring. However, such significant relationships disappear if ENSO is considered. After removing ENSO impacts, the possible mechanisms through which the DJF SPT impacts the following MAM rainfall over eastern China are investigated. The positive DJF SPT is associated with the significantly positive SSTA in the tropical western Pacific, which can persist to the following MAM. In response to the positive SSTA in the tropical western Pacific, a wave-like train in the low-level troposphere extends from the tropical western Pacific (an anomalous cyclone) to the western North Pacific (an anomalous anticyclone) during the following MAM. The anomalous anticyclone over the western North Pacific enhances the anomalous southwesterly over eastern China, which can bring more moisture and favor anomalous increased rainfall. It should be pointed out that La Niña (El Niño) could induce an anomalous cyclone (anticyclone) over the western North Pacific, which offsets the MAM anomalous anticyclone (cyclone) caused by the positive (negative) SPT in the preceding DJF and thus weakens the relationship between the SPT and the rainfall over eastern China.

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Qingqing Li, Yihong Duan, Hui Yu, and Gang Fu

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In this study, the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) is used to simulate Typhoon Rananim (2004) at high resolution (2-km grid size). The simulation agrees well with a variety of observations, especially for intensification, maintenance, landfall, and inner-core structures, including the echo-free eye, the asymmetry in eyewall convection, and the slope of the eyewall during landfall. The asymmetric feature of surface winds is also captured reasonably well by the model, as well as changes in surface winds and pressure near the storm center.

The shear-induced vortex tilt and storm-relative asymmetric winds are examined to investigate how vertical shear affects the asymmetric convection in the inner-core region. The inner-core vertical shear is found to be nonunidirectional, and to induce a nonunidirectional vortex tilt. The distribution of asymmetric convection is, however, inconsistent with the typical downshear-left pattern for a deep-layer shear. Qualitative agreement is found between the divergence pattern and the storm-relative flow, with convergence (divergence) generally associated with asymmetric inflow (outflow) in the eyewall. The collocation of the inflow-induced lower-level convergence in the boundary layer and the lower troposphere and the midlevel divergence causes shallow updrafts in the western and southern parts of the eyewall, while the deep and strong upward motion in the southeastern portion of the eyewall is due to the collocation of the net convergence associated with the strong asymmetric flow in the midtroposphere and the inflow near 400 hPa and its associated divergence in the outflow layer above 400 hPa.

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Wenxue Tong, Gang Li, Juanzhen Sun, Xiaowen Tang, and Ying Zhang

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This study examines two strategies for improving the analysis of an hourly update three-dimensional variational data assimilation (3DVAR) system and the subsequent quantitative precipitation forecast (QPF). The first strategy is to assimilate synoptic and radar observations in different steps. This strategy aims to extract both large-scale and convective-scale information from observations typically representing different scales. The second strategy is to add a divergence constraint to the momentum variables in the 3DVAR system. This technique aims at improving the dynamic balance and suppressing noise introduced during the assimilation process. A detailed analysis on how the new techniques impact convective-scale QPF was conducted using a severe storm case over Colorado and Kansas during 8 and 9 August 2008. First, it is demonstrated that, without the new strategies, the QPF initialized with an hourly update analysis performs worse than its 3-hourly counterpart. The implementation of the two-step assimilation and divergence constraint in the hourly update system results in improved QPF throughout most of the 12-h forecast period. The diagnoses of the analysis fields show that the two-step assimilation is able to preserve key convective-scale as well as large-scale structures that are consistent with the development of the real weather system. The divergence constraint is effective in improving the balance between the momentum control variables in the analysis, which leads to less spurious convection and improved QPF scores. The improvements of the new techniques were further verified by eight convective cases in 2014 and shown to be statistically significant.

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Xiaoye Yang, Gang Zeng, Guwei Zhang, and Zhongxian Li

Abstract

The paths of winter cold surge (CS) events in East Asia (EA) from 1979 to 2017 are tracked by the Flexible Particle (FLEXPART) model using ERA-Interim daily datasets, and the probability density distribution of the paths is calculated by the kernel density estimation (KDE) method. The results showed that the paths of CSs are significantly correlated with the intensity of the CSs, which shows an interdecadal transition from weak to strong around 1995. CS paths can be classified into two types, namely, the western path type and the northern path type, which were more likely to occur before and after 1995, respectively. Before 1995, the cold air mainly originated from Europe and moved from west to east, and the synoptic features were associated with the zonal wave train. After 1995, cold air accumulated over western Siberia and then invaded EA along the northern path, and the synoptic features were mainly associated with the blocking structure. The geopotential height (GPH) anomalies over the Arctic were abnormally strong. This paper further analyzes the relationship between CSs and winter sea ice concentration (SIC) in the Arctic. The results show that the intensity of CSs is negatively correlated with the Barents SIC (BSIC). When the BSIC declines, the upward wave flux over the Barents Sea is enhanced and expanded to the midlatitude region. GPH anomalies over the Arctic are positive and form a negative AO-like pattern, which is conducive to the formation of the northern path CS. Furthermore, the observed results are supported by numerical experiments with the NCAR Community Atmosphere Model, version 5.3 (CAM5.3).

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Guangxin He, Gang Li, Xiaolei Zou, and Peter Sawin Ray

Abstract

An improved velocity dealiasing algorithm is developed as an extension of the Next Generation Weather Radar (NEXRAD) dealiasing algorithm. The algorithm described in this paper is evaluated on selected China Next Generation Weather Radar (CINRAD) S-band radar radial velocity observations. This algorithm includes four modules for removing weak signals and determining the starting radial as a prelude to identifying and correcting aliased velocities. The proposed dealiasing algorithm was tested on 14 different weather systems, composed of typhoons, squall lines, and heavy rains. The results show that the algorithm is robust and stable for dealiasing S-band CINRAD radial velocity measurements. The performance levels for the typhoon and heavy rain cases are slightly better than for squall-line cases.

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Shuanglin Li, Jian Lu, Gang Huang, and Kaiming Hu

Abstract

A basin-scale warming is the leading mode of tropical Indian Ocean sea surface temperature (SST) variability on interannual time scales, and it is also the prominent feature of the interdecadal SST trend in recent decades. The influence of the warming on the East Asian summer monsoon (EASM) is investigated through ensemble experiments of several atmospheric general circulation models (AGCMs). The results from five AGCMs consistently suggest that near the surface, the Indian Ocean warming forces an anticyclonic anomaly over the subtropical western Pacific, intensifying the southwesterly winds to East China; and in the upper troposphere, it forces a Gill-type response with the intensified South Asian high, both favoring the enhancement of the EASM. These processes are argued to contribute to the stronger EASM during the summer following the peak of El Niño than monsoons in other years. These model results also suggest that tropical Indian Ocean warming may not have a causal relationship to the synchronous weakening of EASM on interdecadal time scales.

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Pengyuan Li, Gang Fu, Chungu Lu, Dan Fu, and Shuai Wang

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In this paper, a dense sea fog event that occurred over the Yellow Sea (YS) on 9 March 2005 is investigated using the Weather Research and Forecasting Model version 3.1.1 (WRF v3.1.1). It is shown that the WRF can reasonably reproduce the main features of this fog case with a newly implemented planetary boundary layer (PBL) scheme developed by Mellor–Yamada–Nakanishi–Niino (MYNN). The low-level jet (LLJ) associated with this fog episode played an important role in triggering the turbulence. During the fog formation, sea fog extended vertically with the aid of turbulence. The mechanical production term resulting from wind shear contributed to the generation of the turbulence. WRF simulation results showed that the fog layer was thicker in the northeastern part of the YS than that in the southwestern part due to the intensity of the inversion layer and the LLJ. The topography test in which the mountain region in Fujian Province was removed showed that the roles of topography were to prevent the moisture from extending to land, to intensify the inversion layer, and to enhance the intensity of LLJ, as well as to elevate its altitude.

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Guangxin He, Gang Li, Xiaolei Zou, and Peter Sawin Ray

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A velocity dealiasing algorithm is developed for C-band (5-cm wavelength) Doppler radars. With the shorter wavelength, C-band radars operating in a single PRF mode have a Nyquist interval about one-half that of S-band radars. As a proxy for C-band radar data, S-band Doppler data were sampled to produce radial velocity patterns as they would have been observed by a C-band Doppler radar. This algorithm includes six modules, some of which are modifications to previously existing methods. The modules (i) remove weak signals and determine the reference radials and dealias the radial wind field with a continuity constraint, (ii) perform a bulk continuity check, (iii) perform a “box” check to provide local consistency, (iv) perform a linear consistency fit in the radial direction, (v) provide quadratic least squares fit in the azimuthal direction, and (vi) perform quadratic least squares fit for elevation angle above 6.0° only for data from typhoons. The proposed dealiasing algorithm was tested on data from four typhoons and one squall line observed in China. When compared to the dealiasing results of the synthetic C-band data by the existing Next Generation Weather Radar (NEXRAD) algorithm, it was consistently better, particularly with smaller Nyquist cointervals. The algorithm correctly dealiased 91.56% of the aliased radial velocity data in all test cases.

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Danhong Dong, Weichen Tao, William K. M. Lau, Zhanqing Li, Gang Huang, and Pengfei Wang

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The present study investigates the interdecadal variation of precipitation over the Hengduan Mountains (HM) during rainy seasons from various reanalysis and observational datasets. Based on a moving t test and Lepage test, an obvious rainfall decrease is identified around 2004/05. The spatial distribution of the rainfall changes exhibits large and significant precipitation deficits over the southern HM, with notable anomalous lower-level easterly divergent winds along the southern foothills of the Himalayas (SFH). The anomalous easterlies are located at the northern edge of two cyclones, with two centers of positive rainfall anomalies over the west coast of India (WCI) and the Bay of Bengal (BOB). Observational evidence and numerical experiments demonstrate that the decadal changes of SST over the WP and WIO suppress rainfall over the eastern Indian Ocean (EIO) through large-scale circulation adjustment. The EIO dry anomalies trigger the cross-equatorial anticyclonic wind anomalies as a Rossby wave response, and further cause anomalous meridional circulation and moisture transport over the WCI and BOB, favoring the rainfall increase there. The anomalous easterlies at the northern edge of two cyclones induced by the wet anomalies–related heating cause the divergence anomalies along the SFH, resulting in the reduction of precipitation in the HM. In turn, the two anomalous cyclones and dry anomalies have positive feedback on the wet and easterly wind anomalies, respectively, emphasizing the importance of the circulation–heating interaction.

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