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2016 Vol. 33, No. 8

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Scale-dependent Regional Climate Predictability over North America Inferred from CMIP3 and CMIP5 Ensemble Simulations
Fuqing ZHANG, Wei LI, Michael E. MANN
2016, 33(8): 905-918. doi: 10.1007/s00376-016-6013-2
Through the analysis of ensembles of coupled model simulations and projections collected from CMIP3 and CMIP5, we demonstrate that a fundamental spatial scale limit might exist below which useful additional refinement of climate model predictions and projections may not be possible. That limit varies among climate variables and from region to region. We show that the uncertainty (noise) in surface temperature predictions (represented by the spread among an ensemble of global climate model simulations) generally exceeds the ensemble mean (signal) at horizontal scales below 1000 km throughout North America, implying poor predictability at those scales. More limited skill is shown for the predictability of regional precipitation. The ensemble spread in this case tends to exceed or equal the ensemble mean for scales below 2000 km. These findings highlight the challenges in predicting regionally specific future climate anomalies, especially for hydroclimatic impacts such as drought and wetness.
Intensified Impact of Northern Tropical Atlantic SST on Tropical Cyclogenesis Frequency over the Western North Pacific after the Late 1980s
Xi CAO, Shangfeng CHEN, Guanghua CHEN, Renguang WU
2016, 33(8): 919-930. doi: 10.1007/s00376-016-5206-z
Previous studies suggest that spring SST anomalies over the northern tropical Atlantic (NTA) affect the tropical cyclone (TC) activity over the western North Pacific (WNP) in the following summer and fall. The present study reveals that the connection between spring NTA SST and following summerfall WNP TC genesis frequency is not stationary. The influence of spring NTA SST on following summerfall WNP TC genesis frequency is weak and insignificant before, but strong and significant after, the late 1980s. Before the late 1980s, the NTA SST anomaly-induced SST anomalies in the tropical central Pacific are weak, and the response of atmospheric circulation over the WNP is not strong. As a result, the connection between spring NTA SST and following summerfall WNP TC genesis frequency is insignificant in the former period. In contrast, after the late 1980s, NTA SST anomalies induce pronounced tropical central Pacific SST anomalies through an Atlantic Pacific teleconnection. Tropical central Pacific SST anomalies further induce favorable conditions for WNP TC genesis, including vertical motion, mid-level relative humidity, and vertical zonal wind shear. Hence, the connection between NTA SST and WNP TC genesis frequency is significant in the recent period. Further analysis shows that the interdecadal change in the connection between spring NTA SST and following summer-fall WNP TC genesis frequency may be related to the climatological SST change over the NTA region.
Observed Changes in Aerosol Physical and Optical Properties before and after Precipitation Events
Xingmin LI, Yan DONG, Zipeng DONG, Chuanli DU, Chuang CHEN
2016, 33(8): 931-944. doi: 10.1007/s00376-016-5178-z
Precipitation scavenging of aerosol particles is an important removal process in the atmosphere that can change aerosol physical and optical properties. This paper analyzes the changes in aerosol physical and optical properties before and after four rain events using in situ observations of mass concentration, number concentration, particle size distribution, scattering and absorption coefficients of aerosols in June and July 2013 at the Xianghe comprehensive atmospheric observation station in China. The results show the effect of rain scavenging is related to the rain intensity and duration, the wind speed and direction. During the rain events, the temporal variation of aerosol number concentration was consistent with the variation in mass concentration, but their size-resolved scavenging ratios were different. After the rain events, the increase in aerosol mass concentration began with an increase in particles with diameter 0.8 m [measured using an aerodynamic particle sizer (APS)], and fine particles with diameter 0.1 m [measured using a scanning mobility particle sizer (SMPS)]. Rainfall was most efficient at removing particles with diameter 0.6 m and greater than 3.5 m. The changes in peak values of the particle number distribution (measured using the SMPS) before and after the rain events reflect the strong scavenging effect on particles within the 100-120 nm size range. The variation patterns of aerosol scattering and absorption coefficients before and after the rain events were similar, but their scavenging ratios differed, which may have been related to the aerosol particle size distribution and chemical composition.
Determination of the Effect of Initial Inner-Core Structure on Tropical Cyclone Intensification and Track on a Beta Plane
Guanghua CHEN
2016, 33(8): 945-954. doi: 10.1007/s00376-016-5241-9
The sensitivity of TC intensification and track to the initial inner-core structure on a β plane is investigated using a numerical model. The results show that the vortex with large inner-core winds (CVEX-EXP) experiences an earlier intensification than that with small inner-core winds (CCAVE-EXP), but they have nearly the same intensification rate after spin-up. In the early stage, the convective cells associated with surface heat flux are mainly confined within the inner-core region in CVEX-EXP, whereas the vortex in CCAVE-EXP exhibits a considerably asymmetric structure with most of the convective vortices being initiated to the northeast in the outer-core region due to the β effect. The large inner-core inertial stability in CVEX-EXP can prompt a high efficiency in the conversion from convective heating to kinetic energy. In addition, much stronger straining deformation and PBL imbalance in the inner-core region outside the primary eyewall ensue during the initial development stage in CVEX-EXP than in CCAVE-EXP, which is conducive to the rapid axisymmetrization and early intensification in CVEX-EXP. The TC track in CVEX-EXP sustains a northwestward displacement throughout the integration, whereas the TC in CCAVE-EXP undergoes a northeastward recurvature when the asymmetric structure is dominant. Due to the enhanced asymmetric convection to the northeast of the TC center in CCAVE-EXP, a pair of secondary gyres embedded within the large-scale primary β gyres forms, which modulates the ventilation flow and thus steers the TC to move northeastward.
Impact of Cyclone Nilam on Tropical Lower Atmospheric Dynamics
2016, 33(8): 955-968. doi: 10.1007/s00376-016-5285-x
A deep depression formed over the Bay of Bengal on 28 October 2012, and developed into a cyclonic storm. After landfall near the south coast of Chennai, cyclone Nilam moved north-northwestwards. Coordinated experiments were conducted from the Indian stations of Gadanki (13.5°N, 79.2°E) and Hyderabad (17.4°N, 78.5°E) to study the modification of gravity-wave activity and turbulence by cyclone Nilam, using GPS radiosonde and mesosphere-stratosphere-troposphere radar data. The horizontal velocities underwent large changes during the closest approach of the storm to the experimental sites. Hodograph analysis revealed that inertia gravity waves (IGWs) associated with the cyclone changed their directions from northeast (control time) to northwest following the path of the cyclone. The momentum flux of IGWs and short-period gravity waves (1-8 h) enhanced prior to, and during, the passage of the storm (0.05 m2 s-2 and 0.3 m2 s-2, respectively), compared to the flux after its passage. The corresponding body forces underwent similar changes, with values ranging between 2-4 m s-1 d-1 and 12-15 m s-1 d-1. The turbulence refractivity structure constant (Cn2) showed large values below 10 km before the passage of the cyclone when humidity in the region was very high. Turbulence and humidity reduced during the passage of the storm when a turbulent layer at ∼17 km became more intense. Turbulence in the lower troposphere and near the tropopause became weak after the passage of the cyclone.
Evaluation of a Micro-scale Wind Model's Performance over Realistic Building Clusters Using Wind Tunnel Experiments
Ning ZHANG, Yunsong DU, Shiguang MIAO, Xiaoyi FANG
2016, 33(8): 969-978. doi: 10.1007/s00376-016-5273-1
The simulation performance over complex building clusters of a wind simulation model (Wind Information Field Fast Analysis model, WIFFA) in a micro-scale air pollutant dispersion model system (Urban Microscale Air Pollution dispersion Simulation model, UMAPS) is evaluated using various wind tunnel experimental data including the CEDVAL (Compilation of Experimental Data for Validation of Micro-Scale Dispersion Models) wind tunnel experiment data and the NJU-FZ experiment data (Nanjing University-Fang Zhuang neighborhood wind tunnel experiment data). The results show that the wind model can reproduce the vortexes triggered by urban buildings well, and the flow patterns in urban street canyons and building clusters can also be represented. Due to the complex shapes of buildings and their distributions, the simulation deviations/discrepancies from the measurements are usually caused by the simplification of the building shapes and the determination of the key zone sizes. The computational efficiencies of different cases are also discussed in this paper. The model has a high computational efficiency compared to traditional numerical models that solve the Navier-Stokes equations, and can produce very high-resolution (1-5 m) wind fields of a complex neighborhood scale urban building canopy (∼ 1 km × 1 km) in less than 3 min when run on a personal computer.
Impacts of Two Types of El Niño on the MJO during Boreal Winter
Bo PANG, Zesheng CHEN, Zhiping WEN, Riyu LU
2016, 33(8): 979-986. doi: 10.1007/s00376-016-5272-2
The features of the MJO during two types of El Niño events are investigated in this paper using the daily NCEP-2 reanalysis data, OLR data from NOAA, and Real-time Multivariate MJO index for the period 1979-2012. The results indicate that the MJO exhibits distinct features during eastern Pacific (EP) El Niño events, as compared to central Pacific (CP) El Niño events. First, the intensity of the MJO is weakened during EP El Niño winters from the tropical eastern Indian Ocean to the western Pacific, but enhanced during CP El Niño winters. Second, the range of the MJO eastward propagation is different during the two types of El Niño events. During EP El Niño winters, the MJO propagates eastwards to 120°W, but only to 180° during CP El Niño winters. Finally, the frequency in eight phases of the MJO may be affected by the two types of El Niño. Phases 2 and 3 display a stronger MJO frequency during EP El Niño winters, but phases 4 and 5 during CP El Niño winters.
The Dynamics of Barotropic Vortex Merging
Chanh KIEU
2016, 33(8): 987-995. doi: 10.1007/s00376-016-6006-1
The merging of multiple vortices is a fundamental process of the dynamics of Earth's atmosphere and oceans. In this study, the interaction of like-signed vortices is analytically and numerically examined in a framework of two-dimensional inviscid barotropic flows. It is shown that barotropic vortex interaction turns out to be more intricate than simple merging scenarios often assumed in previous studies. Some particular configurations exist in which the vortex merging process is never complete despite strong interaction of like-signed vortices, regardless of the strengths or distances between the vortices. While the conditions for a complete vortex merging process introduced in this study appear to be too strict for most practical applications, this study suggests that careful criteria for vortex mergers should be properly defined when simulating the interaction of vortices, because the merging may not always result in a final enhanced circulation at the end of the interaction, as usually assumed in the literature.
Distinctive Precursory Air-Sea Signals between Regular and Super El Niños
Lin CHEN, Tim LI, Swadhin K. BEHERA, Takeshi DOI
2016, 33(8): 996-1004. doi: 10.1007/s00376-016-5250-8
Statistically different precursory air-sea signals between a super and a regular El Niño group are investigated, using observed SST and rainfall data, and oceanic and atmospheric reanalysis data. The El Niño events during 1958-2008 are first separated into two groups: a super El Niño group (S-group) and a regular El Niño group (R-group). Composite analysis shows that a significantly larger SST anomaly (SSTA) tendency appears in S-group than in R-group during the onset phase [April-May(0)], when the positive SSTA is very small. A mixed-layer heat budget analysis indicates that the tendency difference arises primarily from the difference in zonal advective feedback and the associated zonal current anomaly (u'). This is attributed to the difference in the thermocline depth anomaly (D') over the off-equatorial western Pacific prior to the onset phase, as revealed by three ocean assimilation products. Such a difference in D' is caused by the difference in the wind stress curl anomaly in situ, which is mainly regulated by the anomalous SST and precipitation over the Maritime Continent and equatorial Pacific.