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2017 Vol. 34, No. 6

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Optimal Precursors Triggering the Kuroshio Extension State Transition Obtained by the Conditional Nonlinear Optimal Perturbation Approach
Xing ZHANG, Mu MU, Qiang WANG, Stefano PIERINI
2017, 34(6): 685-699. doi: 10.1007/s00376-017-6263-7
In this study, the initial perturbations that are the easiest to trigger the Kuroshio Extension (KE) transition connecting a basic weak jet state and a strong, fairly stable meandering state, are investigated using a reduced-gravity shallow water ocean model and the CNOP (Conditional Nonlinear Optimal Perturbation) approach. This kind of initial perturbation is called an optimal precursor (OPR). The spatial structures and evolutionary processes of the OPRs are analyzed in detail. The results show that most of the OPRs are in the form of negative sea surface height (SSH) anomalies mainly located in a narrow band region south of the KE jet, in basic agreement with altimetric observations. These negative SSH anomalies reduce the meridional SSH gradient within the KE, thus weakening the strength of the jet. The KE jet then becomes more convoluted, with a high-frequency and large-amplitude variability corresponding to a high eddy kinetic energy level; this gradually strengthens the KE jet through an inverse energy cascade. Eventually, the KE reaches a high-energy state characterized by two well defined and fairly stable anticyclonic meanders. Moreover, sensitivity experiments indicate that the spatial structures of the OPRs are not sensitive to the model parameters and to the optimization times used in the analysis.
Effects of Tropical Cyclone Activity on the Boundary Moisture Budget over the Eastern China Monsoon Region
Xiaobin LIN, Zhiping WEN, Wen ZHOU, Renguang WU, Ruidan CHEN
2017, 34(6): 700-712. doi: 10.1007/s00376-017- 6191-6
In summer, water vapor over the eastern China monsoon region (ECMR) comes mainly from low latitudes and is modulated by tropical cyclone (TC) activity in East Asia (EA). This study examines the variability of water vapor transport over the ECMR, especially of the moisture inflow via the southern and eastern boundaries. The results of composite and correlation analyses, using data from 1979 to 2010, reveal significant differences in moisture budgets along the boundaries between TC days and non-TC days. Almost 80% of the water vapor transport via the eastern boundary occurs during TC days, while at the southern boundary most inflow occurs on non-TC days. The ratio of the total water vapor transport between TC and non-TC days is about 4:6. In addition, the El Niño-Southern Oscillation (ENSO) exhibits a remarkable influence on moisture transport over EA and the contributions of moisture inflow on TC days increase (reduce) in El Niño (La Niña) years. Moreover, different types of TCs, based on their tracks, have different effects on the moisture budgets along the southern and eastern boundaries. When TCs enter EA (but not the ECMR), they favor the moisture inflow via the eastern boundary and hinder the moisture inflow via the southern boundary. After TCs enter the ECMR, the inhibition of moisture inflow via the southern boundary will be weakened, and more water vapor can be brought into the ECMR. For some recurving TCs with an increase in TC activity in the midlatitudes, the influence is uncertain in different cases. The results herein suggest that TC activity is an important factor that influences the boundary moisture budgets in the ECMR.
A Trend towards a Stable Warm and Windless State of the Surface Weather Conditions in Northern and Northeastern China during 1961-2014
Bo SUN, Huijun WANG
2017, 34(6): 713-726. doi: 10.1007/s00376-017-6252-x
This study investigates the trends in the mean state and the day-to-day variability (DDV) of the surface weather conditions over northern and northeastern China (NNEC) during 1961-2014 using CN05.1 observational data. In this study, we show that the surface temperature (wind speed) has increased (decreased) over NNEC and that the DDV of the surface temperatures and wind speeds has decreased, indicating a trend towards a stable warm and windless state of the surface weather conditions over NNEC. This finding implies a trend towards more persistent hot and windless episodes, which threaten human health and aggravate environmental problems. The trends are also examined in reanalysis data. Both the ERA-40 and the NCEP data show an increasing (decreasing) trend in the mean state of the surface temperatures (wind speeds). However, the reanalysis data show a consistent decreasing trend in the DDV of the surface weather conditions only in the spring. The underlying reason for the decreased DDV of the surface weather conditions is further analyzed, focusing on the spring season. Essentially, the decreased DDV of the surface weather conditions can be attributed to a decrease in synoptic-scale wave activity, which is caused by a decrease in the baroclinic instability. There is a contrasting change in the baroclinic instability over East Asia, showing a decreasing (increasing) trend north (south) of 40°N. This contrasting change in the baroclinic instability is primarily caused by a tropospheric cooling zone over East Asia at approximately 40°N, which influences the meridional temperature gradient over East Asia.
Statistical Characteristics of Raindrop Size Distribution in the Tibetan Plateau and Southern China
Yahao WU, Liping LIU
2017, 34(6): 727-736. doi: 10.1007/s00376-016-5235-7
The characteristics of raindrop size distribution (DSD) over the Tibetan Plateau and southern China are studied in this paper, using the DSD data from April to August 2014 collected by HSC-PS32 disdrometers in Nagqu and Yangjiang, comprising a total of 9430 and 6366 1-min raindrop spectra, respectively. The raindrop spectra, characteristics of parameter variations with rainfall rate, and the relationships between reflectivity factor (Z) and rainfall rate (R) are analyzed, as well as their DSD changes with precipitation type and rainfall rate. The results show that the average raindrop spectra appear to be one-peak curves, the number concentration for larger drops increase significantly with rainfall rate, and its value over southern China is much higher, especially in convective rain. Standardized Gamma distributions better describe DSD for larger drops, especially for convective rain in southern China. All three Gamma parameters for stratiform precipitation over the Tibetan Plateau are much higher, while its shape parameter (μ) and mass-weighted mean diameter (D m), for convective precipitation, are less. In terms of parameter variation with rainfall rate, the normalized intercept parameter (N w) over the Tibetan Plateau for stratiform rain increases with rainfall rate, which is opposite to the situation in convective rain. The μ over the Tibetan Plateau for stratiform and convective precipitation types decreases with an increase in rainfall rate, which is opposite to the case for D m variation. In Z-R relationships, like "Z=ARb", the coefficient A over the Tibetan Plateau is smaller, while its b is higher, when the rain type transfers from stratiform to convective ones. Furthermore, with an increase in rainfall rate, parameters A and b over southern China increase gradually, while A over the Tibetan Plateau decreases substantially, which differs from the findings of previous studies. In terms of geographic location and climate over the Tibetan Plateau and southern China, the precipitation in the pre-flood seasons is dominated by strong convective rain, while weak convective rain occurs frequently in northern Tibet with lower humidity and higher altitude.
Nonlinearity Modulating Intensities and Spatial Structures of Central Pacific and Eastern Pacific El Niño Events
Wansuo DUAN, Chaoming HUANG, Hui XU
2017, 34(6): 737-756. doi: 10.1007/s00376-017-6148-9
This paper compares data from linearized and nonlinear Zebiak-Cane model, as constrained by observed sea surface temperature anomaly (SSTA), in simulating central Pacific (CP) and eastern Pacific (EP) El Niño. The difference between the temperature advections (determined by subtracting those of the linearized model from those of the nonlinear model), referred to here as the nonlinearly induced temperature advection change (NTA), is analyzed. The results demonstrate that the NTA records warming in the central equatorial Pacific during CP El Niño and makes fewer contributions to the structural distinctions of the CP El Niño, whereas it records warming in the eastern equatorial Pacific during EP El Niño, and thus significantly promotes EP El Niño during El Niño-type selection. The NTA for CP and EP El Niño varies in its amplitude, and is smaller in CP El Niño than it is in EP El Niño. These results demonstrate that CP El Niño are weakly modulated by small intensities of NTA, and may be controlled by weak nonlinearity; whereas, EP El Niño are significantly enhanced by large amplitudes of NTA, and are therefore likely to be modulated by relatively strong nonlinearity. These data could explain why CP El Niño are weaker than EP El Niño. Because the NTA for CP and EP El Niño differs in spatial structures and intensities, as well as their roles within different El Niño modes, the diversity of El Niño may be closely related to changes in the nonlinear characteristics of the tropical Pacific.
A High-resolution Simulation of Supertyphoon Rammasun (2014) —— Part I: Model Verification and Surface Energetics Analysis
Xinghai ZHANG, Yihong DUAN, Yuqing WANG, Na WEI, Hao HU
2017, 34(6): 757-770. doi: 10.1007/s00376-017-6255-7
A 72-h high-resolution simulation of Supertyphoon Rammasun (2014) is performed using the Advanced Research Weather Research and Forecasting model. The model covers an initial 18-h spin-up, the 36-h rapid intensification (RI) period in the northern South China Sea, and the 18-h period of weakening after landfall. The results show that the model reproduces the track, intensity, structure of the storm, and environmental circulations reasonably well. Analysis of the surface energetics under the storm indicates that the storm's intensification is closely related to the net energy gain rate (ε g), defined as the difference between the energy production (P D) due to surface entropy flux and the energy dissipation (D S) due to surface friction near the radius of maximum wind (RMW). Before and during the RI stage, the ε g is high, indicating sufficient energy supply for the storm to intensify. However, the ε g decreases rapidly as the storm quickly intensifies, because the D S increases more rapidly than the P D near the RMW. By the time the storm reaches its peak intensity, the D S is about 20% larger than the P D near the RMW, leading to a local energetics deficit under the eyewall. During the mature stage, the P D and D S can reach a balance within a radius of 86 km from the storm center (about 2.3 times the RMW). This implies that the local P D under the eyewall is not large enough to balance the D S, and the radially inward energy transport from outside the eyewall must play an important role in maintaining the storm's intensity, as well as its intensification.
Different Impact of Central Pacific and Eastern Pacific El Niño on the Duration of Sudden Stratospheric Warming
Yuanpu LI, Wenshou TIAN
2017, 34(6): 771-782. doi: 10.1007/s00376-017-6286-0
The NCEP-NCAR reanalysis dataset and the HadISST dataset (1959-2014) are used to analyze the impact of two types of El Niño events, i.e., eastern Pacific El Niño (EP-El Niño) and central Pacific El Niño (CP-El Niño) events, on the duration of major and minor sudden stratospheric warmings (SSWs) in Northern Hemisphere winter (November to February). Although the frequency of major and minor SSWs during different types of El Niño shows no distinct differences, the duration of both major and minor SSWs during CP-El Niño is shorter than that during EP-El Niño. The spatial distribution of geopotential height anomalies preceding major SSWs resembles the western Pacific (WP) teleconnection pattern, while the spatial distribution of geopotential height anomalies preceding minor SSWs bears similarity to the Pacific-North America (PNA) teleconnection pattern. An enhancement of the strength of both wavenumber 1 and wavenumber 2 is found before major SSWs. Before minor SSWs, wavenumber 1 is also strengthened, but wavenumber 2 is weakened. The analysis also reveals that EP-El Niño tends to induce positive phases of PNA and WP teleconnections, while CP-El Niño induces negative-phase WP teleconnection. As the positive phases of the PNA and WP teleconnections are related to the strengthening of wavenumber 1, EP-El Niño causes an enhancement of wavenumber 1 in the high-latitude upper troposphere and an enhancement of the upward wave flux in the high-latitude stratosphere, accompanied by a negative anomaly in Eliassen-Palm flux divergence in the subpolar stratosphere, which accounts for the longer SSW duration during EP-El Niño than during CP-El Niño.
Intensive Radiosonde Measurements of Summertime Convection over the Inner Mongolia Grassland in 2014: Difference between Shallow Cumulus and Other Conditions
Hongrong SHI, Hongbin CHEN, Xiang'ao XIA, Xuehua FAN, Jinqiang ZHANG, Jun LI, Chao LING
2017, 34(6): 783-790. doi: 10.1007/s00376-017-6284-2
Using radiosonde measurements from 26 July to 30 July 2014 at Baiqi over the Inner Mongolia grassland of China, the vertical structure of shallow cumulus (SCu) clouds and associated environmental conditions were investigated. The cloud base height and the cloud top height of SCu was 3.4 km and 5 km, respectively. The temperature of the SCu layer was less than 0°C. The horizontal advection of specific humidity was smaller than the vertical transport in the atmosphere below 5 km. Above 5 km, the thermodynamic structure of the atmosphere remained stable. At the interface of the cloud layer and free air atmosphere, there was obvious wind shear and a temperature inversion (∼2.9°C). Comparisons of environmental parameters associated with cumulus congestus, rain and clear days, showed that the formation of SCu was characterized by a higher Bowen ratio (high sensible heat flux and low latent heat flux), which indicated intensive turbulence in the boundary layer. The formation of SCu was associated with the boundary layer height exceeding the lifting condensation level. The maintenance of SCu was likely associated with the lower convective available potential energy, weak wind shear, and weak subsidence of the synoptic system, which did not favor the dramatic vertical development of SCu and thereby the transformation of SCu to cumulus congestus.
Initial Error-induced Optimal Perturbations in ENSO Predictions, as Derived from an Intermediate Coupled Model
Ling-Jiang TAO, Rong-Hua ZHANG, Chuan GAO
2017, 34(6): 791-803. doi: 10.1007/s00376-017-6266-4
The initial errors constitute one of the main limiting factors in the ability to predict the El Niño-Southern Oscillation (ENSO) in ocean-atmosphere coupled models. The conditional nonlinear optimal perturbation (CNOP) approach was employed to study the largest initial error growth in the El Niño predictions of an intermediate coupled model (ICM). The optimal initial errors (as represented by CNOPs) in sea surface temperature anomalies (SSTAs) and sea level anomalies (SLAs) were obtained with seasonal variation. The CNOP-induced perturbations, which tend to evolve into the La Niña mode, were found to have the same dynamics as ENSO itself. This indicates that, if CNOP-type errors are present in the initial conditions used to make a prediction of El Niño, the El Niño event tends to be under-predicted. In particular, compared with other seasonal CNOPs, the CNOPs in winter can induce the largest error growth, which gives rise to an ENSO amplitude that is hardly ever predicted accurately. Additionally, it was found that the CNOP-induced perturbations exhibit a strong spring predictability barrier (SPB) phenomenon for ENSO prediction. These results offer a way to enhance ICM prediction skill and, particularly, weaken the SPB phenomenon by filtering the CNOP-type errors in the initial state. The characteristic distributions of the CNOPs derived from the ICM also provide useful information for targeted observations through data assimilation. Given the fact that the derived CNOPs are season-dependent, it is suggested that seasonally varying targeted observations should be implemented to accurately predict ENSO events.