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2017 Vol. 34, No. 4

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Atmospheric Circulation and Dynamic Mechanism for Persistent Haze Events in the Beijing-Tianjin-Hebei Region
Ping WU, Yihui DING, Yanju LIU
2017, 34(4): 429-440. doi: 10.1007/s00376-016-6158-z
In this study, regional persistent haze events (RPHEs) in the Beijing-Tianjin-Hebei (BTH) region were identified based on the Objective Identification Technique for Regional Extreme Events for the period 1980-2013. The formation mechanisms of the severe RPHEs were investigated with focus on the atmospheric circulation and dynamic mechanisms. Results indicated that: (1) 49 RPHEs occurred during the past 34 years. (2) The severe RPHEs could be categorized into two types according to the large-scale circulation, i.e. the zonal westerly airflow (ZWA) type and the high-pressure ridge (HPR) type. When the ZWA-type RPHEs occurred, the BTH region was controlled by near zonal westerly airflow in the mid-upper troposphere. Southwesterly winds prevailed in the lower troposphere, and near-surface wind speeds were only 1-2 m s-1. Warm and humid air originating from the northwestern Pacific was transported into the region, where the relative humidity was 70% to 80%, creating favorable moisture conditions. When the HPR-type RPHEs appeared, northwesterly airflow in the mid-upper troposphere controlled the region. Westerly winds prevailed in the lower troposphere and the moisture conditions were relatively weak. (3) Descending motion in the mid-lower troposphere caused by the above two circulation types provided a crucial dynamic mechanism for the formation of the two types of RPHEs. The descending motion contributed to a reduction in the height of the planetary boundary layer (PBL), which generated an inversion in the lower troposphere. This inversion trapped the abundant pollution and moisture in the lower PBL, leading to high concentrations of pollutants.
Performance of RegCM4 over Major River Basins in China
Xuejie GAO, Ying SHI, Zhenyu HAN, Meili WANG, Jia WU, Dongfeng ZHANG, Ying XU, Filippo GIORGI
2017, 34(4): 441-455. doi: 10.1007/s00376-016-6179-7
A long-term simulation for the period 1990-2010 is conducted with the latest version of the International Centre for Theoretical Physics' Regional Climate Model (RegCM4), driven by ERA-Interim boundary conditions at a grid spacing of 25 km. The Community Land Model (CLM) is used to describe land surface processes, with updates in the surface parameters, including the land cover and surface emissivity. The simulation is compared against observations to evaluate the model performance in reproducing the present day climatology and interannual variability over the 10 main river basins in China, with focus on surface air temperature and precipitation. Temperature and precipitation from the ERA-Interim reanalysis are also considered in the model assessment. Results show that the model reproduces the present day climatology over China and its main river basins, with better performances in June-July-August compared to December-January-February (DJF). In DJF, we find a warm bias at high latitudes, underestimated precipitation in the south, and overestimated precipitation in the north. The model in general captures the observed interannual variability, with greater skill for temperature. We also find an underestimation of heavy precipitation events in eastern China, and an underestimation of consecutive dry days in northern China and the Tibetan Plateau. Similar biases for both mean climatology and extremes are found in the ERA-Interim reanalysis, indicating the difficulties for climate models in simulating extreme monsoon climate events over East Asia.
An Abrupt Rainfall Decrease over the Asian Inland Plateau Region around 1999 and the Possible Underlying Mechanism
Jinling PIAO, Wen CHEN, Ke WEI, Yong LIU, Hans-F. GRAF, Joong-Bae AHN, Alexander POGORELTSEV
2017, 34(4): 456-468. doi: 10.1007/s00376-016-6136-5
A decadal change in summer rainfall in the Asian inland plateau (AIP) region is identified around 1999. This decadal change is characterized by an abrupt decrease in summer rainfall of about 15.7% of the climatological average amount, leading to prolonged drought in the Asian inland plateau region. Both the surface air temperature and potential evapotranspiration in the AIP show a significant increase, while the soil moisture exhibits a decrease, after the late 1990s. Furthermore, the normalized difference vegetation index shows an apparent decreasing trend during 1999-2007. Three different drought indices——the standardized precipitation index, the standardized precipitation evapotranspiration index, and the self-calibrating Palmer drought severity index——present pronounced climate anomalies during 1999-2007, indicating dramatic drought exacerbation in the region after the late 1990s. This decadal change in the summer rainfall may be attributable to a wave-like teleconnection pattern from Western Europe to Asia. A set of model sensitivity experiments suggests that the summer warming sea surface temperature in the North Atlantic could induce this teleconnection pattern over Eurasia, resulting in recent drought in the AIP region.
Application of a Three-dimensional Variational Method for Radar Reflectivity Data Correction in a Mudslide-inducing Rainstorm Simulation
Hongli LI, Xiangde XU
2017, 34(4): 469-481. doi: 10.1007/s00376-016-6010-5
Various types of radars with different horizontal and vertical detection ranges are deployed in China, particularly over complex terrain where radar blind zones are common. In this study, a new variational method is developed to correct three-dimensional radar reflectivity data based on hourly ground precipitation observations. The aim of this method is to improve the quality of observations of various types of radar and effectively assimilate operational Doppler radar observations. A mudslide-inducing local rainstorm is simulated by the WRF model with assimilation of radar reflectivity and radial velocity data using LAPS (Local Analysis and Prediction System). Experiments with different radar data assimilated by LAPS are performed. It is found that when radar reflectivity data are corrected using this variational method and assimilated by LAPS, the atmospheric conditions and cloud physics processes are reasonably described. The temporal evolution of radar reflectivity corrected by the variational method corresponds well to observed rainfall. It can better describe the cloud water distribution over the rainfall area and improve the cloud water analysis results over the central rainfall region. The LAPS cloud analysis system can update cloud microphysical variables and represent the hydrometeors associated with strong convective activities over the rainfall area well. Model performance is improved and the simulation of the dynamical processes and moisture transport is more consistent with observation.
Numerical Simulation of the Rapid Intensification of Hurricane Katrina (2005): Sensitivity to Boundary Layer Parameterization Schemes
Jianjun LIU, Feimin ZHANG, Zhaoxia PU
2017, 34(4): 482-496. doi: 10.1007/s00376-016-6209-5
Accurate forecasting of the intensity changes of hurricanes is an important yet challenging problem in numerical weather prediction. The rapid intensification of Hurricane Katrina (2005) before its landfall in the southern US is studied with the Advanced Research version of the WRF (Weather Research and Forecasting) model. The sensitivity of numerical simulations to two popular planetary boundary layer (PBL) schemes, the Mellor-Yamada-Janjic (MYJ) and the Yonsei University (YSU) schemes, is investigated. It is found that, compared with the YSU simulation, the simulation with the MYJ scheme produces better track and intensity evolution, better vortex structure, and more accurate landfall time and location. Large discrepancies (e.g., over 10 hPa in simulated minimum sea level pressure) are found between the two simulations during the rapid intensification period. Further diagnosis indicates that stronger surface fluxes and vertical mixing in the PBL from the simulation with the MYJ scheme lead to enhanced air-sea interaction, which helps generate more realistic simulations of the rapid intensification process. Overall, the results from this study suggest that improved representation of surface fluxes and vertical mixing in the PBL is essential for accurate prediction of hurricane intensity changes.
Interannual Climate Variability Change during the Medieval Climate Anomaly and Little Ice Age in PMIP3 Last Millennium Simulations
Kaiqing YANG, Dabang JIANG
2017, 34(4): 497-508. doi: 10.1007/s00376-016-6075-1
In this study, we analyzed numerical experiments undertaken by 10 climate models participating in PMIP3 (Paleoclimate Modelling Intercomparison Project Phase 3) to examine the changes in interannual temperature variability and coefficient of variation (CV) of interannual precipitation in the warm period of the Medieval Climate Anomaly (MCA) and the cold period of the Little Ice Age (LIA). With respect to the past millennium period, the MCA temperature variability decreases by 2.0% on average over the globe, and most of the decreases occur in low latitudes. In the LIA, temperature variability increases by a global average of 0.6%, which occurs primarily in the high latitudes of Eurasia and the western Pacific. For the CV of interannual precipitation, regional-scale changes are more significant than changes at the global scale, with a pattern of increased (decreased) CV in the midlatitudes of Eurasia and the northwestern Pacific in the MCA (LIA). The CV change ranges from -7.0% to 4.3% (from -6.3% to 5.4%), with a global average of -0.5% (-0.07%) in the MCA (LIA). Also, the variability changes are considerably larger in December-January-February with respect to both temperature and precipitation.
Observational Evidence of High Ice Concentration in a Shallow Convective Cloud Embedded in Stratiform Cloud over North China
Jiefan YANG, Hengchi LEI, Tuanjie HOU
2017, 34(4): 509-520. doi: 10.1007/s00376-016-6079-x
In this study we observed the microphysical properties, including the vertical and horizontal distributions of ice particles, liquid water content and ice habit, in different regions of a slightly supercooled stratiform cloud. Using aircraft instrument and radar data, the cloud top temperature was recorded as higher than -15°C, behind a cold front, on 9 September 2015 in North China. During the flight sampling, the high ice number concentration area was located in the supercooled part of a shallow convective cloud embedded in a stratiform cloud, where the ambient temperature was around -3°C. In this area, the maximum number concentrations of particles with diameter greater than 100 μm and 500 μm (N100 and N500) exceeded 300 L-1 and 30 L-1, respectively, and were related to large supercooled water droplets with diameter greater than 24 μm derived from cloud-aerosol spectrometer probe measurements. The ice particles types in this region were predominantly columnar, needle, graupel, and some freezing drops, suggesting that the occurrence of high ice number concentrations was likely related to the Hallett-Mossop mechanism, although many other ice multiplication processes cannot be totally ruled out. The maximum ice number concentration obtained during the first penetration was around two to three orders of magnitude larger than that predicted by the Demott and Fletcher schemes when assuming the cloud top temperature was around -15°C. During the second penetration conducted within the stratiform cloud, N100 and N500 decreased by a factor of five to ten, and the presence of columnar and needle-like crystals became very rare.
On the Response of Subduction in the South Pacific to an Intensification of Westerlies and Heat Flux in an Eddy Permitting Ocean Model
Chengyan LIU, Zhaomin WANG, Bingrui LI, Chen CHENG, Ruibin XIA
2017, 34(4): 521-531. doi: 10.1007/s00376-016-6021-2
Based on an eddy permitting ocean general circulation model, the response of water masses to two distinct climate scenarios in the South Pacific is assessed in this paper. Under annually repeating atmospheric forcing that is characterized by different westerlies and associated heat flux, the response of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) is quantitatively estimated. Both SAMW and AAIW are found to be warmer, saltier and denser under intensified westerlies and increased heat loss. The increase in the subduction volume of SAMW and AAIW is about 19.8 Sv (1 Sv = 106 m3 s-1). The lateral induction term plays a dominant role in the changes in the subduction volume due to the deepening of the mixed layer depth (MLD). Furthermore, analysis of the buoyancy budget is used to quantitatively diagnose the reason for the changes in the MLD. The deepening of the MLD is found to be primarily caused by the strengthening of heat loss from the ocean to the atmosphere in the formation region of SAMW and AAIW.
Favorable Environments for the Occurrence of Overshooting Tops in Tropical Cyclones
Liangxiao SUN, Xiaoyong ZHUGE, Yuan WANG
2017, 34(4): 532-544. doi: 10.1007/s00376-016-6122-y
Based on Multifunctional Transport Satellite data and the infrared window-texture detection algorithm, the level of overshooting top (OT) activity within a tropical cyclone (TC), which is defined as the hourly mean number of OT occurrence, was statistically investigated in the western North Pacific basin for the period 2005-12. Based on the level of OT activity, the samples were divided into OT and non-OT cases or high-activity-OT (HA-OT) and low-activity-OT (LA-OT) cases. The differences in large-scale environmental variables between OT (HA-OT) and non-OT (LA-OT) cases were examined 12 hours prior to the OT occurrence. Statistical analysis showed that environmental differences did exist between the OT and non-OT cases. The OTs were more skewed towards the early stage of the TC life cycle, and mostly concentrated in low latitudes. Meanwhile, a sufficiently deep warm-water layer, large temperature difference between the upper- and lower-level troposphere, large humidity at the middle and upper levels, and large atmospheric instability, were favorable for OT occurrence. The differences in large-scale environmental characteristics between HA-OTs and LA-OTs were not as significant as those between OTs and non-OTs, but the HA-OT samples tended to occur when the vertical shear was weak and the TC intensity was low. Finally, statistical models were designed to predict the OT and HA-OT. When at least three OT (HA-OT) predictor thresholds were satisfied, the Peirce skill score reached a maximum value of 0.49 (0.30).
A Ka-band Solid-state Transmitter Cloud Radar and Data Merging Algorithm for Its Measurements
Liping LIU, Jiafeng ZHENG, Jingya WU
2017, 34(4): 545-558. doi: 10.1007/s00376-016-6044-8
This study concerns a Ka-band solid-state transmitter cloud radar, made in China, which can operate in three different work modes, with different pulse widths, and coherent and incoherent integration numbers, to meet the requirements for cloud remote sensing over the Tibetan Plateau. Specifically, the design of the three operational modes of the radar (i.e., boundary mode M1, cirrus mode M2, and precipitation mode M3) is introduced. Also, a cloud radar data merging algorithm for the three modes is proposed. Using one month's continuous measurements during summertime at Naqu on the Tibetan Plateau, we analyzed the consistency between the cloud radar measurements of the three modes. The number of occurrences of radar detections of hydrometeors and the percentage contributions of the different modes' data to the merged data were estimated. The performance of the merging algorithm was evaluated. The results indicated that the minimum detectable reflectivity for each mode was consistent with theoretical results. Merged data provided measurements with a minimum reflectivity of -35 dBZ at the height of 5 km, and obtained information above the height of 0.2 km. Measurements of radial velocity by the three operational modes agreed very well, and systematic errors in measurements of reflectivity were less than 2 dB. However, large discrepancies existed in the measurements of the linear depolarization ratio taken from the different operational modes. The percentage of radar detections of hydrometeors in mid- and high-level clouds increased by 60% through application of pulse compression techniques. In conclusion, the merged data are appropriate for cloud and precipitation studies over the Tibetan Plateau.
Analysis of a Vortex Precipitation Event over Southwest China Using AIRS and In Situ Measurements
Chengcheng NI, Guoping LI, Xiaozhen XIONG
2017, 34(4): 559-570. doi: 10.1007/s00376-016-5262-4
A strong precipitation event caused by the southwest vortex (SWV), which affected Sichuan Province and Chongqing municipality in Southwest China on 10-14 July 2012, is investigated. The SWV is examined using satellite observations from AIRS (Atmospheric Infrared Sounder), in situ measurements from the SWV intensive observation campaign, and MICAPS (Marine Interactive Computer-Aided Provisioning System) data. Analysis of this precipitation process revealed that: (2) heavy rain occurred during the development phase, and cloud water content increased significantly after the dissipation of the SWV; (3) the area with low outgoing longwave radiation values from AIRS correlated well with the SWV; (4) variation of the temperature of brightness blackbody (TBB) from AIRS reflected the evolution of the SWV, and the values of TBB reduced significantly during the SWV's development; and (5) strong temperature and water vapor inversions were noted during the development of the SWV. The moisture profile displayed large vertical variation during the SWV's puissant phase, with the moisture inversion occurring at low levels. The moisture content during the receding phase was significantly reduced compared with that during the developing and puissant phases. The vertical flux of vapor divergence explained the variation of the moisture profile. These results also indicate the potential for using AIRS products in studying severe weather over the Tibetan Plateau and its surroundings, where in situ measurements are sparse.