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2017 Vol. 34, No. 1

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3rd ANtarctic Gravity Wave Instrument Network (ANGWIN) Science Workshop
2017, 34(1): 1-3. doi: 10.1007/s00376-016-6197-5
Relationship between Indian and East Asian Summer Rainfall Variations
Renguang WU
2017, 34(1): 4-15. doi: 10.1007/s00376-016-6216-6
The Indian and East Asian summer monsoons are two components of the whole Asian summer monsoon system. Previous studies have indicated in-phase and out-of-phase variations between Indian and East Asian summer rainfall. The present study reviews the current understanding of the connection between Indian and East Asian summer rainfall. The review covers the relationship of northern China, southern Japan, and South Korean summer rainfall with Indian summer rainfall; the atmospheric circulation anomalies connecting Indian and East Asian summer rainfall variations; the long-term change in the connection between Indian and northern China rainfall and the plausible reasons for the change; and the influence of ENSO on the relationship between Indian and East Asian summer rainfall and its change. While much progress has been made about the relationship between Indian and East Asian summer rainfall variations, there are several remaining issues that need investigation. These include the processes involved in the connection between Indian and East Asian summer rainfall, the non-stationarity of the connection and the plausible reasons, the influences of ENSO on the relationship, the performance of climate models in simulating the relationship between Indian and East Asian summer rainfall, and the relationship between Indian and East Asian rainfall intraseasonal fluctuations.
A Cloud Detection Scheme for the Chinese Carbon Dioxide Observation Satellite (TANSAT)
Xi WANG, Zheng GUO, Yipeng HUANG, Hongjie FAN, Wanbiao LI
2017, 34(1): 16-25. doi: 10.1007/s00376-016-6033-y
Cloud detection is an essential preprocessing step for retrieving carbon dioxide from satellite observations of reflected sunlight. During the pre-launch study of the Chinese Carbon Dioxide Observation Satellite (TANSAT), a cloud-screening scheme was presented for the Cloud and Aerosol Polarization Imager (CAPI), which only performs measurements in five channels located in the visible to near-infrared regions of the spectrum. The scheme for CAPI, based on previous cloud-screening algorithms, defines a method to regroup individual threshold tests for each pixel in a scene according to the derived clear confidence level. This scheme is proven to be more effective for sensors with few channels. The work relies upon the radiance data from the Visible and Infrared Radiometer (VIRR) onboard the Chinese FengYun-3A Polar-orbiting Meteorological Satellite (FY-3A), which uses four wavebands similar to that of CAPI and can serve as a proxy for its measurements. The scheme has been applied to a number of the VIRR scenes over four target areas (desert, snow, ocean, forest) for all seasons. To assess the screening results, comparisons against the cloud-screening product from MODIS are made. The evaluation suggests that the proposed scheme inherits the advantages of schemes described in previous publications and shows improved cloud-screening results. A seasonal analysis reveals that this scheme provides better performance during warmer seasons, except for observations over oceans, where results are much better in colder seasons.
Intraseasonal Variation of Visibility in Hong Kong
Wen ZHOU, Richard C. Y. LI, Eric C. H. CHOW
2017, 34(1): 26-38. doi: 10.1007/s00376-016-6056-4
Visibility is one of the parameters for indicating air pollution. In this study, visibility variation in Hong Kong during summer and winter is investigated. Visibility in Hong Kong has clear intraseasonal variation. Examination of different environmental parameters suggests that the intraseasonal component dominates the overall circulation anomalies in both summer and winter. Associated with the intraseasonal variation of environmental parameters, obvious variation in visibility impairment is found in both summer and winter. In summer, local visibility and air quality are found to be significantly affected by the (MJO) and the 10-30-day intraseasonal oscillation (ISO) through modulation of associated atmospheric circulations. In winter, the modulation effects appear to be weaker due to the southward shift of the associated convection. The results in this study highlight the importance of the ISO in contributing to the overall variation in visibility in Hong Kong, and provide useful implications for the development of possible mitigation strategies associated with visibility impairment and air pollution in Hong Kong.
Comparison of Beijing MST Radar and Radiosonde Horizontal Wind Measurements
Yufang TIAN, Daren LÜ
2017, 34(1): 39-53. doi: 10.1007/s00376-016-6129-4
To determine the performance and data accuracy of the 50 MHz Beijing Mesosphere-Stratosphere-Troposphere (MST) radar, comparisons of radar measured horizontal winds in the height range 3-25 km with radiosonde observations were made during 2012. A total of 427 profiles and 15 210 data pairs were compared. There was very good agreement between the two types of measurement. Standard deviations of difference (mean difference) for wind direction, wind speed, zonal wind and meridional wind were 24.86° (0.77°), 3.37 (-0.44), 3.33 (-0.32) and 3.58 (-0.25) m s-1, respectively. The annual standard deviations of differences for wind speed were within 2.5-3 m s-1 at all heights apart from 10-15 km, the area of strong winds, where the values were 3-4 m s-1. The relatively larger differences were mainly due to wind field variations in height regions with larger wind speeds, stronger wind shear and the quasi-zero wind layer. A lower MST radar SNR and a lower percentage of data pairs compared will also result in larger inconsistencies. Importantly, this study found that differences between the MST radar and radiosonde observations did not simply increase when balloon drift resulted in an increase in the real-time distance between the two instruments, but also depended on spatiotemporal structures and their respective positions in the contemporary synoptic systems. In this sense, the MST radar was shown to be a unique observation facility for atmospheric dynamics studies, as well as an operational meteorological observation system with a high temporal and vertical resolution.
Strengthened Relationship between the Antarctic Oscillation and ENSO After the Mid-1990s during Austral Spring
Tingting HAN, Huijun WANG, Jianqi SUN
2017, 34(1): 54-65. doi: 10.1007/s00376-016-6143-6
This paper documents a decadal strengthened co-variability of the Antarctic Oscillation (AAO) and ENSO in austral spring after the mid-1990s. During the period 1979-93, the ENSO (AAO) spatial signatures are restricted to the tropics-midlatitudes (Antarctic-midlatitudes) of the Southern Hemisphere (SH), with a weak connection between the two oscillations. Comparatively, after the mid-1990s, the El Niño-related atmospheric anomalies project on a negative AAO pattern with a barotropic structure in the mid-high latitudes of the SH. The expansion of El Niño-related air temperature anomalies have a heightened impact on the meridional thermal structure of the SH, contributing to a weakened circumpolar westerly and strengthened subtropical jet. Meanwhile, the ENSO-related southern three-cell circulations expand poleward and then strongly couple the Antarctic and the tropics. Numerical simulation results suggest that the intensified connection between ENSO and SST in the South Pacific since the mid-1990s is responsible for the strengthened AAO-ENSO relationship.
Impact of Coastal Radar Observability on the Forecast of the Track and Rainfall of Typhoon Morakot (2009) Using WRF-based Ensemble Kalman Filter Data Assimilation
Jian YUE, Zhiyong MENG, Cheng-Ku YU, Lin-Wen CHENG
2017, 34(1): 66-78. doi: 10.1007/s00376-016-6028-8
This study explored the impact of coastal radar observability on the forecast of the track and rainfall of Typhoon Morakot (2009) using a WRF-based ensemble Kalman filter (EnKF) data assimilation (DA) system. The results showed that the performance of radar EnKF DA was quite sensitive to the number of radars being assimilated and the DA timing relative to the landfall of the tropical cyclone (TC). It was found that assimilating radial velocity (Vr) data from all the four operational radars during the 6 h immediately before TC landfall was quite important for the track and rainfall forecasts after the TC made landfall. The TC track forecast error could be decreased by about 43% and the 24-h rainfall forecast skill could be almost tripled. Assimilating Vr data from a single radar outperformed the experiment without DA, though with less improvement compared to the multiple-radar DA experiment. Different forecast performances were obtained by assimilating different radars, which was closely related to the first-time wind analysis increment, the location of moisture transport, the quasi-stationary rainband, and the local convergence line. However, only assimilating Vr data when the TC was farther away from making landfall might worsen TC track and rainfall forecasts. Besides, this work also demonstrated that Vr data from multiple radars, instead of a single radar, should be used for verification to obtain a more reliable assessment of the EnKF performance.
Comparison between the Response of the Northwest Pacific Ocean and the South China Sea to Typhoon Megi (2010)
Zi-Liang LI, Ping WEN
2017, 34(1): 79-87. doi: 10.1007/s00376-016-6027-9
The upper-ocean responses to Typhoon Megi (2010) are investigated using data from ARGO floats and the satellite TMI. The experiments are conducted using a three-dimensional Princeton Ocean Model (POM) to assess the storm, which affected the Northwest Pacific Ocean (NWP) and the South China Sea (SCS). Results show that the upwelling and entrainment experiment together account for 93% of the SST anomalies, where typhoon-induced upwelling may cause strong ocean cooling. In addition, the anomalous SST cooling is stronger in the SCS than in the NWP. The most striking feature of the ocean response is the presence of a two-layer inertial wave in the SCS——a feature that is absent in the NWP. The near-inertial oscillations can be generated as typhoon wakes, which have maximum flow velocity in the surface mixed layer and may last for a few days, after the typhoon's passage. Along the typhoon tracks, the horizontal currents in the upper ocean show a series of alternating negative and positive anomalies emanating from the typhoon.
Diagnosis of Moist Vorticity and Moist Divergence for a Heavy Precipitation Event in Southwestern China
Gang LI, Daoyong YANG, Xiaohua JIANG, Jing PAN, Yanke TAN
2017, 34(1): 88-100. doi: 10.1007/s00376-016-6124-9
A regional heavy precipitation event that occurred over Sichuan Province on 8-9 September 2015 is analyzed based on hourly observed precipitation data obtained from weather stations and NCEP FNL data. Two moist dynamic parameters, i.e., moist vorticity (mζ) and moist divergence (mδ), are used to diagnose this heavy precipitation event. Results show that the topography over southwestern China has a significant impact on the ability of these two parameters to diagnose precipitation. When the impact of topography is weak (i.e., low altitude), mζ cannot exactly depict the location of precipitation in the initial stage of the event. Then, as the precipitation develops, its ability to depict the location improves significantly. In particular, mζ coincides best with the location of precipitation during the peak stage of the event. Besides, the evolution of the mζ center shows high consistency with the evolution of the precipitation center. For mδ, although some false-alarm regions are apparent, it reflects the location of precipitation almost entirely during the precipitation event. However, the mδ center shows inconsistency with the precipitation center. These results suggest that both mζ and mδ have a significant ability to predict the location of precipitation. Moreover, mζ has a stronger ability than mδ in terms of predicting the variability of the precipitation center. However, when the impact of topography is strong (i.e., high altitude), both of these two moist dynamic parameters are unable to depict the location and center of precipitation during the entire precipitation event, suggesting their weak ability to predict precipitation over complex topography.
Impact of Long-range Desert Dust Transport on Hydrometeor Formation over Coastal East Asia
Zhenxi ZHANG, Wen ZHOU, Mark WENIG, Liangui YANG
2017, 34(1): 101-115. doi: 10.1007/s00376-016-6157-0
Model simulations and hydrological reanalysis data for 2007 are applied to investigate the impact of long-range desert dust transport on hydrometeor formation over coastal East Asia. Results are analyzed from Hong Kong and Shanghai, which are two representative coastal cities of East Asia. Long-range desert dust transport impacts mainly spring and summer clouds and precipitation over coastal East Asia. In spring, clouds and precipitation come mainly from large-scale condensation and are impacted mainly by dust from the Gobi, Sahara, and Thar deserts. These desert dusts can participate in the precipitation within and below the clouds. At lower latitudes, the dust particles act mainly as water nuclei. At higher latitudes, they act as both water nuclei and ice nuclei. The effect of Gobi, Sahara, and Thar dust on large-scale clouds and precipitation becomes stronger at higher latitudes. In summer, clouds and precipitation over coastal East Asia come mainly from convection and are impacted mainly by dust from the Taklamakan, Arabian, and Karakum-Kavir deserts. Most Taklamakan dust particles can participate in precipitation within convective clouds as ice nuclei, while Arabian and Karakum-Kavir dust particles participate only as water nuclei in precipitation below the clouds. The effect of Taklamakan dust on convective clouds and precipitation becomes stronger at lower latitudes. Of all the desert dusts, that from the Gobi and Taklamakan deserts has the relatively largest impact. Gobi dust impacts climate change in coastal East Asia by affecting spring water clouds at higher latitudes.
Influence of October Eurasian Snow on Winter Temperature over Northeast China
Huanlian LI, Huijun WANG, Dabang JIANG
2017, 34(1): 116-126. doi: 10.1007/s00376-016-5274-0
This paper addresses the interannual variation of winter air temperature over Northeast China and its connection to preceding Eurasian snow cover. The results show that there is a significant negative correlation between October Eurasian snow cover and following-winter air temperature over Northeast China. The snow cover located in eastern Siberia and to the northeast of Lake Baikal plays an important role in the winter air temperature anomaly. More (less) eastern Siberia snow in October can cause an atmospheric circulation anomaly pattern in which the atmospheric pressure is higher (lower) than normal in the polar region and lower (higher) in the northern mid-high latitudes. Due to the persistence of the eastern Siberia snow from October to the following winter, the winter atmospheric anomaly is favorable (unfavorable) to the widespread movement of cold air masses from the polar region toward the northern mid-high latitudes and, hence, lower (higher) temperature over Northeast China. Simultaneously, when the October snow cover is more (less), the SST in the northwestern Pacific is continuously lower (higher) as a whole; then, the Aleutian low and the East Asia trough are reinforced (weakened), favoring the lower (higher) temperature over Northeast China.