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2020 Vol. 37, No. 2

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News & Views
Advanced Radiative Transfer Modeling System (ARMS): A New-Generation Satellite Observation Operator Developed for Numerical Weather Prediction and Remote Sensing Applications
Fuzhong WENG, Xinwen YU, Yihong DUAN, Jun YANG, Jianjie WANG
2020, 37(2): 131-136. doi: 10.1007/s00376-019-9170-2
News & Views
Record-Setting Ocean Warmth Continued in 2019
Lijing CHENG, John ABRAHAM, Jiang ZHU, Kevin E. TRENBERTH, John FASULLO, Tim BOYER, Ricardo LOCARNINI, Bin ZHANG, Fujiang YU, Liying WAN, Xingrong CHEN, Xiangzhou SONG, Yulong LIU, Michael E. MANN
2020, 37(2): 137-142. doi: 10.1007/s00376-020-9283-7
Original Paper
A 38-Year Climatology of Explosive Cyclones over the Northern Hemisphere
Gang FU, Yawen SUN, Jilin SUN, Pengyuan LI
2020, 37(2): 143-159. doi: 10.1007/s00376-019-9106-x
Explosive cyclones (ECs) over two basins in the Northern Hemisphere (20°−90°N) from January 1979 to December 2016 are investigated using ERA-Interim and Optimum Interpolation Sea Surface Temperature (OISST) data. The classical definition of an EC is modified considering not only the rapid drop of the central sea level pressure of the cyclone, but also the strong wind speed at the height of 10 m in which maximum wind speeds greater than 17.2 m s−1 are included. According to the locations of the northern Atlantic and northern Pacific, the whole Northern Hemisphere is divided into the “A region” (20°−90°N, 90°W−90°E) and “P region” (20°−90°N, 90°E−90°W). Over both the A and P regions, the climatological features of ECs, such as their spatial distribution, intensity, seasonal variation, interannual variation, and moving tracks, are documented.
The Linkage between Two Types of El Niño Events and Summer Streamflow over the Yellow and Yangtze River Basins
Dan WANG, Aihui WANG, Lianlian XU, Xianghui KONG
2020, 37(2): 160-172. doi: 10.1007/s00376-019-9049-2
It is generally agreed that El Niño can be classified into East Pacific (EP) and Central Pacific (CP) types. Nevertheless, little is known about the relationship between these two types of El Niño and land surface climate elements. This study investigates the linkage between EP/CP El Niño and summer streamflow over the Yellow and Yangtze River basins and their possible mechanisms. Over the Yellow River basin, the anomalous streamflow always manifests as positive (negative) in EP (CP) years, with a correlation coefficient of 0.39 (−0.37); while over the Yangtze River basin, the anomalous streamflow shows as positive in both EP and CP years, with correlation coefficients of 0.72 and 0.48, respectively. Analyses of the surface hydrological cycle indicate that the streamflow is more influenced by local evapotranspiration (ET) than precipitation over the Yellow River basin, while it is dominantly affected by precipitation over the Yangtze River basin. The different features over these two river basins can be explained by the anomalous atmospheric circulation, which is cyclonic (anticyclonic) north (south) of 30°N over East Asia. EP years are dominated by two anticyclones, which bring strong water vapor convergence and induce more precipitation but less ET, and subsequently increase streamflow and flooding risks. In CP years, especially over the Yellow River basin, two cyclones dominate and lead to water vapor divergence and reduce moisture arriving. Meanwhile, the ET enhances mainly due to local high surface air temperature, which further evaporates water from the soil. As a result, the streamflow decreases, which will then increase the drought risk.
Vertical Structure of Interannual Variability in Cross-Equatorial Flows over the Maritime Continent and Indian Ocean in Boreal Summer
Xiaoxuan ZHAO, Riyu LU
2020, 37(2): 173-186. doi: 10.1007/s00376-019-9103-0
This study investigates the vertical structure of variability in the cross-equatorial flows (CEFs) over the Maritime Continent and Indian Ocean in boreal summer, based on three reanalysis datasets: ERA-Interim, JRA-55 and NCEP-2. The results show a high consistency in the interannual variability among the reanalysis datasets, especially between ERA-Interim and JRA-55, while great uncertainty exists in the decadal or long-term changes of CEFs. Further analyses on the interannual variability in CEFs indicate that there is a significant negative relationship between the upper- and lower-level CEFs over the Maritime Continent—that is, the northerlies at the upper level and southerlies at the lower level are both enhanced or weakened. This seesaw pattern is also significantly related to the CEFs over the Indian Ocean at the upper level and lower level (i.e., the Somali jet). This close relationship between the upper- and lower-level CEFs and between the Maritime Continent and Indian Ocean is manifested as the leading mode of equatorial meridional winds in the vertical−zonal section over the Maritime Continent and Indian Ocean. Finally, it is found that ENSO is closely related to the vertical leading mode, and to all the CEFs at the upper and lower levels over the Maritime Continent and Indian Ocean.
Analysis of an Ensemble of High-Resolution WRF Simulations for the Rapid Intensification of Super Typhoon Rammasun (2014)
Xun LI, Noel E. DAVIDSON, Yihong DUAN, Kevin J. TORY, Zhian SUN, Qinbo CAI
2020, 37(2): 187-210. doi: 10.1007/s00376-019-8274-z
Diagnostics are presented from an ensemble of high-resolution forecasts that differed markedly in their predictions of the rapid intensification (RI) of Typhoon Rammasun. We show that the basic difference stems from subtle differences in initializations of (a) 500−850-hPa environmental winds, and (b) midlevel moisture and ventilation. We then describe how these differences impact on the evolving convective organization, storm structure, and the timing of RI. As expected, ascent, diabatic heating and the secondary circulation near the inner-core are much stronger in the member that best forecasts the RI. The evolution of vortex cloudiness from this member is similar to the actual imagery, with the development of an inner cloud band wrapping inwards to form the eyewall. We present evidence that this structure, and hence the enhanced diabatic heating, is related to the tilt and associated dynamics of the developing inner-core in shear. For the most accurate ensemble member: (a) inhibition of ascent and a reduction in convection over the up-shear sector allow moistening of the boundary-layer air, which is transported to the down-shear sector to feed a developing convective asymmetry; (b) with minimal ventilation, undiluted clouds and moisture from the down-shear left quadrant are then wrapped inwards to the up-shear left quadrant to form the eyewall cloud; and (c) this process seems related to a critical down-shear tilt of the vortex from midlevels, and the vertical phase-locking of the circulation over up-shear quadrants. For the member that forecasts a much-delayed RI, these processes are inhibited by stronger vertical wind shear, initially resulting in poor vertical coherence of the circulation, lesser moisture and larger ventilation. Our analysis suggests that ensemble prediction is needed to account for the sensitivity of forecasts to a relatively narrow range of environmental wind shear, moisture and vortex inner-structure.
Roles of Synoptic to Quasi-Monthly Disturbances in Generating Two Pre-Summer Heavy Rainfall Episodes over South China
Zhina JIANG, Da-Lin ZHANG, Hongbo LIU
2020, 37(2): 211-228. doi: 10.1007/s00376-019-8156-4
In this study, power spectral analysis and bandpass filtering of daily meteorological fields are performed to explore the roles of synoptic to quasi-monthly disturbances in influencing the generation of pre-summer heavy rainfall over South China. Two heavy rainfall episodes are selected during the months of April−June 2008−15, which represent the collaboration between the synoptic and quasi-biweekly disturbances and the synoptic and quasi-monthly disturbances, respectively. Results show that the first heavy rainfall episode takes place in a southwesterly anomalous flow associated with an anticyclonic anomaly over the South China Sea (SCS) at the quasi-biweekly scale with 15.1% variance contributions, and at the synoptic scale in a convergence zone between southwesterly and northeasterly anomalous flows associated with a southeastward-moving anticyclonic anomaly on the leeside of the Yungui Plateau and an eastward-propagating anticyclonic anomaly from higher latitudes with 35.2% variance contribution. In contrast, the second heavy rainfall episode takes place in southwest-to-westerly anomalies converging with northwest-to-westerly anomalies at the quasi-monthly scale with 23.2% variance contributions to the total rainfall variance, which are associated with an anticyclonic anomaly over the SCS and an eastward-propagating cyclonic anomaly over North China, respectively. At the synoptic scale, it occurs in south-to-southwesterly anomalies converging with a cyclonic anomaly on the downstream of the Yungui Plateau with 49.3% variance contributions. In both cases, the lower-tropospheric mean south-to-southwesterly flows provide ample moisture supply and potentially unstable conditions; it is the above synoptic, quasi-biweekly or quasi-monthly disturbances that determine the general period and distribution of persistent heavy rainfall over South China.
Effect of Aerosol Particles on Orographic Clouds: Sensitivity to Autoconversion Schemes
Hui XIAO, Yan YIN, Pengguo ZHAO, Qilin WAN, Xiantong LIU
2020, 37(2): 229-238. doi: 10.1007/s00376-019-9037-6
Aerosol particles can serve as cloud condensation nuclei (CCN) to influence orographic clouds. Autoconversion, which describes the initial formation of raindrops from the collision of cloud droplets, is an important process for aerosol−cloud−precipitation systems. In this study, seven autoconversion schemes are used to investigate the impact of CCN on orographic warm-phase clouds. As the initial cloud droplet concentration is increased from 100 cm−3 to 1000 cm−3 (to represent an increase in CCN), the cloud water increases and then the rainwater is suppressed due to a decrease in the autoconversion rate, leading to a spatial shift in surface precipitation. Intercomparison of the results from the autoconversion schemes show that the sensitivity of cloud water, rainwater, and surface precipitation to a change in the concentration of CCN is different from scheme to scheme. In particular, the decrease in orographic precipitation due to increasing CCN is found to range from −87% to −10% depending on the autoconversion scheme. Moreover, the surface precipitation distribution also changes significantly by scheme or CCN concentration, and the increase in the spillover (ratio of precipitation on the leeward side to total precipitation) induced by increased CCN ranges from 10% to 55% under different autoconversion schemes. The simulations suggest that autoconversion parameterization schemes should not be ignored in the interaction of aerosol and orographic cloud.