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2019 Vol. 36, No. 5

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Causes of the Extreme Hot Midsummer in Central and South China during 2017: Role of the Western Tropical Pacific Warming
Ruidan CHEN, Zhiping WEN, Riyu LU, Chunzai WANG
2019, 36(5): 465-478. doi: 10.1007/s00376-018-8177-4
This study investigates why an extreme hot midsummer occurred in Central and South China (CSC) during 2017. It is shown that the western North Pacific subtropical high (WNPSH) was abnormally intensified and westward-extending, resulting in anomalous high pressure and consequent extreme heat over CSC. The abnormal WNPSH was favored by the warming of the western tropical Pacific (WTP), which was unrelated to ENSO and manifested its own individual effect. The WTP warming enhanced the convection in-situ and led to anomalous high pressure over CSC via a local meridional circulation. The influence of the WTP was confirmed by CAM4 model experiments. A comparison between the 2017 midsummer and 2010 midsummer (with a stronger WNPSH but weaker extreme heat) indicated that the influence of the WNPSH on extreme heat can be modulated by the associated precipitation in the northwestern flank. The role of the WTP was verified by regression analyses on the interannual variation of the WTP sea surface temperature anomaly (SSTA). On the other hand, the WTP has undergone prominent warming during the past few decades, resulting from decadal to long-term changes and favoring extreme warm conditions. Through a mechanism similar to the interannual variation, the decadal to long-term changes have reinforced the influence of WTP warming on the temperature over CSC, contributing to the more frequent hot midsummers recently. It is estimated that more than 50% of the temperature anomaly over CSC in the 2017 midsummer was due to the WTP warming, and 40% was related to the decadal to long-term changes of the WTP SSTA.
Sub-seasonal to Seasonal Hindcasts of Stratospheric Sudden Warming by BCC_CSM1.1(m): A Comparison with ECMWF
Jian RAO, Rongcai REN, Haishan CHEN, Xiangwen LIU, Yueyue YU, Yang YANG
2019, 36(5): 479-494. doi: 10.1007/s00376-018-8165-8
This study focuses on model predictive skill with respect to stratospheric sudden warming (SSW) events by comparing the hindcast results of BCC_CSM1.1(m) with those of the ECMWF's model under the sub-seasonal to seasonal prediction project of the World Weather Research Program and World Climate Research Program. When the hindcasts are initiated less than two weeks before SSW onset, BCC_CSM and ECMWF show comparable predictive skill in terms of the temporal evolution of the stratospheric circumpolar westerlies and polar temperature up to 30 days after SSW onset. However, with earlier hindcast initialization, the predictive skill of BCC_CSM gradually decreases, and the reproduced maximum circulation anomalies in the hindcasts initiated four weeks before SSW onset replicate only 10% of the circulation anomaly intensities in observations. The earliest successful prediction of the breakdown of the stratospheric polar vortex accompanying SSW onset for BCC_CSM (ECMWF) is the hindcast initiated two (three) weeks earlier. The predictive skills of both models during SSW winters are always higher than that during non-SSW winters, in relation to the successfully captured tropospheric precursors and the associated upward propagation of planetary waves by the model initializations. To narrow the gap in SSW predictive skill between BCC_CSM and ECMWF, ensemble forecasts and error corrections are performed with BCC_CSM. The SSW predictive skill in the ensemble hindcasts and the error corrections are improved compared with the previous control forecasts.
Differences between Convective and Stratiform Precipitation Budgets in a Torrential Rainfall Event
Yongjie HUANG, Yaping WANG, Xiaopeng CUI
2019, 36(5): 495-509. doi: 10.1007/s00376-019-8159-1
Differences in rainfall budgets between convective and stratiform regions of a torrential rainfall event were investigated using high-resolution simulation data produced by the Weather Research and Forecasting (WRF) model. The convective and stratiform regions were reasonably separated by the radar-based convective-stratiform partitioning method, and the three-dimensional WRF-based precipitation equation combining water vapor and hydrometeor budgets was further used to analyze the rainfall budgets. The results showed that the magnitude of precipitation budget processes in the convective region was one order larger than that in the stratiform region. In convective/stratiform updraft regions, precipitation was mainly from the contribution of moisture-related processes, with a small negative contribution from cloud-related processes. In convective/stratiform downdraft regions, cloud-related processes played positive roles in precipitation, while moisture-related processes made a negative contribution. Moisture flux convergence played a dominant role in the moisture-related processes in convective or stratiform updraft regions, which was closely related to large-scale dynamics. Differences in cloud-related processes between convective and stratiform regions were more complex compared with those in moisture-related processes. Both liquid- and ice-phase microphysical processes were strong in convective/stratiform updraft regions, and ice-phase processes were dominant in convective/stratiform downdraft regions. There was strong net latent heating within almost the whole troposphere in updraft regions, especially in the convective updraft region, while the net latent heating (cooling) mainly existed above (below) the zero-layer in convective/stratiform downdraft regions.
Microphysical Properties of Rainwater in Typhoon Usagi (2013): A Numerical Modeling Study
Lin DENG, Wenhua GAO, Yihong DUAN, Yuqing WANG
2019, 36(5): 510-526. doi: 10.1007/s00376-019-8170-6
A 2-km resolution simulation using the Weather Research and Forecasting model with Morrison microphysics was employed to investigate the rainwater microphysical properties during different stages of Typhoon Usagi (2013) in the inner-core and outer region. The model reproduced the track, intensity, and overall structure of Usagi (2013) reasonably. The simulated raindrop size distribution showed a rapid increase in small-size raindrop concentration but an oscillated decrease in large-size ones in the inner-core region, corresponding well with the upward motion. It was found that there existed two levels (1.25 and 5.25 km) of maximum number concentration of raindrops. The ice-related microphysics at high levels was stronger than the warm-rain processes at low levels. The larger raindrops formed by self-collection in the inner-core suffered from significant breakup, but the raindrops outside the eyewall did not experience evident breakup. Model results indicated that the dominant terms in the water vapor budget were the horizontal moisture flux convergence (HFC) and local condensation and deposition. The evaporation from the ocean surface (PBL) was ~10% of the HFC in the inner core, but up to 40% in the outer region as the air therein was far from saturation. Furthermore, water vapor in the outer region was obtained equally through evaporation from the cloud and inward transportation from the environment. An earlier start of cloud microphysical processes in the inner-core region was evident during the intensification stage, and the continuous decreasing of condensation in both the inner-core and outer regions might imply the beginning of the storm weakening.
Coupling of a Regional Climate Model with a Crop Development Model and Evaluation of the Coupled Model across China
Jing ZOU, Zhenghui XIE, Chesheng ZHAN, Feng CHEN, Peihua QIN, Tong HU, Jinbo XIE
2019, 36(5): 527-540. doi: 10.1007/s00376-018-8160-0
In this study, the CERES (Crop Estimation through Resource and Environment Synthesis) crop model was coupled with CLM3.5, the land module of the regional climate model RegCM4. The new coupled model was named RegCM4_CERES; and in this model, crop type was further divided into winter wheat, spring wheat, spring maize, summer maize, early rice, late rice, single rice, and other crop types based on each distribution fraction. The development of each crop sub-type was simulated by the corresponding crop model separately, with each planting and harvesting date. A simulation test using RegCM4_CERES was conducted across China from 1999 to 2008; a control test was also performed using the original RegCM4. Data on crop LAI (leaf area index), soil moisture at 10 cm depth, precipitation, and 2 m air temperature were collected to evaluate the performance of RegCM4_CERES. The evaluation provided comparison of single-station time series, regional distributions, seasonal variations, and statistical indices for RegCM4_CERES. The results revealed that the coupled model had an excellent ability to simulate the phonological changes and spatial variations in crops. The consideration of dynamic crop development in RegCM4_CERES corrected the wet bias of the original RegCM4 over North China and the cold bias over South China. However, the degree of improvement was minimal and the statistical indices for RegCM4_CERES were roughly the same as the original RegCM4.
Convective Bursts Episode of the Rapidly Intensified Typhoon Mujigae (2015)
Shuai YANG, Xiba TANG, Shuixin ZHONG, Bin CHEN, Yushu ZHOU, Shouting GAO, Chengxin WANG
2019, 36(5): 541-556. doi: 10.1007/s00376-019-8142-x
Convective burst (CB) characteristics at distinct stages of a rapidly intensified Typhoon Mujigae (2015), are investigated based on a 72-h simulation. The spatial features show that almost all CB elements develop in the eyewall. The number of CBs in the inner-core region within a 100 km radius——which account for a large proportion of the total CBs, with a sharp increase about 6 h before the onset of rapid intensification (RI)——provides some indication of the RI of the typhoon. The CBs during pre-RI and RI are examined from dynamic and thermodynamic viewpoints. The combination of lower-level convergent inflow and upper-level divergent outflow pushes a relay-race-like transmission of convective activity, favorable for the development of deep convection. A double warm-core structure is induced by the centripetal outflow sinking and warming associated with CBs, which directly accelerates RI by a sudden decrease in hydrostatic pressure. By utilizing the convection activity degree (CAD) index derived from the local total energy anomaly, a correlation formula between CBs and CAD is deduced. Furthermore, an intense CAD (ICAD) signal threshold (with a value equal to 100) to predict CBs is obtained. It is verified that this ICAD threshold is effective for estimating the occurrence of a CB episode and predicting RI of a typhoon. Therefore, this threshold may be a valuable tool for identifying CB episodes and forecasting rapidly intensified typhoons.
Latitudinal and Scan-dependent Biases of Microwave Humidity Sounder Measurements and Their Dependences on Cloud Ice Water Path
Anqi CAI, Xiaolei ZOU
2019, 36(5): 557-569. doi: 10.1007/s00376-019-8190-2
The relationship between differences in microwave humidity sounder (MHS)-channel biases which represent measured brightness temperatures and model-simulated brightness temperatures, and cloud ice water path (IWP) as well as the influence of the cloud liquid water path (LWP) on the relationship is examined. Seven years (2011-17) of NOAA-18 MHS-derived measured brightness temperatures and IWP/LWP data generated by the NOAA Comprehensive Large Array-data Stewardship System Microwave Surface and Precipitation Products System are used. The Community Radiative Transfer Model, version 2.2.4, is used to simulate model-simulated brightness temperatures using European Center for Medium-Range Weather Forecasts reanalysis data as background fields. Scan-angle deviations of the MHS window channel biases range from -1.7 K to 1.0 K. The relationships between channels 2, 4, and 5 biases and scan angle are symmetrical about the nadir. The latitude-dependent deviations of MHS window channel biases are positive and range from 0-7 K. For MHS non-window channels, the latitudinal deviations between measured brightness temperatures and model-simulated brightness temperatures are larger when the detection height is higher. No systematic warm or cold deviations are found in the global spatial distribution of difference between measured brightness temperatures and model-simulated brightness temperatures over oceans after removing scan-angle and latitudinal deviations. The corrected biases of five different MHS channels decrease differently with respect to the increase in IWP. This decrease is stronger when LWP values are higher.