2022 Vol. 28, No. 4

Improved Tropical Cyclone Forecasts with Increased Vertical Resolution in the TRAMS Model
XU Dao-sheng, LIANG Jia-hao, LU Ze-bin, ZHANG Yan-xia, HUANG Fei, FENG Ye-rong, ZHANG Bang-lin
2022, 28(4): 377-387. doi: 10.46267/j.1006-8775.2022.028
Abstract(198) HTML (97) PDF [2703KB](128)
The numerical simulation of typhoons has been found to be very sensitive to the vertical resolution of the model. During the updating of the TRAMS model from version 1.0 to 3.0, the horizontal resolution has been increased from 36 km to 9 km, while the vertical layer number only increased from 55 to 65 layers. The lack of high vertical resolution limits the performance of the TRAMS model in typhoon forecasting to a certain extent. In order to study the potential improvement of typhoon forecasting by increasing the vertical resolution, this paper increases the vertical resolution of the TRAMS model from 65 to 125 layers for the first time for a comparative simulation test. The results of the case study with Typhoon Hato (2017) show that the model with high vertical resolution can significantly enhance the warm structure caused by water vapor flux convergence and vertical transport, thus accurately simulating the rapid strengthening process of the typhoon. Meanwhile, the model with 125-layer vertical resolution can simulate the asymmetric structural characteristics of the wind field, which are closer to the observations and can help to reduce the bias in typhoon track forecasting. The improvement of vertical resolution is also trialed by using the batch test results of several landfalling typhoons in 2016-2017. The experimental results show that the typhoon forecast of the model becomes consistent with the observations only when the number of vertical layers of the model increases to about 125 layers, which in turn causes a large computational burden. In the next step, we will try to solve the computational burden problem caused by ultra-high vertical resolution with the top boundary nesting technique, and realize the application of high vertical resolution in the actual operation of the TRAMS model.
Characteristics of Radar Echo Parameters and Microphysical Structure Simulation of a Short-Time Heavy Precipitation Supercell
ZHAO Gui-xiang, WANG Yi-jie, LIAN Zhi-luan
2022, 28(4): 388-404. doi: 10.46267/j.1006-8775.2022.029
Abstract(111) HTML (53) PDF [4790KB](28)
By using the conventional observations, radar data, NCEP/NCAR FNL 1º×1º reanalysis data and numerical simulation data and with the construction and calculation of radar echo parameters, this paper presents the structural characteristics and physical processes of a short-time heavy precipitation supercell that occurred in the squall line process in Shanxi Province on 24 June 2020. The results show that this squall line event occurred in front of a surface cold front, combined with infiltration of low-level cold air and continuous increase of near-surface humidity in the afternoon. The surface mesoscale convergence line and mesoscale dew point front contributed to the development and systemization of the squall line by a large degree. The short-time extremely heavy precipitation in Pingshun County was caused by the development of a supercell from thunderstorm cells on the front side of the squall line. The characteristics of sharp increase in vertical integral liquid water content, persistent increase in reflectivity factor and continuous rise in the echo top height appeared about 23 min earlier than the severe precipitation, which has qualitative indicating significance for the nowcasting of short-time heavy precipitation. A quantitative analysis of the radar echo parameters suggests that the "sudden drop"of FV40 was a precursor signal of cells'coalescence and rapid development to the mature stage. The areal change of the echo core at the 6 km height was highly subject to the merging and developing of cells, the rapid change of hydrometeor particles in clouds and the precipitation intensity. Changes in the cross-sectional area of convective cells at different heights can indirectly reflect the changes of liquid particles and ice particles in clouds, which is indicatively meaningful for predicting the coalescing and developing-to-maturing of cells and heavy precipitation 30-45 min earlier. A comprehensive echo parameter prediction model constructed by the random forest principle can predict the magnitude of short-time heavy precipitation 40-50 min in advance. Numerical simulation reveals that large amounts of water vapor existed in the near-surface atmosphere, and that the cells rapidly obtained moisture from the ambient atmosphere and developed rapidly through maternal feeding. The cold cloud zone was narrow, upright and had a high stretch height. The upward motion in clouds was strong and deep, and very rich in liquid water content. The graupel particles had a large vertical distribution range, the coexistence area of graupel and snow was large, the height of raindrops was close to the surface with a wide horizontal scale, and the precipitation efficiency was high. These may be the important elements responsible for the occurrence of the short-time heavy precipitation that exceeded historical extreme values. On the basis of the above analyses, a comprehensive parameter (CP) prediction model is worked out, which can estimate the developing trend of supercells and the intensity of short-time heavy precipitation about 1 h in advance.
ConvLSTM Based Temperature Forecast Modification Model for North China
GENG Huan-tong, HU Zhong-yan, WANG Tian-lei
2022, 28(4): 405-412. doi: 10.46267/j.1006-8775.2022.030
Abstract(128) HTML (57) PDF [1137KB](37)
The correction of model forecast is an important step in evaluating weather forecast results. In recent years, post-processing models based on deep learning have become prominent. In this paper, a deep learning model named ED-ConvLSTM based on encoder-decoder structure and ConvLSTM is developed, which appears to be able to effectively correct numerical weather forecasts. Compared with traditional post-processing methods and convolutional neural networks, ED-ConvLSTM has strong collaborative extraction ability to effectively extract the temporal and spatial features of numerical weather forecasts and fit the complex nonlinear relationship between forecast field and observation field. In this paper, the post-processing method of ED-ConvLSTM for 2 m temperature prediction is tested using The International Grand Global Ensemble dataset and ERA5-Land data from the European Centre for Medium-Range Weather Forecasts (ECMWF). Root mean square error and temperature prediction accuracy are used as evaluation indexes to compare ED-ConvLSTM with the method of model output statistics, convolutional neural network postprocessing methods, and the original prediction by the ECMWF. The results show that the correction effect of ED-ConvLSTM is better than that of the other two postprocessing methods in terms of the two indexes, especially in the long forecast time.
Climatology of Shear Line and Related Rainstorm over the Southern Yangtze River Valley Based on an Improved Intelligent Identification Method
LIU Jin-qing, CHEN He, XU Jing-yu
2022, 28(4): 413-424. doi: 10.46267/j.1006-8775.2022.031
Based on four reanalysis datasets including CMA-RA, ERA5, ERA-Interim, and FNL, this paper proposes an improved intelligent method for shear line identification by introducing a second-order zonal-wind shear. Climatic characteristics of shear lines and related rainstorms over the Southern Yangtze River Valley (SYRV) during the summers (June-August) from 2008 to 2018 are then analyzed by using two types of unsupervised machine learning algorithm, namely the t-distributed stochastic neighbor embedding method (t-SNE) and the k-means clustering method. The results are as follows: (1) The reproducibility of the 850 hPa wind fields over the SYRV using China's reanalysis product CMA-RA is superior to that of European and American products including ERA5, ERA-Interim, and FNL. (2) Theory and observations indicate that the introduction of a second-order zonal-wind shear criterion can effectively eliminate the continuous cyclonic curvature of the wind field and identify shear lines with significant discontinuities. (3) The occurrence frequency of shear lines appearing in the daytime and nighttime is almost equal, but the intensity and the accompanying rainstorm have a clear diurnal variation: they are significantly stronger during daytime than those at nighttime. (4) Half (47%) of the shear lines can cause short-duration rainstorms (≥20 mm (3h)-1), and shear line rainstorms account for one-sixth (16%) of the total summer short-duration rainstorms. Rainstorms caused by shear lines are significantly stronger than that caused by other synoptic forcing. (5) Under the influence of stronger water vapor transport and barotropic instability, shear lines and related rainstorms in the north and middle of the SYRV are stronger than those in the south.
Formation and Replacement Mechanism of the Concentric Eyewall of Super Typhoon Muifa (1109)
HUANG Lang, WEN Ying-fang, LIU Yu-di
2022, 28(4): 425-444. doi: 10.46267/j.1006-8775.2022.032
Based on high-fidelity numerical simulation by using the Weather Research and Forecast (WRF) model, we analyzed the formation and replacement mechanism of the concentric eyewall of Super Typhoon Muifa (1109) from the aspects of the potential vorticity (PV), dynamic/thermodynamic structure change, sea surface flux, and water vapor content. Observational data and sensitivity tests were also adopted to verify the results. We found that: (1) The abnormal increase of the PV in the rain zone is mainly due to the condensation latent heat. Sufficient water vapor conditions are beneficial to the formation of the outer eyewall structure, and when the environmental water vapor content is larger, the intensity of the outer eyewall becomes greater. (2) After the formation of the typhoon's outer eyewall, in the area where the outer eyewall is located, the increase of inertial stability contributes to the decrease of the intensity of the inner eyewall. When the intensity of the outer eyewall is larger, the divergence and subsidence motion in the upper layer of the outer eyewall has a greater weakening effect on the intensity of the inner eyewall. (3) The increase of potential temperature of the outer eyewall is mainly due to the condensation latent heat release and the warming of dry air subsidence motion in the moat area. (4) The increase of sea surface heat flux can prolong the concentric eyewall replacement process.
Pre-summer Persistent Heavy Rain over Southern China and Its Relationship with Intra-seasonal Oscillation of Tropical Atmosphere
QIU Di, YAO Su-xiang, XIA Yi-cong
2022, 28(4): 445-456. doi: 10.46267/j.1006-8775.2022.033
Abstract(106) HTML (49) PDF [2727KB](17)
Based on daily precipitation data supplied by the Chinese meteorological administration, hourly reanalysis datasets provided by the ECMWF and daily outgoing long wave radiation supplied by the NOAA, the evolution regularity of continuous heavy precipitation over Southern China (SC) from April to June in 1979-2020 was systematically analyzed. The interaction between specific humidity and circulation field at the background-scale, the intra-seasonal-scale and the synoptic-scale, and its influence on persistent heavy precipitation over the SC during the April-June rainy season were quantitatively diagnosed and analyzed. The results are as follows. Persistent heavy rainfall events (PHREs) over the SC during the April-June rainy season occur frequently from mid-May to mid- and late-June, exhibiting significant intra-seasonal oscillation (10-30-day) features. Vertically integrated moisture flux convergence (VIMFC) can well represent the variation of the PHREs. A multiscale quantitative diagnosis of the VIMFC shows that the pre-summer PHREs over the SC are mainly affected by the background water vapor (greater than 30 days), intra-seasonal circulation disturbance (10-30-day) and background circulation (greater than 30 days), and water vapor convergences are the main factor. The SC is under the control of a warm and humid background and a strong intra-seasonal cyclonic circulation, with strong convergence and ascending movements and abundant water vapor conditions during the period of the PHREs. Meanwhile, the westward inter-seasonal oscillation of tropical atmosphere keeps the precipitation system over the SC for several consecutive days, eventually leading to the occurrence, development and persistence of heavy precipitation.
Impacts of the Boreal Summer Intraseasonal Oscillation on the Warm-Season Rainfall over Hainan Island
MA Wen-lan, ZHU Lei, FENG Xiao, ZHANG Ke-ying
2022, 28(4): 457-472. doi: 10.46267/j.1006-8775.2022.034
Abstract(109) HTML (52) PDF [5893KB](10)
This study investigates the roles of the boreal summer intraseasonal oscillation (BSISO) in the diurnal rainfall cycle over Hainan Island during the warm season (April-September) using 20-year satellite-based precipitation, ERA5 and the outgoing longwave radiation data with the phase composite analysis method. Results show that the spatial distributions of the hourly rainfall anomaly significantly change under the BSISO phases 1-8 while no clear variations are found on the daily and anomaly daily area-averaged rainfall over the island. During the BSISO phase 1, the rainfall anomaly distinctly increases in the morning over the southwest and late afternoon over the northeast of the island, while suppressed convection occurs in the early afternoon over the southwest area. Under this circumstance, strong low-level westerly winds bring abundant moisture into the island, which helps initiate the nocturnal-morning convection over the south coastal area, and drives the convergence region of sea breeze fronts to concentrate into the northwest. Opposite to Phase 1, an almost completely reversed diurnal cycle of rainfall anomaly is found in Phase 5, whereas a positive anomalous rainfall peak is observed in the early afternoon over the center while negative peaks are found in the morning and late afternoon over the southwest and northeast, owing to a strong low-level northeasterly anomaly flow, which causes relatively low moisture and enlarges a sea-breeze convergence area over the island. During Phase 8, strongest moisture is found over the island all through the day, which tends to produce highest rainfall in the afternoon with enhanced anomalous northerly. These results further indicate that multiscale interactions between the large-scale circulations and local land-sea breeze circulations play important roles in modulating diurnal precipitation cycles over the tropical island.
Temporal and Spatial Distribution Characteristics of Lightning Activity over the Pacific Ocean and Adjacent Land and Its Relationship with AOD and CAPE
CHU Yu, GUO Feng-xia, ZHANG Zhi-wei, ZOU Di-ke, WU Ze-yi, LIU Zhou, LU Xian
2022, 28(4): 473-488. doi: 10.46267/j.1006-8775.2022.035
Based on LIS/OTD gridded lightning climatology data, ERA5 reanalysis data, and MODIS atmosphere monthly global products, we examined latitudinal and daily variations of lightning activity over land, offshore areas, open sea, and all marine areas (i.e., the aggregate of open sea and offshore areas) for different seasons over the Pacific Ocean and the adjacent land areas at 65° N-50° S, 99° E-78° W, and analysed the relationships of lightning activity with CAPE (Convective Available Potential Energy) and AOD (Aerosol Optical Depth). At any given latitude, the lightning density is the highest over land, followed by offshore areas, all marine areas and the open sea in sequence. The lightning density over land is approximately an order of magnitude greater than that over all marine areas. Lightning activity over land, offshore areas, open sea, and all marine areas varies with season. The diurnal variation of lightning density over land has a single-peak pattern. Over the offshore area, open sea, and all marine areas, lightning densities have two maxima per day. The magnitude of the daily variation in mean lightning density is the largest over land and the smallest over the open sea. The lightning density over the Pacific Ocean and adjacent land areas is significantly and positively correlated with CAPE. The correlation is the strongest over land and the weakest over the open sea. Cloud Base Height (CBH) may affect the efficiency of CAPE conversion to updraft. CAPE has a positive effect on lightning activity and has a greater impact on land than on the ocean. Over the sea, both CAPE and AOD can contribute to lightning activity, but the magnitudes of the influence of CAPE and AOD on lightning activity remain to be determined. Lightning activity over land and sea is a result of the combined action of AOD and CAPE.