Combined with the multiple solution scheme (MSS) and the rain considered Geophysical model function (GMF+Rain), the two-dimensional variational (2DVAR) ambiguity removal technique is applied to the cyclone wind retrieval under rain condition with QuikSCAT scatterometer data. With the GMF+Rain model, the retrieved wind speed is effectively improved, but large wind direction error still exists when the background is in large error. In this paper, a changeable multi-parameter error model is introduced in the 2DVAR to reduce the wind direction error, and the sensitivity experiments of 2DVAR to its error model parameters are studied with cyclone Yagi QuikSCAT data, to choose the best parameters setting for cyclone wind retrieval with theoretical explanation. Numerical results show that 2DVAR is more effective in wind direction ambiguity removal with the proposed multi-parameter error model when the gross error probability in the multi-parameter error model is set to zero in comparison of the standard setting. The influence of the background is decreased with increasing backround error variance, decreasing the background error correlation length, or decreasing the gross error probabilities in multi-parameter error model.
Dust particles in the atmosphere play an important role in air pollution, climate change, and biogeochemical cycles. Some of the dominant sources of dust in mid-latitude regions are in Asia. An intense dust storm engulfed Northern China at the beginning of May 2017, and PM10 mass concentrations of 1500–2000 μg m−3 were measured near the dust source region. We combined numerical simulations, air quality monitoring data, and satellite retrievals to investigate dust emission and transport during this event. We found that the event was closely related to cold front activity, characterized by increased wind speed, which increased dust emission. We improved the dust scheme using a local dust size distribution to better simulate the dust emission flux. We found that accurate parametrization of the dust size distribution was important to effectively simulate both dust emission and ambient particle concentration. We showed that using a local dust size distribution substantially improved the accuracy of the simulation, allowing both the spatial distribution of pollution caused by the dust storm and temporal variability in the pollution to be captured.
Four distinctive but poorly documented landforms in the Badain Jaran megadunes were studied: arcuate steps, multi-stage fans, depressions formed by runoff erosion, and groundwater overflow zones around lakes. The development of these four landform types indicates the following: (1) The hydrological balance in the sand layers of the megadune areas is positive; (2) After evaporation and transpiration, precipitation is able to infiltrate the deep sand layers; (3) Precipitation is a source for the groundwater and for many of the lakes of the area. The groundwater recharge mechanism is characterized by intense precipitation events that provide a water source, high infiltration rate, shallow evaporation depth, and low water retention. These factors together enable the precipitation to be transformed into groundwater. The energy of gravity water and the high water film pressure of adsorbed water together provide the forces necessary for effective water recharge.
Abstract On the basis of global positioning system dropsonde data, Japan Meteorology Agency Regional Spectral Model analysis data, National Centers for Environmental Prediction reanalysis data, satellite products from the Naval Research Laboratory, and best-track tropical-cyclone (TC) datasets from the Shanghai Typhoon Institute, the statistical characteristics of the ducts induced by TCs (TC ducts) over the western North Pacific Ocean were analyzed for the period from September 2003 to September 2006, and two typical strong-duct cases with remarkable differences in formation cause were analyzed and compared. Of the total of 357 dropsondes, there are 212 cases that show ducting conditions, with an occurrence percentage of ~59%. Of the 212 TC-duct events, profiles with multiple ducting layers make up nearly one-half, with the main type of ducts being elevated ducts; in contrast, weak ducts make up over one-half, resulting in a weak median duct strength and thickness. Ducts formed in the transition zone, especially on the left side of TC tracks, tend to be much stronger and thicker than those formed inside TCs. The former are induced by the interaction between TCs and their surrounding systems, such as the inrush of dry and cold air from the north on the left side of TC tracks. The latter are associated with successive subsidence in the gaps between spiral cloud bands. With increasing TC intensities, the associated ducts inside TCs tend to be much stronger and thicker and to appear at higher altitudes.
This study systematically evaluates the accuracy, trends, and error sources for western North Pacific tropical cyclone intensity forecasts between 2005 and 2018. The study uses homogeneous samples from tropical cyclone (TC) intensity official forecasts issued by the China Meteorological Administration (CMA), Joint Typhoon Warning Center (JTWC), and Regional Specialized Meteorological Center Tokyo-Typhoon Center (RSMC-Tokyo). The TC intensity forecast accuracy performances are as follows: 24–48 h, JTWC > RSMC-Tokyo > CMA; 72 h, JTWC > CMA > RSMC-Tokyo; and 96–120 h, JTWC > CMA. Improvements in TC intensity forecasting are marginal but steady for all three centers. The 24–72 h improvement rate is approximately 1–2 % yr−1. The improvement rates are statistically significant at the 95 % level for almost half of the verification times from 0–120 h. The three centers tend to overestimate weak TCs over the northern South China Sea, but strong TCs are sometimes underestimated over the area east of the Philippines. The three centers generally have higher skill scores associated with forecasting of rapid weakening (RW) events than rapid intensification (RI) events. Overall, the three centers are not skillful in forecasting RI events more than three days in advance. Fortunately, RW events could be forecasted five days in advance with an accuracy order of CMA > RSMC-Tokyo > JTWC.
Abstract Extremely persistent heavy rainfall (EPHR) is characterized by high rainfall intensity and long durations, which can cause destructive natural disasters. In this study, hourly merged precipitation data for 2009–2019 are used to investigate the spatiotemporal distribution of EPHR over China. Reanalysis data are used to classify synoptic patterns related to EPHR. Observations show that the EPHR process is substantially asymmetrical, occurring mostly in South China, whereas its strongest intensity takes place in North China. There is a tendency for EPHR to occur near coastal and mountainous areas, basins, and urban agglomerations. Monthly and diurnal patterns of EPHR exhibit complex regional disparities in frequency and cumulative precipitation. EPHR exhibits a bimodal structure during summer in South and Southwest China, whereas a unimodal structure is observed in Huang‐Huai valleys, and early morning peaks are substantial elsewhere. Furthermore, the synoptic patterns associated with EPHR also differ by region. Using an objective synoptic classification method, typical synoptic patterns are classified in each region. Combining the effect of land‐sea contrast, the low vortices system, low‐level jets (LLJs), and shear lines play a key role in supporting EPHR in South China. The location of EPHR south of the Yangtze River is substantially controlled by the intensity of southwest vortex, LLJs, and the shear line, while the southwest vortex dominates in Southwest China. The effects of the low vortex, shear lines, and terrain are prominent in Jianghuai region. Furthermore, EPHR in the northern region is closely related to western Pacific subtropic high and local vortices.
Abstract Official forecasts of tropical cyclone (TC) tracks issued by the China Meteorological Administration (CMA); the Regional Specialized Meteorological Centre in Tokyo, Japan; and the Joint Typhoon Warning Center (JTWC) were used to evaluate the accuracies, biases, and trends of TC track forecasts during 2005–14 over the western North Pacific. Overall, the JTWC demonstrated the best forecasting performance. However, the CMA showed the most significant rate of improvement. Two main zones were discovered in the regional distribution of forecast errors: a low-latitude zone that comprises the South China Sea and the sea region east of the Philippines, and a midlatitude zone comprising the southern Sea of Japan and the sea region east of Japan. When TCs moved into the former zone, there were both translational speed and direction biases in the forecast tracks, whereas slow biases were predominated in the latter zone. Twelve synoptic flow patterns of TCs with the largest error have been identified based on the steering flow theory. Among them, the most two common pattern are the pattern with the combination of cyclonic circulations, subtropical ridges, and midlatitude troughs (CRT, 26 TCs), and the pattern of the TCs’ track that cannot be explained by steering flow (NSF, 6 TCs). In the CRT pattern, TCs move northwestward forced by the cyclonic circulations and the subtropical ridges and then turn poleward and eastward under the influence of the midlatitude troughs. In the NSF pattern, storms embedded in the southwest flow by the cyclonic circulation and the steering flow suggest TCs should turn to the right and move northeastward but instead TCs persisted in moving northwestward.
Abstract Extremely persistent heavy rainfall (EPHR) generated by landfalling tropical cyclones (LTCs) can cause destructive natural disasters. In this study, hourly precipitation data and best‐track data set for 2009–2019 are used to investigate the basic characteristics and preliminary causes of LTC‐generated EPHR over China. Using the relative threshold method, heavy precipitation lasting more than 7 hr is defined as EPHR. The result shows that most of the EPHR events occur in the south and southeast China, especially in the northwest of Hainan Island and the coastal areas of Zhejiang. The maximum duration of EPHR can reach 13 hr with a maximum cumulative rainfall of 668.9 mm. EPHR in the intermediate region (150–300 km away from the tropical cyclone (TC) center) of the LTCs tends to have a higher frequency and greater intensity, and the development process of the EPHR events exhibits a significant asymmetrical feature. Although EPHR accounts for a relatively small proportion (29.4%) of duration in EPHR events, it accounts for a large proportion (61.0%) of cumulative precipitation. The combination effects of topographic forcing and tracks of the TCs directly determines the high incidence areas of EPHR. And the preliminary causes of EPHR are mainly due to the upliftment or blocking effects of the topography. In addition, the translation speed and intensity of the TC can also restrict the generation of EPHR. In general, a stronger and slower TC is more likely to cause EPHR events when it passes through north of Hainan Island or makes landfall at Zhejiang‐Fujian area.
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