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2011 Vol. 28, No. 6

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CCSC (climate change study in China)
Evaluation of Heavy Rainfall Model Forecasts over the Korean Peninsula Using Different Physical Parameterization Schemes and Horizontal Resolution
Jeong-Gyun PARK, Dong-Kyou LEE
2011, 28(6): 1233-1245. doi: 10.1007/s00376-011-0058-z
In this study, the accuracy of a Pennsylvania State University--National Center for Atmospheric Research mesoscale model (PSU/NCAR MM5) for predicting heavy summer precipitation over the Korean Peninsula was investigated. A total of 1800 simulations were performed using this model for 30 heavy rainfall events employing four cumulus parameterization schemes (CPS), two grid-scale resolvable precipitation schemes (GRS), and two planetary boundary layer (PBL) schemes in three model resolutions (90 km, 30 km, and 10 km). The heavy rainfall events were mesoscale convective systems developed under the influence of mid-latitude baroclinic systems with low-level moisture transport from the ocean. The predictive accuracy for maximum rainfall was approximately 80% for 10-km resolution and was 60% for 30-km resolution. The predictive accuracy for rainfall position extended to ~150 km from the observed position for both resolutions. Simulated rainfall was most sensitive to CPS, then to PBL schemes, and then to GRS. In general, the Grell (GR) scheme and the Anthes and Kuo (AK) scheme showed a better prediction capability for heavy rainfall than did the Betts-Miller (BM) scheme and the Kain-Fritsch (KF) scheme. The GR scheme also performed well in the 24-h and 12-h precipitation predictions: the parameterized convective rainfall in GR is directly related to synoptic-scale forcing. The models without CPS performed better for rainfall amounts but worse for rainfall position than those with CPS. The MM5 model demonstrated substantial predictive capacity using synoptic-scale initial conditions and lateral boundary data because heavy summer rainfall in Korea occurs in a strong synoptic-scale environment.
Weakening of Interannual Variability in the Summer East Asian Upper-tropospheric Westerly Jet since the Mid-1990s
LU Riyu, YE Hong, Jong-Ghap JHUN
2011, 28(6): 1246-1258. doi: 10.1007/s00376-011-0222-5
In this study, we found that the intensity of interannual variability in the summer upper-tropospheric zonal wind has significantly weakened over Northeast Asia and the subtropical western North Pacific (WNP) since the mid-1990s, concurrent with the previously documented decrease of the westerly jet over North China and Northwest China. Corresponding to this weakening of zonal wind variability, the meridional displacement of the East Asian westerly jet (EAJ) manifested as the leading mode of zonal wind variability over the WNP and East Asia (WNP--EA) before the mid-1990s but not afterward. The energetics of the anomalous pattern associated with the meridional displacement of the EAJ suggests that barotropic energy conversion, from basic flow to anomalous patterns, has led to the weakening of the variability in the EAJ meridional displacement and to a change in the leading dominant mode since the mid-1990s. The barotropic energy conversion efficiently maintained the anomalies associated with the variability in the EAJ meridional displacement during 1979--1993 but acted to dampen the anomalies during 1994--2008. A further investigation of the energetics suggests that the difference in the patterns of the circulation anomaly associated with either the first leading mode or the meridional displacement of the EAJ, i.e., a southwest--northeast tilted pattern during 1979--1993 and a zonally oriented pattern during 1994--2008, has contributed greatly to the change in barotropic energy conversion.
Has the East Asian Westerly Jet Experienced a Poleward Displacement in Recent Decades?
ZHANG Yaocun, HUANG Danqing
2011, 28(6): 1259-1265. doi: 10.1007/s00376-011-9185-9
Previous studies have suggested a poleward shift of the zonally averaged jet stream due to rapid warming over continents. However, the regional characteristics of the change in the jet stream are not yet understood. Here, we present evidence suggesting that the East Asian westerly jet did not shift poleward in past decades (1980--2004 relative to 1958--1979), both in winter and summer. Rather, the jet axis has moved southward in summer, but its meridional position is steady in winter. The main change of the jet stream in winter is the enhancement of its intensity. These changes in both summer and winter are consistent with the corresponding changes in the large meridional tropospheric temperature-gradient zone. Based on these results, we suggest that the changes of the jet stream over East Asia are unique and are different from the zonal mean jet stream over the Northern Hemisphere and over the North Atlantic region.
Response of a Grassland Ecosystem to Climate Change in a Theoretical Model
SUN Guodong, MU Mu
2011, 28(6): 1266-1278. doi: 10.1007/s00376-011-0169-6
The response of a grassland ecosystem to climate change is discussed within the context of a theoretical model. An optimization approach, a conditional nonlinear optimal perturbation related to parameter (CNOP-P) approach, was employed in this study. The CNOP-P, a perturbation of moisture index in the theoretical model, represents a nonlinear climate perturbation. Two kinds of linear climate perturbations were also used to study the response of the grassland ecosystem to different types of climate changes. The results show that the extent of grassland ecosystem variation caused by the CNOP-P-type climate change is greater than that caused by the two linear types of climate change. In addition, the grassland ecosystem affected by the CNOP-P-type climate change evolved into a desert ecosystem, and the two linear types of climate changes failed within a specific amplitude range when the moisture index recovered to its reference state. Therefore, the grassland ecosystem response to climate change was nonlinear. This study yielded similar results for a desert ecosystem seeded with both living and wilted biomass litter. The quantitative analysis performed in this study also accounted for the role of soil moisture in the root zone and the shading effect of wilted biomass on the grassland ecosystem through nonlinear interactions between soil and vegetation. The results of this study imply that the CNOP-P approach is a potentially effective tool for assessing the impact of nonlinear climate change on grassland ecosystems.
Effect of Stochastic MJO Forcing on ENSO Predictability
PENG Yuehua, DUAN Wansuo, XIANG Jie
2011, 28(6): 1279-1290. doi: 10.1007/s00376-011-0126-4
Within the frame of the Zebiak-Cane model, the impact of the uncertainties of the Madden--Julian Oscillation (MJO) on ENSO predictability was studied using a parameterized stochastic representation of intraseasonal forcing. The results show that the uncertainties of MJO have little effect on the maximum prediction error for ENSO events caused by conditional nonlinear optimal perturbation (CNOP); compared to CNOP-type initial error, the model error caused by the uncertainties of MJO led to a smaller prediction uncertainty of ENSO, and its influence over the ENSO predictability was not significant. This result suggests that the initial error might be the main error source that produces uncertainty in ENSO prediction, which could provide a theoretical foundation for the data assimilation of the ENSO forecast.
A Statistical-Dynamical Scheme for the Extraseasonal Prediction of Summer Rainfall for 160 Observation Stations across China
LANG Xianmei, ZHENG Fei
2011, 28(6): 1291-1300. doi: 10.1007/s00376-011-0177-6
The purpose of this study was to design and test a statistical--dynamical scheme for the extraseasonal (one season in advance) prediction of summer rainfall at 160 observation stations across China. The scheme combined both valuable information from the preceding observations and dynamical information from synchronous numerical predictions of atmospheric circulation factors produced by an atmospheric general circulation model. First, the key preceding climatic signals and synchronous atmospheric circulation factors that were not only closely related to summer rainfall but also numerically predictable were identified as the potential predictors. Second, the extraseasonal prediction models of summer rainfall were constructed using a multivariate linear regression analysis for 15 subregions and then 160 stations across China. Cross-validation analyses performed for the period 1983--2008 revealed that the performance of the prediction models was not only high in terms of interannual variation, trend, and sign but also was stable during the whole period. Furthermore, the performance of the scheme was confirmed by the accuracy of the real-time prediction of summer rainfall during 2009 and 2010.
Impacts of Land Process on the Onset and Evolution of Asian Summer Monsoon in the NCEP Climate Forecast System
Song YANG, WEN Min, Rongqian YANG, Wayne HIGGINS, ZHANG Renhe
2011, 28(6): 1301-1317. doi: 10.1007/s00376-011-0167-8
Impacts of land models and initial land conditions (ICs) on the Asian summer monsoon, especially its onset, were investigated using the NCEP Climate Forecast System (CFS). Two land models, the Oregon State University (OSU) land model and the NCEP, OSU, Air Force, and Hydrologic Research Laboratory (Noah) land model, were used to get parallel experiments. The experiments also used land ICs from the NCEP/Department of Energy (DOE) Global Reanalysis 2 (GR2) and the Global Land Data Assimilation System (GLDAS). Previous studies have demonstrated that, a systematic weak bias appears in the modeled monsoon, and this bias may be related to a cold bias over the Asian land mass. Results of the current study show that replacement of the OSU land model by the Noah land model improved the model's cold bias and produced improved monsoon precipitation and circulation patterns. The CFS predicted monsoon with greater proficiency in El Nino years, compared to La Nina years, and the Noah model performed better than the OSU model in monsoon predictions for individual years. These improvements occurred not only in relation to monsoon onset in late spring but also to monsoon intensity in summer. Our analysis of the monsoon features over the India peninsula, the Indo-China peninsula, and the South Chinese Sea indicates different degrees of improvement. Furthermore, a change in the land models led to more remarkable improvement in monsoon prediction than did a change from the GR2 land ICs to the GLDAS land ICs.
Formation of the Summertime Ozone Valley over the Tibetan Plateau: The Asian Summer Monsoon and Air Column Variations
BIAN Jianchun, YAN Renchang, CHEN Hongbin, Lu Daren, Steven T. MASSIE
2011, 28(6): 1318-1325. doi: 10.1007/s00376-011-0174-9
The summertime ozone valley over the Tibetan Plateau is formed by two influences, the Asian summer monsoon (ASM) and air column variations. Total ozone over the Tibetan Plateau in summer was $\sim $33 Dobson units (DU) lower than zonal mean values over the ocean at the same latitudes during the study period 2005--2009. Satellite observations of ozone profiles show that ozone concentrations over the ASM region have lower values in the upper troposphere and lower stratosphere (UTLS) than over the non-ASM region. This is caused by frequent convective transport of low-ozone air from the lower troposphere to the UTLS region combined with trapping by the South Asian High. This offset contributes to a $\sim$20-DU deficit in the ozone column over the ASM region. In addition, along the same latitude, total ozone changes identically with variations of the terrain height, showing a high correlation with terrain heights over the ASM region, which includes both the Tibetan and Iranian plateaus. This is confirmed by the fact that the Tibetan and Iranian plateaus have very similar vertical distributions of ozone in the UTLS, but they have different terrain heights and different total-column ozone levels. These two factors (lower UTLS ozone and higher terrain height) imply 40 DU in the lower-ozone column, but the Tibetan Plateau ozone column is only ~33 DU lower than that over the non-ASM region. This fact suggests that the lower troposphere has higher ozone concentrations over the ASM region than elsewhere at the same latitude, contributing ~7 DU of total ozone, which is consistent with ozonesonde and satellite observations.
Seasonal Variation of Columnar Aerosol Optical Properties in Yangtze River Delta in China
YU Xingna, ZHU Bin, YIN Yan, FAN Shuxian, CHEN Aijun
2011, 28(6): 1326-1335. doi: 10.1007/s00376-011-0158-9
In order to understand the seasonal variation of aerosol optical properties in the Yangtze River Delta, 5 years of measurements were conducted during September 2005 to December 2009 at Taihu, China. The monthly averages of aerosol optical depth were commonly >0.6; the maximum seasonal average (0.93) occurred in summer. The magnitude of the Angstrom exponent was found to be high throughout the year; the highest values occurred in autumn (1.33) and were the lowest in spring (1.08). The fine modes of volume size distribution showed the maxima (peaks) at a radius of ~0.15 μm in spring, autumn, and winter; at a radius of ~0.22 μm in summer. The coarse modes showed the maxima (peaks) at a radius of 2.9 μm in spring, summer, and autumn and at a radius of 3.8 μm in winter. The averages of single-scattering albedo were 0.92 (spring), 0.92 (summer), 0.91 (autumn), and 0.88 (winter). The averages of asymmetry factor were found to be larger in summer than during other seasons; they were taken as 0.66 at 440--1020 nm over Taihu. The real part of the refractive index showed a weak seasonal variation, with averages of 1.48 (spring), 1.43 (summer), 1.45 (autumn), and 1.48 (winter). The imaginary parts of the refractive index were higher in winter (0.013) than in spring (0.0076), summer (0.0092), and autumn (0.0091), indicating that the atmosphere in the winter had higher absorbtivity.
Radiative Forcing and Climate Response Due to Black Carbon in Snow and Ice
WANG Zhili, ZHANG Hua, SHEN Xueshun
2011, 28(6): 1336-1344. doi: 10.1007/s00376-011-0117-5
The radiative forcing and climate response due to black carbon (BC) in snow and/or ice were investigated by integrating observed effects of BC on snow/ice albedo into an atmospheric general circulation model (BCC_AGCM2.0.1) developed by the National Climate Center (NCC) of the China Meteorological Administration (CMA). The results show that the global annual mean surface radiative forcing due to BC in snow/ice is +0.042 W m-2, with maximum forcing found over the Tibetan Plateau and regional mean forcing exceeding +2.8 W m-2. The global annual mean surface temperature increased 0.071oC due to BC in snow/ice. Positive surface radiative forcing was clearly shown in winter and spring and increased the surface temperature of snow/ice in the Northern Hemisphere. The surface temperatures of snow-covered areas of Eurasia and North America in winter (spring) increased by 0.83oC (0.6oC) and 0.83oC (0.46oC), respectively. Snowmelt rates also increased greatly, leading to earlier snowmelt and peak runoff times. With the rise of surface temperatures in the Arctic, more water vapor could be released into the atmosphere, allowing easier cloud formation, which could lead to higher thermal emittance in the Arctic. However, the total cloud forcing could decrease due to increasing cloud cover, which will offset some of the positive feedback mechanism of the clouds.
Relationship Between East Asian Winter Monsoon and Summer Monsoon
YAN Hongming, YANG Hui, YUAN Yuan, LI Chongyin
2011, 28(6): 1345-1356. doi: 10.1007/s00376-011-0014-y
Using National Centers for Environmental Prediction/National Centre for Atmospheric Research (NCEP/NCAR) reanalysis data and monthly Hadley Center sea surface temperature (SST) data, and selecting a representative East Asian winter monsoon (EAWM) index, this study investigated the relationship between EAWM and East Asian summer monsoon (EASM) using statistical analyses and numerical simulations. Some possible mechanisms regarding this relationship were also explored. Results indicate a close relationship between EAWM and EASM: a strong EAWM led to a strong EASM in the following summer, and a weak EAWM led to a weak EASM in the following summer. Anomalous EAWM has persistent impacts on the variation of SST in the tropical Indian Ocean and the South China Sea, and on the equatorial atmospheric thermal anomalies at both lower and upper levels. Through these impacts, the EAWM influences the land--sea thermal contrast in summer and the low-level atmospheric divergence and convergence over the Indo-Pacific region. It further affects the meridional monsoon circulation and other features of the EASM. Numerical simulations support the results of diagnostic analysis. The study provides useful information for predicting the EASM by analyzing the variations of preceding EAWM and tropical SST.
An Improved, Downscaled, Fine Model for Simulation of Daily Weather States
JIANG Zhihong, DING Yuguo, ZHENG Chunyu, CHEN Weilin
2011, 28(6): 1357-1366. doi: 10.1007/s00376-011-0086-8
In this study, changes in daily weather states were treated as a complex Markov chain process, based on a continuous-time watershed model (soil water assessment tool, SWAT) developed by the Agricultural Research Service at the U.S. Department of Agriculture (USDA-ARS). A finer classification using total cloud amount for dry states was adopted, and dry days were classified into three states: clear, cloudy, and overcast (rain free). Multistate transition models for dry- and wet-day series were constructed to comprehensively downscale the simulation of regional daily climatic states. The results show that the finer, improved, downscaled model overcame the oversimplified treatment of a two-weather state model and is free of the shortcomings of a multistate model that neglects finer classification of dry days (i.e., finer classification was applied only to wet days). As a result, overall simulation of weather states based on the SWAT greatly improved, and the improvement in simulating daily temperature and radiation was especially significant.
Impact of the South China Sea Throughflow on the Pacific Low-Latitude Western Boundary Current: A Numerical Study for Seasonal and Interannual Time Scales
WANG Weiwen, WANG Dongxiao, ZHOU Wen, LIU Qinyan, YU Yongqiang, LI Chao
2011, 28(6): 1367-1376. doi: 10.1007/s00376-011-0142-4
Prior studies have revealed that, as a part of the Pacific tropical gyre, the South China Sea throughflow (SCSTF) is strongly influenced by the Pacific low-latitude western boundary current (LLWBC). In this study, ocean general circulation model (OGCM) experiments with and without connection to the South China Sea (SCS) were performed to investigate the impact of the SCSTF on the Pacific LLWBC. These model experiments show that if the SCS is blocked, seasonal variability of the Kuroshio and Mindanao Current becomes stronger, and the meridional migration of the North Equatorial Current (NEC) bifurcation latitude is enhanced. Both in seasonal and interannual time scales, stronger Luzon Strait transport (LST) induces a stronger Kuroshio transport combined with a southward shift of the NEC bifurcation, which is unfavorable for a further increase of the LST; a weaker LST induces a weaker Kuroshio transport and a northward shifting NEC bifurcation, which is also unfavorable for the continuous decrease of the LST.
Characteristics of the Boundary Layer Structure of Sea Fog on the Coast of Southern China
LIU Hongnian, JIANG Weimei, HUANG Jian, MAO Weikang
2011, 28(6): 1377-1389. doi: 10.1007/s00376-011-0191-8
Using boundary layer data with regard to sea fog observed at the Science Experiment Base for Marine Meteorology at Bohe, Guangdong Province, the structure of the atmospheric boundary layer and the characteristics of the tops of the fog and the clouds were analyzed. In addition, the effects of advection, radiation, and turbulence during sea fog were also investigated. According to the stability definition of saturated, wet air, the gradient of the potential pseudo-equivalent temperature equal to zero was defined as the thermal turbulence interface. There is evidence to suggest that two layers of turbulence exist in sea fog. Thermal turbulence produced by long-wave radiation is prevalent above the thermal turbulence interface, whereas mechanical turbulence aroused by wind shear is predominant below the interface. The height of the thermal turbulence interface was observed between 180 m and 380 m. Three important factors are closely related to the development of the top of the sea fog: (1) the horizontal advection of the water vapor, (2) the long-wave radiation of the fog top, and (3) the movement of the vertical turbulence. Formation, development, and dissipation are the three possible phases of the evolution of the boundary-layer structure during the sea fog season. In addition, the thermal turbulence interface is the most significant turbulence interface during the formation and development periods; it is maintained after sea fog rises into the stratus layer.
A Comparison of Precipitation Distribution of Two Landfalling Tropical Cyclones during the Extratropical Transition
CHEN Guanghua
2011, 28(6): 1390-1404. doi: 10.1007/s00376-011-0148-y
The precipitation distributions associated with two landfalling tropical cyclones (TCs) during extratropical transition (ET) were examined in this study. Their distinction is that the bulk of precipitation fell to the left of the TC track in one TC and to the right in the other. The analyses indicate that, for the TC Haima (2004) case, accompanied by the approach of a deep midlatitude trough throughout the depth of the troposphere, the warm and moist air advection by the southeasterly flow north of TC was favorable for warm advection and frontogenesis to the northwest of the TC. Due to the steepening of equivalent potential temperature (θe, the air-parcel uplift along the θe, surface, in collaboration with thermally direct circulation related to frontogenesis, led to enhanced precipitation northwest of the TC. In contrast, for TC Matsa (2005) embedded within a moister environment, a weak midlatitude trough was situated at the mid-upper level. The convection was triggered by the conditional instability at the lower level and then sustained by dynamic forcing at the mid-upper level so that the heavy precipitation occurred to the northeast of TC. For the two TC cases, the precipitation enhancement was also linked to the upper-level anomalous divergence associated with the jet-related forcing on the right side of the jet entrance. From the quasigeostrophic perspective, the advection of geostrophic absolute vorticity by the thermal wind most likely served as an indication reflecting the displacement of the vertical motion relative to the center of the TC.
Water Cycle and Microphysical Processes Associated with a Mesoscale Convective Vortex System in the Dabie Mountain Area
WANG Xiaokang, NI Yunqi, XU Wenhui, GU Chunli, QIU Xuexing
2011, 28(6): 1405-1422. doi: 10.1007/s00376-011-0089-5
The water vapor budget and the cloud microphysical processes associated with a heavy rainfall system in the Dabie Mountain area in June 2008 were analyzed using mesoscale reanalysis data (grid resolution 0.03o× 0.03o, 22 vertical layers, 1-h intervals), generated by amalgamating the local analysis and prediction system (LAPS). The contribution of each term in the water vapor budget formula to precipitation was evaluated. The characteristics of water vapor budget and water substances in various phase states were evaluated and their differences in heavy and weak rainfall areas were compared. The precipitation calculated from the total water vapor budget accounted for 77% of actual precipitation; surface evaporation is another important source of water vapor. Water vapor within the domain of interest mainly came from the lower level along the southern boundary and the lower--middle level along the western boundary. This altitude difference for water vapor flux was caused by different weather systems. The decrease of local water vapor in the middle--lower layer in the troposphere during the system development stage also contributed to precipitation. The strength and the layer thickness of water vapor convergence and the content of various water substances in the heavy rainfall areas were obviously larger than in the weak rainfall areas. The peak values of lower-level water vapor convergence, local water vapor income, and the concentration of cloud ice all preceded the heaviest surface rainfall by a few hours.
Taiwan Yushan Snowfall Activity and Its Association with Atmospheric Circulation from 1979 to 2009
TSAI Li-Min, LIU Koung-Ying, WANG Zifa
2011, 28(6): 1423-1432. doi: 10.1007/s00376-011-0178-5
Yushan is the most famous location for snow in Taiwan, while snowfall in the subtropical zone is rare. When it is snowing in Yushan, people are experiencing unusually cold and wet weather elsewhere in Taiwan. In this study, Yushan snowfall activity from 1979 to 2009 and the related atmosphere circulation were examined with the Taiwan Central Weather Bureau's Yushan weather station observations and the National Centers for Environmental Prediction/Department of Energy (NCEP/DOE) reanalysis atmospheric data. To provide a quantitative measure of snowfall events, a snowfall activity index (SAI) was defined in this study. The time series of yearly SAIs shows that Yushan snowfall activity for an active year, such as 1983 (SAI =39 153) was ~118 times larger than for an inactive year, such as 1999 (SAI=331). Our analyses show that snowfall activity is closely related to the position of the East Asian Trough (EAT) and the strength of the West Pacific High (WPH). In active years, when the EAT shifted eastward and the strength of WPH increased, an anomalous anticyclone occurred in the West Pacific. This anticyclone introduced anomalous southwesterly flows along the southeastern cost of mainland China and over Taiwan, resulting in a wetter-than-normal atmosphere that favored snowfall. Alternatively, in inactive years, a drier-than-normal atmosphere resulted in sluggish snowfall seasons.
Computing Streamfunction and Velocity Potential in a Limited Domain of Arbitrary Shape. Part I: Theory and Integral Formulae
Qin XU, CAO Jie, GAO Shouting
2011, 28(6): 1433-1444. doi: 10.1007/s00376-011-0185-6
The non-uniqueness of solution and compatibility between the coupled boundary conditions in computing velocity potential and streamfunction from horizontal velocity in a limited domain of arbitrary shape are revisited theoretically with rigorous mathematic treatments. Classic integral formulas and their variants are used to formulate solutions for the coupled problems. In the absence of data holes, the total solution is the sum of two integral solutions. One is the internally induced solution produced purely and uniquely by the domain internal divergence and vorticity, and its two components (velocity potential and streamfunction) can be constructed by applying Green's function for Poisson equation in unbounded domain to the divergence and vorticity inside the domain. The other is the externally induced solution produced purely but non-uniquely by the domain external divergence and vorticity, and the non-uniqueness is caused by the harmonic nature of the solution and the unknown divergence and vorticity distributions outside the domain. By setting either the velocity potential (or streamfunction) component to zero, the other component of the externally induced solution can be expressed by the imaginary (or real) part of the Cauchy integral constructed using the coupled boundary conditions and solvability conditions that exclude the internally induced solution. The streamfunction (or velocity potential) for the externally induced solution can also be expressed by the boundary integral of a double-layer (or single-layer) density function. In the presence of data holes, the total solution includes a data-hole--induced solution in addition to the above internally and externally induced solutions.
Computing Streamfunction and Velocity Potential in a Limited Domain of Arbitrary Shape. Part II: Numerical Methods and Test Experiments
CAO Jie, Qin XU
2011, 28(6): 1445-1458. doi: 10.1007/s00376-011-0186-5
Built on the integral formulas in Part I, numerical methods are developed for computing velocity potential and streamfunction in a limited domain. When there is no inner boundary (around a data hole) inside the domain, the total solution is the sum of the internally and externally induced parts. For the internally induced part, three numerical schemes (grid-staggering, local-nesting and piecewise continuous integration) are designed to deal with the singularity of the Green's function encountered in numerical calculations. For the externally induced part, by setting the velocity potential (or streamfunction) component to zero, the other component of the solution can be computed in two ways: (1) Solve for the density function from its boundary integral equation and then construct the solution from the boundary integral of the density function. (2) Use the Cauchy integral to construct the solution directly. The boundary integral can be discretized on a uniform grid along the boundary. By using local-nesting (or piecewise continuous integration), the scheme is refined to enhance the discretization accuracy of the boundary integral around each corner point (or along the entire boundary). When the domain is not free of data holes, the total solution contains a data-hole--induced part, and the Cauchy integral method is extended to construct the externally induced solution with irregular external and internal boundaries. An automated algorithm is designed to facilitate the integrations along the irregular external and internal boundaries. Numerical experiments are performed to evaluate the accuracy and efficiency of each scheme relative to others.
A Modeling Study of Surface Rainfall Processes Associated with a Torrential Rainfall Event over Hubei, China, during July 2007
ZHOU Yushu, CUI Chunguang
2011, 28(6): 1459-1470. doi: 10.1007/s00376-010-0119-8
The surface rainfall processes associated with the torrential rainfall event over Hubei, China, during July 2007 were investigated using a two-dimensional cloud-resolving model. The model integrated the large-scale vertical velocity and zonal wind data from National Centers for Environmental Prediction (NCEP)/Global Data Assimilation System (GDAS) for 5 days. The time and model domain mean surface rain rate was used to identify the onset, mature, and decay periods of rainfall. During the onset period, the descending motion data imposed in the lower troposphere led to a large contribution of stratiform rainfall to the model domain mean surface rainfall. The local atmospheric drying and transport of rain from convective regions mainly contributes to the stratiform rainfall. During the mature periods, the ascending motion data integrated into the model was so strong that water vapor convergence was the dominant process for both convective and stratiform rainfall. Both convective and stratiform rainfalls made important contributions to the model domain mean surface rainfall. During the decay period, descending motion data input into the model prevailed, making stratiform rainfall dominant. Stratiform rainfall was mainly caused by the water vapor convergence over raining stratiform regions.
The Application of Sea-Surface Wind Detection with Doppler Lidar in Olympic Sailing
WANG Gaili, LIU Liping, LIU Zhishen, LU Bo, MU Rong
2011, 28(6): 1471-1480. doi: 10.1007/s00376-011-9189-5
The mobile incoherent Doppler lidar (MIDL), which was jointly developed by State Key Laboratory of Severe Weather (LaSW) of the Chinese Academy of Meteorological Sciences (CAMS) and Ocean University of China, provided meteorological services during the Olympic sailing events in Qingdao in 2008. In this study, two experiments were performed based on these measurements. First, the capabilities of MIDL detection of sea-surface winds were investigated by comparing its radial velocities with those from a sea buoy. MIDL radial velocity was almost consistent with sea-buoy data; both reflected the changes in wind with time. However, the MIDL data was 0.5 m s-1 lower on average than the sea-buoy data due to differences in detection principle, sample volume, sample interval, spatial and temporal resolution. Second, the wind fields during the Olympic sailing events were calculated using a four-dimensional variation data assimilation (4DVAR) algorithm and were evaluated by comparing them with data from a sea buoy. The results show that the calculations made with the 4DVAR wind retrieval method are able to simulate the fine retrieval of sea-surface wind data---the retrieved wind fields were consistent with those of sea-buoy data. Overall, the correlation coefficient of wind direction was 0.93, and the correlation coefficient of wind speed was 0.70. The distribution of retrieval wind fields was consistent with that of MIDL radial velocity; the root-mean-square error between them had an average of only 1.52 m s-1.