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2007 Vol. 24, No. 5

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Impacts of Systematic Precipitation Bias on Simulations of Water and Energy Balances in Northwest America
Youlong XIA, XU Guoqiang
2007, 24(5): 739-749. doi: 10.1007/s00376-007-0739-9
At high latitudes and in mountainous areas, evaluation and validation of water and energy flux simulations are greatly affected by systematic precipitation errors. These errors mainly come from topographic effects and undercatch of precipitation gauges. In this study, the Land Dynamics (LaD) land surface model is used to investigate impacts of systematic precipitation bias from topography and wind-blowing on water and energy flux simulation in Northwest America. The results show that topographic and wind adjustment reduced bias of streamflow simulations when compared with observed streamflow at 14 basins. These systematic biases resulted in a -50%--100% bias for runoff simulations, a -20%--20% bias for evapotranspiration, and a -40%--40% bias for sensible heat flux, subject to different locations and adjustments, when compared with the control run. Uncertain gauge adjustment leads to a 25% uncertainty for precipitation, a 20%--100% uncertainty for runoff simulation, a less-than-10% uncertainty for evapotranspiration, and a less-than-20% uncertainty for sensible heat flux.
Implementation of a Surface Runoff Model with Horton and Dunne Mechanisms into the Regional Climate Model RegCM_NCC
SHI Xueli, XIE Zhenghui, LIU Yiming, YANG Hongwei
2007, 24(5): 750-764. doi: 10.1007/s00376-007-0750-1
A surface runoff parameterization scheme that dynamically represents both Horton and Dunne runoff generation mechanisms within a model grid cell together with a consideration of the subgrid-scale soil heterogeneity, is implemented into the National Climate Center regional climate model (RegCM_NCC). The effects of the modified surface runoff scheme on RegCM_NCC performance are tested with an abnormal heavy rainfall process which occurred in summer 1998. Simulated results show that the model with the original surface runoff scheme (noted as CTL) basically captures the spatial pattern of precipitation, circulation and land surface variables, but generally overestimates rainfall compared to observations. The model with the new surface runoff scheme (noted as NRM) reasonably reproduces the distribution pattern of various variables and effectively diminishes the excessive precipitation in the CTL. The processes involved in the improvement of NRM-simulated rainfall may be as follows: with the new surface runoff scheme, simulated surface runoff is larger, soil moisture and evaporation (latent heat flux) are decreased, the available water into the atmosphere is decreased; correspondingly, the atmosphere is drier and rainfall is decreased through various processes. Therefore, the implementation of the new runoff scheme into the RegCM_NCC has a significant effect on results at not only the land surface, but also the overlying atmosphere.
Sensitivity of the Upper Ocean Temperature and Circulation in the Equatorial Pacific to Solar Radiation Penetration Due to Phytoplankton
LIN Pengfei, LIU Hailong, ZHANG Xuehong
2007, 24(5): 765-780. doi: 10.1007/s00376-007-0765-7
Solar radiation penetration in the upper ocean is strongly modulated by phytoplankton, which impacts the upper ocean temperature structure, especially in the regions abundant with phytoplankton. In the paper, a new solar radiation penetration scheme, based on the concentration of chlorophyll-a, was introduced into the LASG/IAP (State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics/Institute of Atmospheric Physics) Climate system Ocean Model (LICOM). By comparing the simulations using this new scheme with those using the old scheme that included the constant e-folding attenuation depths in LICOM, it was found that the sea surface temperature (SST) and circulation in the central and eastern equatorial Pacific were both sensitive to the amount of phytoplankton present. Distinct from other regions, the increase of chlorophyll-a concentration would lead to SST decrease in the central and eastern equatorial Pacific. The higher chlorophyll-a concentration at the equator in comparison to the off-equator regions can enlarge the subsurface temperature gradient, which in turn strengthens the upper current near the equator and induces an enhancing upwelling. The enhancing upwelling can then lead to a decrease in the SST in the central and eastern equatorial Pacific. The results of these two sensitive experiments testify to the fact that the meridional gradient in the chlorophyll-a concentration can result in an enhancement in the upper current and a decrease in the SST, along with the observation that a high chlorophyll-a concentration at the equator is one of the predominant reasons leading to a decrease in the SST. This study points out that these results can be qualitatively different simply because of the choice of the solar radiation penetration schemes for comparison. This can help explain previously reported contradictory conclusions.
Seasonal Phase-Locking of Peak Events in the Eastern Indian Ocean
2007, 24(5): 781-798. doi: 10.1007/s00376-007-0781-7
The sea surface temperature (SST) anomaly of the eastern Indian Ocean (EIO) exhibits cold anomalies in the boreal summer or fall during El Nino development years and warm anomalies in winter or spring following the El Nino events. There also tend to be warm anomalies in the boreal summer or fall during La Nina development years and cold anomalies in winter or spring following the La Nina events. The seasonal phase-locking of SST change in the EIO associated with El Nino/Southern Oscillation is linked to the variability of convection over the maritime continent, which induces an atmospheric Rossby wave over the EIO. Local air-sea interaction exerts different effects on SST anomalies, depending on the relationship between the Rossby wave and the mean flow related to the seasonal migration of the buffer zone, which shifts across the equator between summer and winter. The summer cold events start with cooling in the Timor Sea, together with increasing easterly flow along the equator. Negative SST anomalies develop near Sumatra, through the interaction between the atmospheric Rossby wave and the underneath sea surface. These SST anomalies are also contributed to by the increased upwelling of the mixed layer and the equatorward temperature advection in the boreal fall. As the buffer zone shifts across the equator towards boreal winter, the anomalous easterly flow tends to weaken the mean flow near the equator, and the EIO SST increases due to the reduction of latent heat flux from the sea surface. As a result, wintertime SST anomalies appear with a uniform and nearly basin-wide pattern beneath the easterly anomalies. These SST anomalies are also caused by the increase in solar radiation associated with the anticyclonic atmospheric Rossby wave over the EIO. Similarly, the physical processes of the summer warm events, which are followed by wintertime cold SST anomalies, can be explained by the changes in atmospheric and oceanic fields with opposite signs to those anomalies described above.
Monthly Changes in the Influence of the Arctic Oscillation on Surface Air Temperature over China
HUANG Jiayou, TAN Benkui, SUO Lingling, HU Yongyun
2007, 24(5): 799-807. doi: 10.1007/s00376-007-0799-x
Partial Least Squares Regression (PLSR) is used to study monthly changes in the influence of the Arctic Oscillation (AO) on spring, summer and autumn air temperature over China with the January 500 hPa geopotential height data from 1951 to 2004 and monthly temperature data from January to November at 160 stations in China. Several AO indices have been defined with the 500-hPa geopotential data and the index defined as the first principal component of the normalized geopotential data is best to be used to study the influence of the AO on SAT (surface air temperature) in China. There are three modes through which the AO in winter influences SAT in China. The influence of the AO on SAT in China changes monthly and is stronger in spring and summer than in autumn. The main influenced regions are Northeast China and the Changjiang River drainage area.
Interdecadal Variability of the East Asian Summer Monsoon in an AGCM
HAN Jinping, WANG Huijun
2007, 24(5): 808-818. doi: 10.1007/s00376-007-0808-0
It is well known that significant interdecadal variation of the East Asian summer monsoon (EASM) occurred around the end of the 1970s. Whether these variations can be attributed to the evolution of global sea surface temperature (SST) and sea ice concentration distribution is investigated with an atmospheric general circulation model (AGCM). The model is forced with observed monthly global SST and sea ice evolution through 1958--1999. A total of four integrations starting from different initial conditions are carried out. It is found that only one of these reproduces the observed interdecadal changes of the EASM after the 1970s, including weakened low-level meridional wind, decreased surface air temperature and increased sea level pressure in central China, as well as the southwestward shift of the western Pacific subtropical high ridge and the strengthened 200-hPa westerlies. This discrepancy among these simulated results suggests that the interdecadal variation of the EASM cannot be accounted for by historical global SST and sea ice evolution. Thus, the possibility that the interdecadal timescale change of monsoon is a natural variability of the coupled climate system evolution cannot be excluded.
Influence of Topography and Large-scale Forcing on the Occurrence of Katabatic Flow Jumps in Antarctica: Idealized Simulations
YU Ye, Xiaoming CAI, QIE Xiushu
2007, 24(5): 819-832. doi: 10.1007/s00376-007-0819-x
The Regional Atmospheric Modeling System (RAMS), which is a non-hydrostatic numerical model, has been used to investigate the impact of terrain shape and large-scale forcing on the Antarctic surface-wind regime, focusing on their roles in establishing favorable flow conditions for the formation of katabatic flow jumps. A series of quasi-2D numerical simulations were conducted over idealized slopes representing the slopes of Antarctica during austral winter conditions. Results indicate that the steepness and variations of the underlying slope play a role in the evolution of near-surface flows and thus the formation of katabatic flow jumps. However, large-scale forcing has a more noticeable effect on the occurrence of this small-scale phenomenon by establishing essential upstream and downstream flow conditions, including the upstream supercritical flow, the less stably stratified or unstable layer above the cold katabatic layer, as well as the cold-air pool located near the foot of the slope through an interaction with the underlying topography. Thus, the areas with steep and abrupt change in slopes, e.g. near the coastal areas of the eastern Antarctic, are preferred locations for the occurrence of katabatic flow jumps, especially under supporting synoptic conditions.
ENSO Phase-Locking in an Ocean-tmosphere Coupled Model FGCM-1.0
ZHENG Weipeng, YU Yongqiang
2007, 24(5): 833-844. doi: 10.1007/s00376-007-0833-z
The mean climatology and the basic characteristics of the ENSO cycle simulated by a coupled model FGCM-1.0 are investigated in this study. Although with some common model biases as in other directly coupled models, FGCM-1.0 is capable of producing the interannual variability of the tropical Pacific, such as the ENSO phenomenon. The mechanism of the ENSO events in the coupled model can be explained by ``delayed oscillator" and ``recharge-discharge" hypotheses. Compared to the observations, the simulated ENSO events show larger amplitude with two distinctive types of phase-locking: one with its peak phase-locked to boreal winter and the other to boreal summer. These two types of events have a similar frequency of occurrence, but since the second type of event is seldom observed, it may be related to the biases of the coupled model. Analysis show that the heat content anomalies originate from the central south Pacific in the type of events peaking in boreal summer, which can be attributed to a different background climatology from the normal events. The mechanisms of their evolutions are also discussed.
Simulating Crop Net Primary Production in China from 2000 to 2050 by Linking the Crop-C model with a FGOALS's Model Climate Change Scenario
ZHANG Wen, HUANG Yao, SUN Wenjuan, YU Yongqiang
2007, 24(5): 845-854. doi: 10.1007/s00376-007-0845-8
Net primary production (NPP) of crop represents the capacity of sequestrating atmospheric CO2 in agro-ecosystem, and it plays an important role in terrestrial carbon cycling. By linking the Crop-C model with climate change scenario projected by a coupled GCM FGOALS via geographical information system (GIS) techniques, crop NPP in China was simulated from 2000 to 2050. The national averaged surface air temperature from FGOALS is projected to increase by 1.0C over this period and the corresponding atmospheric CO2 concentration is 535 ppm by 2050 under the IPCC A1B scenario. With a spatial resolution of 10x10 km2, model simulation indicated that an annual average increase of 0.6 Tg C yr-1 (Tg=1012 g) would be possible under the A1B scenario. The NPP in the late 2040s would increase by 5\% (30 Tg C) within the 98x06 hm2 cropland area in contrast with that in the early 2000s. A further investigation suggested that changes in the NPP would not be evenly distributed in China. A higher increase would occur in a majority of regions located in eastern and northwestern China, while a slight reduction would appear in Hebei and Tianjin in northern China. The spatial characteristics of the crop NPP change are attributed primarily to the uneven distribution of temperature change.
Validating the Runoff from the PRECIS Model Using a Large-Scale Routing Model
CAO Lijuan, DONG Wenjie, XU Yinlong, ZHANG Yong, Michael SPARROW
2007, 24(5): 855-862. doi: 10.1007/s00376-007-0855-6
The streamflow over the Yellow River basin is simulated using the PRECIS (Providing REgional Climates for Impacts Studies) regional climate model driven by 15-year (1979--1993) ECMWF reanalysis data as the initial and lateral boundary conditions and an off-line large-scale routing model (LRM). The LRM uses physical catchment and river channel information and allows streamflow to be predicted for large continental rivers with a 1ox1o spatial resolution. The results show that the PRECIS model can reproduce the general southeast to northwest gradient distribution of the precipitation over the Yellow River basin. The PRECIS-LRM model combination has the capability to simulate the seasonal and annual streamflow over the Yellow River basin. The simulated streamflow is generally coincident with the naturalized streamflow both in timing and in magnitude.
Validation of the Polar MM5 for Use in the Simulation of the Arctic River Basins
MA Yan, CHEN Shang
2007, 24(5): 863-874. doi: 10.1007/s00376-007-0863-6
The simulations were performed using a modified mesoscale model, the Polar MM5, which was adapted for use within polar regions. The objective of the study was to illustrate the skill of the Polar MM5 in simulating atmospheric behavior over the Arctic river basins. Automatic weather station data, global atmospheric analyses, as well as near-surface and upper-air observations were used to verify the simulation. Parallel simulations of the Polar MM5 and the original MM5 within the period 19--29 April 1997 simulations revealed that Polar MM5 reproduced better near-surface variables forecasts than the original MM5 for the region located over the North American Arctic regions. The well predicted near-surface temperature and mixing ratio by the Polar MM5 confirmed the modified physical parameterization schemes that were used in this model are appropriate for the Arctic river regions. Then the extended evaluations of the Polar MM5 simulations over both the North American and Eurasian domains during 15 December 2002 to 15 May 2003 were then carried out. The time series plots and statistical analyses from the observations and the Polar MM5 simulations at 16 stations for the near-surface and vertical profiles at 850 hPa and 500 hPa variables were analyzed. The model was found to reproduce the observed atmospheric state both at magnitude and variability with a high degree of accuracy, especially for temperature and near-surface winds, although there was a slight cold bias that existed near the surface.
Measurements of Nighttime Nitrate Radical Concentrations in the Atmosphere by Long-Path Differential Optical Absorption Spectroscopy
LI Suwen, LIU Wenqing, XIE Pinhua, LI Ang, QIN Min, DOU Ke
2007, 24(5): 875-880. doi: 10.1007/s00376-007-0875-2
The long-path differential optical absorption spectroscopy (LP-DOAS) technique was developed to measure nighttime atmospheric nitrate radical (NO3) concentrations. An optimized retrieval method, resulting in a small residual structure and low detection limits, was developed to retrieve NO3. The time series of the NO3 concentration were collected from 17 to 24 March, 2006, where a nighttime average value of 15.8 ppt was observed. The interfering factors and errors are also discussed. These results indicate that the DOAS technique provides an essential tool for the quantification of NO3 concentration and in the study of its effects upon nighttime chemistry.
Impacts of the Thermal Effects of Sub-grid Orography on the Heavy Rainfall Events Along the Yangtze River Valley in 1991
FENG Lei, ZHANG Yaocun
2007, 24(5): 881-892. doi: 10.1007/s00376-007-0881-4
A P-sigma regional climate model using a parameterization scheme to account for the thermal effects of the sub-grid scale orography was used to simulate the three heavy rainfall events that occurred within the Yangtze River Valley during the mei-yu period of 1991. The simulation results showed that by considering the sub-grid scale topography scheme, one can significantly improve the performance of the model for simulating the rainfall distribution and intensity during these three heavy rainfall events, most especially the second and third. It was also discovered that the rainfall was mainly due to convective precipitation. The comparison between experiments, either with and without the sub-grid scale topography scheme, showed that the model using the scheme reproduced the convergence intensity and distribution at the 850 hPa level and the ascending motion and moisture convergence center located at 500 hPa over the Yangtze River valley. However, some deviations still exist in the simulation of the atmospheric moisture content, the convergence distribution and the moisture transportation route, which mainly result in lower simulated precipitation levels. Further analysis of the simulation results demonstrated that the sub-grid topography scheme modified the distribution of the surface energy budget components, especially at the south and southwest edges of the Tibetan Plateau, leading to the development and eastward propagation of the negative geopotential height difference and positive temperature-lapse rate difference at 700 hPa, which possibly led to an improved precipitation simulation over eastern China.
An Improved Velocity Volume Processing Method
LI Nan, WEI Ming, TANG Xiaowen, PAN Yujie
2007, 24(5): 893-906. doi: 10.1007/s00376-007-0893-0
Velocity volume processing (VVP) retrieval of single Doppler radar is an effective method which can be used to obtain many wind parameters. However, due to the problem of an ill-conditioned matrix arising from the coefficients of equations not being easily resolved, the VVP method has not been applied adequately and effectively in operation. In this paper, an improved scheme, SVVP (step velocity volume processing), based on the original method, is proposed. The improved algorithm retrieves each group of components of the wind field through a stepwise procedure, which overcomes the problem of an ill-conditioned matrix, which currently limits the application of the VVP method. Variables in a six-parameter model can be retrieved even if the analysis volume is very small. In addition, the source and order of errors which exist in the traditional method are analyzed. The improved method is applied to real cases, which show that it is robust and has the capability to obtain the wind field structure of the local convective system. It is very helpful for studying severe storms.
Modeling of Trophospheric Ozone Concentrations Using Genetically Trained Multi-Level Cellular Neural Networks
H. Kurtulus OZCAN, Erdem BILGILI, Ulku SAHIN, O. Nuri UCAN, Cuma BAYAT
2007, 24(5): 907-914. doi: 10.1007/s00376-007-0907-y
Tropospheric ozone concentrations, which are an important air pollutant, are modeled by the use of an artificial intelligence structure. Data obtained from air pollution measurement stations in the city of Istanbul are utilized in constituting the model. A supervised algorithm for the evaluation of ozone concentration using a genetically trained multi-level cellular neural network (ML-CNN) is introduced, developed, and applied to real data. A genetic algorithm is used in the optimization of CNN templates. The model results and the actual measurement results are compared and statistically evaluated. It is observed that seasonal changes in ozone concentrations are reflected effectively by the concentrations estimated by the multilevel-CNN model structure, with a correlation value of 0.57 ascertained between actual and model results. It is shown that the multilevel-CNN modeling technique is as satisfactory as other modeling techniques in associating the data in a complex medium in air pollution applications.
Interdecadal Variability of the East Asian Summer Monsoon and Associated Atmospheric Circulations
ZENG Gang, SUN Zhaobo, Wei-Chyung WANG, MIN Jinzhong
2007, 24(5): 915-926. doi: 10.1007/s00376-007-0915-y
Based on the National Centers for Environmental Prediction and National Center for Atmospheric Research (NCEP/NCAR) reanalysis data from 1950--1999, interdecadal variability of the East Asian Summer Monsoon (EASM) and its associated atmospheric circulations are investigated. The EASM exhibits a distinct interdecadal variation, with stronger (weaker) summer monsoon maintained from 1950--1964 (1976--1997). In the former case, there is an enhanced Walker cell in the eastern Pacific and an anti-Walker cell in the western Pacific. The associated ascending motion resides in the central Pacific, which flows eastward and westward in the upper troposphere, descending in the eastern and western ends of the Pacific basin. At the same time, an anomalous East Asian Hadley Cell (EAHC) is found to connect the low-latitude and mid-latitude systems in East Asia, which strengthens the EASM. The descending branch of the EAHC lies in the west part of the anti-Walker cell, flowing northward in the lower troposphere and then ascending at the south of Lake Baikal (40o--50oN, 95o--115oE) before returning to low latitudes in the upper troposphere, thus strengthening the EASM. The relationship between the EASM and SST in the eastern tropical Pacific is also discussed. A possible mechanism is proposed to link interdecadal variation of the EASM with the eastern tropical Pacific SST. A warmer sea surface temperature anomaly (SSTA) therein induces anomalous ascending motion in the eastern Pacific, resulting in a weaker Walker cell, and at the same time inducing an anomalous Walker cell in the western Pacific and an enhanced EAHC, leading to a weaker EASM. Furthermore, the interdecadal variation of summer precipitation over North China is found to be strongly regulated by the velocity potential over the south of Lake Baikal through enhancing and reducing the regional vertical motions.
Development of a Model for Water and Heat Exchange Between the Atmosphere and a Water Body
SUN Shufen, YAN Jinfeng, XIA Nan, SUN Changhai
2007, 24(5): 927-938. doi: 10.1007/s00376-007-0927-7
A model for studying the heat and mass exchange between the atmosphere and a water body is developed, in which the phase change process of water freezing in winter and melting in summer and the function of the convective mixing process are taken into consideration. The model uses enthalpy rather than temperature as the predictive variable. It helps to set up governing equations more concisely, to deal with the phase change process more easily, and make the numerical scheme simpler. The model is verified by observed data from Lake Kinneret for a non-frozen lake in summer time, and Lake Lower Two Medicine for a frozen lake in winter time. Reasonably good agreements between the model simulations and observed data indicate that the model can serve as a component for a water body in a land surface model. In order to more efficiently apply the scheme in a climate system model, a sensitivity study of various division schemes with less layers in the vertical direction in the water body is conducted. The results of the study show that the division with around 10 vertical layers could produce a prediction accuracy that is comparable to the fine division with around 40 layers.