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Yongqiang Zhang, Francis H. S. Chiew, Lu Zhang, and Hongxia Li

Abstract

This paper explores the use of the Moderate Resolution Imaging Spectroradiometer (MODIS), mounted on the polar-orbiting Terra satellite, to determine leaf area index (LAI), and use actual evapotranspiration estimated using MODIS LAI data combined with the Penman–Monteith equation [remote sensing evapotranspiration (E RS)] in a lumped conceptual daily rainfall–runoff model. The model is a simplified version of the HYDROLOG (SIMHYD) model, which is used to estimate runoff in ungauged catchments. Two applications were explored: (i) the calibration of SIMHYD against both the observed streamflow and E RS, and (ii) the modification of SIMHYD to use MODIS LAI data directly. Data from 2001 to 2005 from 120 catchments in southeast Australia were used for the study. To assess the modeling results for ungauged catchments, optimized parameter values from the geographically nearest gauged catchment were used to model runoff in the ungauged catchment. The results indicate that the SIMHYD calibration against both the observed streamflow and E RS produced better simulations of daily and monthly runoff in ungauged catchments compared to the SIMHYD calibration against only the observed streamflow data, despite the modeling results being assessed solely against the observed streamflow data. The runoff simulations were even better for the modified SIMHYD model that used the MODIS LAI directly. It is likely that the use of other remotely sensed data (such as soil moisture) and smarter modification of rainfall–runoff models to use remotely sensed data directly can further improve the prediction of runoff in ungauged catchments.

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Yi Zhang, Rucong Yu, Jian Li, Weihua Yuan, and Minghua Zhang

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Given the large discrepancies that exist in climate models for shortwave cloud forcing over eastern China (EC), the dynamic (vertical motion and horizontal circulation) and thermodynamic (stability) relations of stratus clouds and the associated cloud radiative forcing in the cold season are examined. Unlike the stratus clouds over the southeastern Pacific Ocean (as a representative of marine boundary stratus), where thermodynamic forcing plays a primary role, the stratus clouds over EC are affected by both dynamic and thermodynamic factors. The Tibetan Plateau (TP)-forced low-level large-scale lifting and high stability over EC favor the accumulation of abundant saturated moist air, which contributes to the formation of stratus clouds. The TP slows down the westerly overflow through a frictional effect, resulting in midlevel divergence, and forces the low-level surrounding flows, resulting in convergence. Both midlevel divergence and low-level convergence sustain a rising motion and vertical water vapor transport over EC. The surface cold air is advected from the Siberian high by the surrounding northerly flow, causing low-level cooling. The cooling effect is enhanced by the blocking of the YunGui Plateau. The southwesterly wind carrying warm, moist air from the east Bay of Bengal is uplifted by the HengDuan Mountains via topographical forcing; the midtropospheric westerly flow further advects the warm air downstream of the TP, moistening and warming the middle troposphere on the lee side of the TP. The low-level cooling and midlevel warming together increase the stability. The favorable dynamic and thermodynamic large-scale environment allows for the formation of stratus clouds over EC during the cold season.

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Yi Zhang, Rucong Yu, Jian Li, Weihua Yuan, and Minghua Zhang
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Guiwan Chen, Jian Ling, Chongyin Li, Yuanwen Zhang, and Chidong Zhang

Abstract

This study explores possible mechanisms for the barrier effect of the Indo-Pacific Maritime Continent (MC) on MJO propagation. In particular, this study examines whether similar mechanisms can be found in both observations and CMIP5 simulations. All models simulate individual MJO events but underestimate the percentage of MJO events propagating into the MC. The simulations are grouped into the top and bottom 50% based on their capability of reproducing the MJO spectral signal. When compared with the observations, the bottom 50% of the simulations significantly underestimate the MJO strength and exaggerate the barrier effect intensity, whereas these discrepancies are not significant in the top 50% of the simulations. From the top 50% of the simulations, the MJO strength, moisture processes, and surface evaporation in the MC all play important roles in constituting the barrier effect. No such evidence is found in observations. The discrepancies may come from small observed sample size and/or misrepresentations of key physical processes in the models. A consistent result is found in the observations and simulations: Whether MJO events can cross the MC depends on the degree to which dominant precipitation over land shifts to over water in the MC as MJO convection centers approach the MC and cross it. This result emphasizes the critical role of precipitation over water in carrying convective signals of the MJO through the MC. The results suggest that diagnosing the model alone on mechanisms for the barrier effect could be misleading; further investigations using a combination of observations, global gridded data, and high-resolution models are needed.

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Li Zhang, Ping Chang, and Michael K. Tippett

Abstract

A novel noise filter is used to effectively reduce internal atmospheric variability in the air–sea fluxes of a coupled model. This procedure allows for a test of the impact of the internal atmospheric variability on ENSO through its effect on the Pacific meridional mode (MM). Three 100-yr coupled experiments are conducted, where the filter is utilized to suppress internal atmospheric variability in 1) both the surface wind stress and the heat flux (fully filtered run), 2) only the surface heat flux (filtered-flux run), and 3) only the surface wind stress (filtered-wind run). The fully filtered run indicates that suppressing internal atmospheric variability weakens the MM, which in turn results in substantially reduced ENSO variability. ENSO is no longer phase locked to the boreal winter. The filtered-flux and filtered-wind experiments reveal that different types of noise affect ENSO in different ways. The noise in the wind stress does not have a significant impact on the MM and its relationship to ENSO. This type of noise, however, tends to broaden the spectral peak of ENSO while shifting it toward lower frequencies. The noise in the heat flux, on the other hand, has a direct impact on the strength of the MM and consequently its ability to influence ENSO. Reducing the effect of heat flux noise yields substantially weakened MM activity and a weakened relationship to ENSO, which leads to altered seasonal phase-locking characteristics.

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Li Zhang, Ping Chang, and Link Ji

Abstract

The occurrence of a boreal spring phenomenon referred to as the Pacific meridional model (MM) is shown to be intimately linked to the development of El Niño–Southern Oscillation (ENSO) in a long simulation of a coupled model. The MM, characterized by an anomalous north–south SST gradient and anomalous surface circulation in the northeasterly trade regime with maximum variance in boreal spring, is shown to be inherent to thermodynamic ocean–atmosphere coupling in the intertropical convergence zone (ITCZ) latitude, and the MM existence is independent of ENSO. The thermodynamic coupling enhances the persistence of the anomalous winds in the deep tropics, forcing energetic equatorially trapped oceanic waves to occur in the central western Pacific, which in turn initiate an ENSO event. The majority of ENSO events in both nature and the coupled model are preceded by MM events.

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Richard J. Greatbatch, Guoqing Li, and Sheng Zhang

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This paper investigates the hindcasting of interdecadal climate events using an ocean circulation model driven by different combinations of time-varying surface flux, sea surface temperature (SST), and sea surface salinity (SSS) data. Data are generated from a control run, against which the subsequent model experiments are compared. The most robust results are obtained using flux boundary conditions on both surface temperature and salinity. For these boundary conditions, model results am relatively insensitive to noise in the surface data and take about 20 years to overcome the imposition of an incorrect initial condition. Model results are much more sensitive to noisy inputs when run using SST and SSS data. To obtain meaningful results, SST data alone are not sufficient; SSS data are also required. This is related to the well-known instability of ocean climate models upon a switch to mixed boundary conditions. Time-varying SSS data cannot be replaced by climatology; using a best-fit TS relation, to calculate anomalies in SSS from those in SST is also found to give disappointing results. The difficulty of trying to correct for inaccuracies in surface heat flux using SST data, while at the same time using a flux boundary condition on surface salinity, is demonstrated.

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Weiqing Zhang, William Perrie, and Weibiao Li

Abstract

A coupled atmosphere–wave–sea spray model system is used to evaluate the combined impacts of spray evaporation and wave drag on midlatitude storms. The focus of this paper is on the role of air–sea fluxes on storm intensity and development, and related impacts on the structure of the atmospheric boundary layer. The composite model system consists of the Canadian Mesoscale Compressible Community atmospheric model coupled to the operational wave model WAVEWATCH III, and a recent bulk parameterization for heat fluxes due to sea spray. The case studies are extratropical Hurricane Earl (in 1998) and two intense winter storms from 2000 and 2002, hereafter denoted “superbomb” and “bomb,” respectively. The results show that sea spray tends to intensify storms, whereas wave-related drag tends to weaken storms. The mechanisms by which spray and wave-related drag can influence storm intensity are quite different. When wind speeds are high and sea surface temperatures warm, spray can significantly increase the surface heat fluxes. By comparison, momentum fluxes related to wave drag are important over regions of the storm where young, newly generated waves are prevalent, for example during the rapid development phase of the storm. These momentum fluxes decrease in areas where the storm waves reach maturity. The collective influence of spray and waves on storm intensity depends on their occurrence in the early stages of a storm’s rapid intensification phase, and their spatial distribution with respect to the storm center. Moreover, for the case of the superbomb, a potential vorticity framework is used to show the relative importance of these surface flux impacts compared with baroclinic processes.

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C-P. Chang, Yongsheng Zhang, and Tim Li

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The interannual relationship between the East Asian summer monsoon and the tropical Pacific SSTs is studied using rainfall data in the Yangtze River Valley and the NCEP reanalysis for 1951–96. The datasets are also partitioned into two periods, 1951–77 and 1978–96, to study the interdecadal variations of this relationship.

A wet summer monsoon is preceded by a warm equatorial eastern Pacific in the previous winter and followed by a cold equatorial eastern Pacific in the following fall. This relationship involves primarily the rainfall during the pre-Mei-yu/Mei-yu season (May–June) but not the post-Mei-yu season (July–August). In a wet monsoon year, the western North Pacific subtropical ridge is stronger as a result of positive feedback that involves the anomalous Hadley and Walker circulations, an atmospheric Rossby wave response to the western Pacific complementary cooling, and the evaporation–wind feedback. This ridge extends farther to the west from the previous winter to the following fall, resulting in an 850-hPa anomalous anticyclone near the southeast coast of China. This anticyclone 1) blocks the pre-Mei-yu and Mei-yu fronts from moving southward thereby extending the time that the fronts produce stationary rainfall; 2) enhances the pressure gradient to its northwest resulting in a more intense front; and 3) induces anomalous warming of the South China Sea surface through increased downwelling, which leads to a higher moisture supply to the rain area. A positive feedback from the strong monsoon rainfall also appears to occur, leading to an intensified anomalous anticyclone near the monsoon region. This SST–subtropical ridge–monsoon rainfall relationship is observed in both the interannual timescale within each interdecadal period and in the interdecadal scale.

The SST anomalies (SSTAs) change sign in northern spring and resemble a tropospheric biennial oscillation (TBO) pattern during the first interdecadal period (1951–77). In the second interdecadal period (1978–96) the sign change occurs in northern fall and the TBO pattern in the equatorial eastern Pacific SST is replaced by longer timescales. This interdecadal variation of the monsoon–SST relationship results from the interdecadal change of the background state of the coupled ocean–atmosphere system. This difference gives rise to the different degrees of importance of the feedback from the anomalous circulations near the monsoon region to the equatorial eastern Pacific.

In a wet monsoon year, the anomalous easterly winds south of the monsoon-enhanced anomalous anticyclone start to propagate slowly eastward toward the eastern Pacific in May and June, apparently as a result of an atmosphere–ocean coupled wave motion. These anomalous easterlies carry with them a cooling effect on the ocean surface. In 1951–77 this effect is insignificant as the equatorial eastern Pacific SSTAs, already change from warm to cold in northern spring, probably as a result of negative feedback processes discussed in ENSO mechanisms. In 1978–96 the equatorial eastern Pacific has a warmer mean SST. A stronger positive feedback between SSTA and the Walker circulation during a warm phase tends to keep the SSTA warm until northern fall, when the eastward-propagating anomalous easterly winds reach the eastern Pacific and reverse the SSTA.

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Yongsheng Zhang, Tim Li, and Bin Wang

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The decadal change in the spring snow depth over the Tibetan Plateau and impact on the East Asian summer monsoon are investigated using station observations of snow depth data and the NCEP–NCAR reanalysis for 1962–93. During spring (March–April), both the domain-averaged snow depth index (SDI) and the first principal component of the empirical orthogonal function (EOF) analysis exhibit a sharp increase in snow depth after the late 1970s, which is accompanied by excessive precipitation and land surface cooling. The correlation between SDI and precipitation shows a coherent remote teleconnection from the Tibetan Plateau–northern India to western Asia.

It is found that the increased snow depth over the plateau after the mid-1970s is concurrent with a deeper India–Burma trough, an intensified subtropical westerly jet as well as enhanced ascending motion over the Tibetan Plateau. Additional factors for the excessive snowfall include more moisture supply associated with the intensification of the southerly flow over the Bay of Bengal and an increase of humidity over the Indian Ocean. While the extensive changes of the circulation in Eurasia and the Indian Ocean are associated with a climate shift in the Northern Hemisphere after the mid-1970s, some regional factors such as the enhanced coupling between the sea surface temperature (SST) warming in the northern Indian Ocean/Maritime Continent and the tropical convective maximum (TCM), as well as local feedback of the land surface cooling due to excessive snow cover and the atmosphere may contribute to the regional circulation changes. The former enhances the western Pacific subtropical in the South China Sea–Philippine Sea through modulation of the local Hadley circulation and results in stronger pressure gradients and fronts in southeastern and eastern Asia.

A close relationship exists between the interdecadal increase of snow depth over the Tibetan Plateau during March–April and a wetter summer rainfall over the Yangtze River valley and a dryer one in the southeast coast of China and the Indochina peninsula. It is proposed that the excessive snowmelt results in a surface cooling over the plateau and neighboring regions and high pressure anomalies that cause a more northwestward extension of the western Pacific subtropical high in the subsequent summer. Additionally, the increased surface moisture supply provides more energy for the development of the eastward-migrating low-level vortex over the eastern flank of the Tibetan Plateau. Both factors lead to a wetter summer in the vicinity of the Yangtze River valley.

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