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Miming Zhang
,
Liqi Chen
,
Guojie Xu
,
Qi Lin
, and
Minyi Liang

Abstract

Multiple year-round aerosol samplings were conducted from February 2005 to October 2008 at Zhongshan Station, a research base in East Antarctica, to study methanesulfonic acid (MSA) and non-sea-salt sulfate (nss-SO4 2−). The concentrations of atmospheric sulfur species exhibited a seasonal cycle; the maximum and minimum concentrations occurred in austral summer and austral winter, respectively. Significant correlations between chlorophyll a (Chl a) in offshore polynyas and both MSA (r = 0.726, n = 52, and p < 0.01) and nss-SO4 2− (r = 0.724, n = 48, and p < 0.01) were found, indicating that the phytoplankton activity had a crucial effect on the sulfur aerosols. The sea ice dynamics in the polynyas and the variations in the polynya area may indirectly influence the sulfur aerosols in austral spring and summer. In austral winter, the sulfur compounds in the atmosphere are primarily originating in long-range transported by-products from remote regions because nearly no phytoplankton activity occurred in the offshore polynyas.

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Chuan-Yao Lin
,
Wan-Chin Chen
,
Pao-Liang Chang
, and
Yang-Fan Sheng

Abstract

To evaluate the impacts of the urban heat island (UHI) effect on precipitation over a complex geographic environment in northern Taiwan, the next-generation mesoscale model, the Weather Research and Forecasting (WRF) model, coupled with the Noah land surface model and urban canopy model (UCM), was used to study this issue. Based on a better land use classification derived from Moderate Resolution Imaging Spectroradiometer (MODIS) satellite data (the MODIS case), it has significantly improved simulation results for the accumulation rainfall pattern as compared with the original U.S. Geological Survey (USGS) 25-category land use classification (the USGS case). The precipitation system was found to develop later but stronger in the urban (MODIS) case than in the nonurban (USGS) case. In comparison with the observation by radar, simulation results predicted reasonably well; not only was the rainfall system enhanced downwind of the city over the mountainous area, but it also occurred at the upwind plain area in the MODIS case. The simulation results suggested that the correct land use classification is crucial for urban heat island modeling study. The UHI effect plays an important role in perturbing thermal and dynamic processes; it affects the location of thunderstorms and precipitation over the complex geographic environment in northern Taiwan.

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Liang Ning
,
Kefan Chen
,
Jian Liu
,
Zhengyu Liu
,
Mi Yan
,
Weiyi Sun
,
Chunhan Jin
, and
Zhengguo Shi

Abstract

The influence and mechanism of volcanic eruptions on decadal megadroughts over eastern China during the last millennium were investigated using a control (CTRL) and five volcanic eruption sensitivity experiments (VOLC) from the Community Earth System Model (CESM) Last Millennium Ensemble (LME) archive. The decadal megadroughts associated with the failures of the East Asian summer monsoon (EASM) are associated with a meridional tripole of sea surface temperature anomalies (SSTAs) in the western Pacific from the equator to high latitudes, suggestive of a decadal-scale internal mode of variability that emerges from empirical orthogonal function (EOF) analysis. Composite analyses further showed that, on interannual time scales, within a decade after an eruption the megadrought was first enhanced but then weakened, due to the change from an El Niño state to a La Niña state. The impacts of volcanic eruptions on the magnitudes of megadroughts are superposed on internal variability. Therefore, the evolution of decadal megadroughts coinciding with strong volcanic eruptions demonstrate that the impacts of internal variability and external forcing can combine to influence hydroclimate.

Open access
Chia-Chi Wang
,
Wei-Liang Lee
,
Yu-Luen Chen
, and
Huang-Hsiung Hsu

Abstract

The double intertropical convergence zone (ITCZ) bias in the eastern Pacific in the Community Earth System Model version 1 with Community Atmosphere Model version 5 (CESM1/CAM5) is diagnosed. In CAM5 standalone, the northern ITCZ is associated with inertial instability and the southern ITCZ is thermally forced. After air–sea coupling, the processes on both hemispheres are switched because the spatial pattern of sea surface temperature (SST) is changed.

Biases occur during boreal spring in both CAM5 and the ocean model. In CAM5 alone, weaker-than-observed equatorial easterly in the tropical eastern South Pacific leads to weaker evaporation and an increase in local SST. The shallow meridional circulation overly converges in the same region in the CAM5 standalone simulation, the planetary boundary layer and middle troposphere are too humid, and the large-scale subsidence is too weak at the middle levels. These biases may result from excessive shallow convection behavior in CAM5. The extra moisture would then fuel stronger convection and a higher precipitation rate in the southeastern Pacific.

In the ocean model, the South Equatorial Current is underestimated and the North Equatorial Countercurrent is located too close to the equator, causing a warm SST bias in the southeastern Pacific and a cold bias in the northeastern Pacific. These SST biases feed back to the atmosphere and further influence convection and the surface wind biases in the coupled simulation. When the convection in the tropical northeastern Pacific becomes thermally forced after coupling, the northern ITCZ is diminished due to colder SST, forming the so-called alternating ITCZ bias.

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Ren Wang
,
Pierre Gentine
,
Longhui Li
,
Jianyao Chen
,
Liang Ning
,
Linwang Yuan
, and
Guonian Lü

Abstract

Land–atmosphere interactions play an important role in the changes of extreme climates, especially in hot spots of land–atmosphere coupling. One of the linkages in land–atmosphere interactions is the coupling between air temperature and surface energy fluxes associated with soil moisture variability, vegetation change, and human water/land management. However, existing studies on the coupling between hot extreme and surface energy fluxes are mainly based on the parameterized solution of climate model, which might not dynamically reflect all changes in the surface energy partitioning due to the effects of vegetation physiological control and human water/land management. In this study, for the first time, we used daily weather observations to identify hot spots where the daily hot extreme (i.e., the 99th percentile of maximum temperature, Tq99th) rises faster than local mean temperature (Tmean) during 1975–2017. Furthermore, we analyzed the relationship between the trends in temperature hot extreme relative to local average (ΔTq99th/ΔTmean) and the trends in evaporative fraction (ΔEF), i.e., the ratio of latent heat flux to surface available energy, using long-term latent and sensible heat fluxes, which are informed by atmospheric boundary layer theory, machine learning, and ground-based observations of flux towers and weather stations. Hot spots of increase in ΔTq99th/ΔTmean are identified to be Europe, southwestern North America, northeast Asia, and southern Africa. The detected significant negative correlations between ΔEF and ΔTq99th/ΔTmean suggested that the hot spot regions are typically affected by annual/summer surface dryness. Our observation-driven findings have great implications in providing realistic observational evidence for the extreme climate change accelerated by surface energy partitioning.

Open access
Chengyan Liu
,
Zhaomin Wang
,
Xi Liang
,
Xiang Li
,
Xichen Li
,
Chen Cheng
, and
Di Qi

Abstract

Warm deep water intrusion over the Antarctic continental shelves threatens the Antarctic ice sheet stability by enhancing the basal melting of ice shelves. In East Antarctica, the Antarctic Slope Current (ASC), along with the Antarctic Slope Front (ASF), acts as a potential vorticity barrier to prevent the warm modified Circumpolar Deep Water (mCDW) from ventilating the cold and fresh shelf. However, mCDW onshore transport is still observed within certain shelf regions, such as submarine troughs running perpendicular to the continental shelf. This study focuses on the dynamic mechanisms governing mCDW intrusion within a submarine trough over the fresh shelf regions, East Antarctica. Based on an idealized eddy-resolving coupled ocean–ice shelf model, two high-resolution process-oriented numerical experiments are conducted to reveal the mechanisms responsible for the mCDW onshore transport. Three dynamic mechanisms governing cross-slope mCDW intrusion are identified: 1) the bottom pressure torque, 2) the topography beta spiral, and 3) the topography Rossby waves. These three mechanisms simultaneously govern the mCDW intrusion together. The bottom pressure torque plays a leading role in driving the time-mean onshore flow whose vertical structure is determined by the topography beta spiral, while the topography Rossby waves contribute to the high-frequency oscillations in the onshore volume and heat transport. The simulated spatial distribution and seasonality of mCDW intrusion qualitatively coincide with the observed mCDW intrusion over fresh shelf regions, East Antarctica. Both the topography beta spiral and the ASC play an important role in governing the seasonality of mCDW intrusion.

Open access
Jau-Ming Chen
,
Pei-Hua Tan
,
Liang Wu
,
Hui-Shan Chen
,
Jin-Shuen Liu
, and
Ching-Feng Shih

Abstract

This study examines the interannual variability of summer tropical cyclone (TC) rainfall (TCR) in the western North Pacific (WNP) depicted by the Climate Forecast System Reanalysis (CFSR). This interannual variability exhibits a maximum region near Taiwan (19°–28°N, 120°–128°E). Significantly increased TCR in this region is modulated by El Niño–Southern Oscillation (ENSO)-related large-scale processes. They feature elongated sea surface temperature warming in the tropical eastern Pacific and a southeastward-intensified monsoon trough. Increased TC movements are facilitated by interannual southerly/southeasterly flows in the northeastern periphery of the intensified monsoon trough to move from the tropical WNP toward the region near Taiwan, resulting in increased TCR. The coherent dynamic relations between interannual variability of summer TCR and large-scale environmental processes justify CFSR as being able to reasonably depict interannual characteristics of summer TCR in the WNP. For intraseasonal oscillation (ISO) modulations, TCs tend to cluster around the center of a 10–24-day cyclonic anomaly and follow its northwestward propagation from the tropical WNP toward the region near Taiwan. The above TC movements are subject to favorable background conditions provided by a northwest–southeasterly extending 30–60-day cyclonic anomaly. Summer TCR tends to increase (decrease) during El Niño (La Niña) years and strong (weak) ISO years. By comparing composite TCR anomalies and correlations with TCR variability, it is found that ENSO is more influential than ISO in modulating the interannual variability of summer TCR in the WNP.

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Zhenghui Xie
,
Fei Yuan
,
Qingyun Duan
,
Jing Zheng
,
Miaoling Liang
, and
Feng Chen

Abstract

This paper presents a methodology for regional parameter estimation of the three-layer Variable Infiltration Capacity (VIC-3L) land surface model with the goal of improving the streamflow simulation for river basins in China. This methodology is designed to obtain model parameter estimates from a limited number of calibrated basins and then regionalize them to uncalibrated basins based on climate characteristics and large river basin domains, and ultimately to continental China. Fourteen basins from different climatic zones and large river basins were chosen for model calibration. For each of these basins, seven runoff-related model parameters were calibrated using a systematic manual calibration approach. These calibrated parameters were then transferred within the climate and large river basin zones or climatic zones to the uncalibrated basins. To test the efficiency of the parameter regionalization method, a verification study was conducted on 19 independent river basins in China. Overall, the regionalized parameters, when evaluated against the a priori parameter estimates, were able to reduce the model bias by 0.4%–249.8% and relative root-mean-squared error by 0.2%–119.1% and increase the Nash–Sutcliffe efficiency of the streamflow simulation by 1.9%–31.7% for most of the tested basins. The transferred parameters were then used to perform a hydrological simulation over all of China so as to test the applicability of the regionalized parameters on a continental scale. The continental simulation results agree well with the observations at regional scales, indicating that the tested regionalization method is a promising scheme for parameter estimation for ungauged basins in China.

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Yu-Kun Qian
,
Chang-Xia Liang
,
Shiqiu Peng
,
Shumin Chen
, and
Sihua Wang

Abstract

A horizontal map of the upper-level forcing index (ULFI) is constructed to show the possible influence of upper-level large-scale environmental flow on the intensity change of tropical cyclones (TCs). The ULFI includes three commonly used diagnostics, that is, 200-hPa eddy flux convergences of both relative (REFC) and planetary angular momentum (PEFC), as well as axisymmetric absolute vorticity as a denominator that rescales the strength of the eddy forcings similar to the outflow-layer inertial stability. A simple procedure is adopted to convert these storm-relative components and the ULFI into Eulerian horizontal maps. Applications of this index map to three selected TC cases clearly demonstrate the process of upper-level TC–environment interaction: when a TC moves into a region of high (low) index, significant upper-level asymmetric forcing is exerted on the TC, leading to the strengthening (weakening) of the TC’s axisymmetric outflow and then possibly its intensity. As such, the horizontal map of ULFI not only provides a quantitative way of estimating the strength of upper-level asymmetric forcing at each grid point, but also serves as an indicator showing where the possible intensity change of a TC would occur under the influence of upper-level environmental flow. The index is thus recommended to be used in future studies of TC–environment interaction.

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Chang-Rong Liang
,
Xiao-Dong Shang
,
Yong-Feng Qi
,
Gui-Ying Chen
, and
Ling-Hui Yu

Abstract

Finescale parameterizations are of great importance to explore the turbulent mixing in the open ocean due to the difficulty of microstructure measurements. Studies based on finescale parameterizations have greatly aided our knowledge of the turbulent mixing in the open ocean. In this study, we introduce a modified finescale parameterization (MMG) based on shear/strain variance ratio R ω and compare it with three existing parameterizations, namely, the MacKinnon–Gregg (MG) parameterization, the Gregg–Henyey–Polzin (GHP) parameterization based on shear and strain variances, and the GHP parameterization based on strain variance. The result indicates that the prediction of MG parameterization is the best, followed by the MMG parameterization, then the shear-and-strain-based GHP parameterization, and finally the strain-based GHP parameterization. The strain-based GHP parameterization is less effective than the shear-and-strain-based GHP parameterization, which is mainly due to its excessive dependence on stratification. The predictions of the strain-based MMG parameterization can be comparable to that of the MG parameterization and better than that of the shear-and-strain-based GHP parameterization. Most importantly, MMG parameterization is even effective over rough topography where the GHP parameterization fails. This modified MMG parameterization with prescribed R ω can be applied to extensive CTD data. It would be a useful tool for researchers to explore the turbulent mixing in the open ocean.

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