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Liang Gui Chen

Abstract

An analysis of the Levitus data is employed to examine Stommel's mixed layer density ratio regulator hypothesis. Three different methods of computing the lateral density ratio (R l≡αΔT/βΔS) are used and the least squares method was found to have the least variance in the density ratio over the temperature range of 7°–17°C. Seasonal mean and annual mean density ratios in the North Pacific Ocean are calculated. The spring season has the highest density ratio of 2.1, and the fall season has the lowest of 1.64. The vertical variation in the lateral density ratio is small, in the upper 50 m, especially during the winter season. Overall, in the world's ocean, the mixed layer annual mean density ratios in the 7°–17°C range show remarkably consistent values, in support of Stommel's hypothesis. However, our estimate shows a systematically lower density ratio than that of Stommel in each ocean. Only the South Pacific and South Atlantic have density ratios equal to or slightly over 2. The North Pacific, North Atlantic, and Indian Oceans have density ratios around 1.7.

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Liang Chen and Paul A. Dirmeyer

ABSTRACT

Recent studies have shown the impacts of historical land-use land-cover changes (i.e., deforestation) on hot temperature extremes; contradictory temperature responses have been found between studies using observations and climate models. However, different characterizations of surface temperature are sometimes used in the assessments: land surface skin temperature T s is more commonly used in observation-based studies while near-surface air temperature T 2m is more often used in model-based studies. The inconsistent use of temperature variables is not inconsequential, and the relationship between deforestation and various temperature changes can be entangled, which complicates comparisons between observations and model simulations. In this study, the responses in the diurnal cycle of summertime T s and T 2m to deforestation are investigated using the Community Earth System Model. For the daily maximum, opposite responses are found in T s and T 2m. Due to decreased surface roughness after deforestation, the heat at the land surface cannot be efficiently dissipated into the air, leading to a warmer surface but cooler air. For the daily minimum, strong warming is found in T 2m, which exceeds daytime cooling and leads to overall warming in daily mean temperatures. After comparing several climate models, we find that the models agree in daytime land surface (T s) warming, but different turbulent transfer characteristics produce discrepancies in T 2m. Our work highlights the need to investigate the diurnal cycles of temperature responses carefully in land-cover change studies. Furthermore, consistent consideration of temperature variables should be applied in future comparisons involving observations and climate models.

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Liang Chen and Oliver W. Frauenfeld

Abstract

Historical temperature variability over China during the twentieth century and projected changes under three emission scenarios for the twenty-first century are evaluated on the basis of a multimodel ensemble of 20 GCMs from phase 5 of the Coupled Model Intercomparison Project (CMIP5) and two observational datasets. Changes relative to phase 3 of the Coupled Model Intercomparison Project (CMIP3) are assessed, and the performance of individual GCMs is also quantified. Compared with observations, GCMs have substantial cold biases over the Tibetan Plateau, especially in the cold season. The timing and location of these biases also correspond to the greatest disagreement among the individual models, indicating GCMs’ limitations in reproducing climatic features in this complex terrain. The CMIP5 multimodel ensemble shows better agreement with observations than CMIP3 in terms of the temperature biases. Both CMIP3 and CMIP5 capture the climatic warming over the twentieth century. However, the magnitude of the annual mean temperature trends is underestimated. There is also limited agreement in the spatial and seasonal patterns of temperature trends over China. Based on six statistical measures, four individual models—the Max Planck Institute Earth System Model, low resolution (MPI-ESM-LR), Second Generation Canadian Earth System Model (CanESM2), Model for Interdisciplinary Research on Climate, Earth System Model (MIROC-ESM), and Community Climate System Model, version 4 (CCSM4)—best represent surface air temperature variability over China. The future temperature projections indicate that the representative concentration pathway (RCP) 8.5 and RCP 4.5 scenarios exhibit a gradual increase in annual temperature during the twenty-first century at a rate of 0.60° and 0.27°C (10 yr)−1, respectively. As the lowest-emission mitigation scenario, RCP 2.6 projects the lowest rate of temperature increase [0.10°C (10 yr)−1]. By the end of the twenty-first century, temperature is projected to increase by 1.7°–5.7°C, with larger warming over northern China and the Tibetan Plateau.

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Liang Chen and Paul A. Dirmeyer

Abstract

This study investigates the impacts of historical land-cover change on summer afternoon precipitation over North America using the Community Earth System Model. Using land–atmosphere coupling metrics, this study examines the sensitivity of afternoon atmospheric conditions to morning land surface states and fluxes that are altered by land-cover changes before and since 1850. The deforestation in the eastern United States prior to 1850 leads to increased latent but decreased sensible heat flux during the morning and a reduction in afternoon precipitation over the southern regions of the U.S. East Coast. The agricultural expansion over the Great Plains since preindustrial times shows similar effects on surface fluxes but results in a significant widespread increase in precipitation over the crop area. The coupling metrics exhibit a strong positive soil moisture–precipitation relationship over the Great Plains. Impacts of land-cover change on precipitation manifest through changes in rainfall frequency, rather than intensity, that are largely controlled by the distribution of CAPE as the trigger of convective precipitation. However, deforestation and later reforestation over the eastern United States, where coupling properties are different than the Great Plains, do not have as dominant an effect on afternoon precipitation. Additionally, precipitation over parts of the U.S. Southwest decreases in this model during the earlier period of East Coast deforestation, owing to changes in the large-scale circulation over North America driven by land-use changes prior to 1850.

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Feng Chen, Xudong Liang, and Hao Ma

Abstract

An improved Doppler radar radial velocity assimilation observation operator is proposed based on the integrating velocity–azimuth process (IVAP) method. This improved operator can ingest both radial wind and its spatial distribution characteristics to deduce the two components of the mean wind within a given area. With this operator, the system can be used to assimilate information from tangential wind and radial wind. On the other hand, because the improved observation operator is defined within a given area, which can be uniformly chosen in both the observation and analysis coordinate systems, it has a thinning function. The traditional observation operator and the improved observation operator, along with their corresponding data processing modules, were implemented in the community Gridpoint Statistical Interpolation analysis system (GSI) to demonstrate the superiority of the improved operator. The results of single analysis unit experiments revealed that the two operators are comparable when the analysis unit is small. When the analysis unit becomes larger, the analysis results of the improved operator are better than those of the traditional operator because the former can ingest more wind information than the latter. The results of a typhoon case study indicated that both operators effectively ingested radial wind information and produced more reasonable typhoon structures than those in the background fields. The tangential velocity relative to the radar was retrieved by the improved operator through ingesting tangential wind information from the spatial distribution characteristics of radial wind. Because of the improved vortex intensity and structure, obvious improvements were seen in both track and intensity predictions when the improved operator was used.

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Liang Gui Chen and William K. Dewar

Abstract

A three-layer, wind-driven, general circulation model involving both subtropical and subpolar gyres has been developed to study intergyre exchange. Following some early studies, the present work allows flow to baroclinically cross the intergyre boundary. This model differs from past work by examining a three-layer fluid. Solutions with both southward and northward subsurface flows are obtained. The two principal objectives of this work are to clarify the structure and maintenance of the permanent thermocline and to aid in understanding the distribution of deep water masses.

A class of thermocline structures at the zero Ekman pumping line has been constructed that permits intergyre exchange, or communication. The zones of exchange are called windows. In this study, the windows have several unique properties relative to those computed elsewhere, and exhibit relatively rich structure. Principally, the addition of an active third layer allows a new second baroclinic window to open. This new window is physically and dynamically distinct from the first window (found in previous studies), and most of the intergyre baroclinic transport can occur through it. Its appearance also supports the conjecture that the number of communication windows increases with the number of active layers.

In addition to the model development, observed potential vorticity distributions have been reexamined within the context of this model. Possible explanations for deep potential vorticity contours in the North Atlantic and North Pacific oceans are proposed.

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Liang Chen, Trent W. Ford, and Priyanka Yadav

Abstract

Flash droughts are noted by their unusually rapid rate of onset or intensification, which makes it difficult to anticipate and prepare for them, thus resulting in severe impacts. Although the development of flash drought can be associated with certain atmospheric conditions, vegetation also plays a role in propagating flash drought. This study examines the climatology of warm season (March–September) flash drought occurrence in the United States between 1979 and 2014, and quantifies the possible impacts of vegetation on flash drought based on a set of sensitivity experiments using the Community Earth System Model, version 2 (CESM2). With atmospheric nudging, CESM2 well captures historical flash drought. Compared with NASA’s Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), and National Climate Assessment–Land Data Assimilation System (NCA-LDAS), CESM2 shows agreement on the high flash drought frequency in the Great Plains and southeastern United States, but overestimates flash drought occurrence in the Midwest. The vegetation sensitivity experiments suggest that vegetation greening can significantly increase the flash drought frequency in the Great Plains and the western United States during the warm seasons through enhanced evapotranspiration. However, flash drought occurrence is not significantly affected by vegetation phenology in the eastern United States and Midwest due to weak land–atmosphere coupling. In response to vegetation greening, the extent of flash drought also increases, but the duration of flash drought is not sensitive to greening. This study highlights the importance of vegetation in flash drought development, and provides insights for improving flash drought monitoring and early warning.

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Yang Yang, X. San Liang, Bo Qiu, and Shuiming Chen

Abstract

Previous studies have found that the decadal variability of eddy kinetic energy (EKE) in the upstream Kuroshio Extension is negatively correlated with the jet strength, which seems counterintuitive at first glance because linear stability analysis usually suggests that a stronger jet would favor baroclinic instability and thus lead to stronger eddy activities. Using a time-varying energetics diagnostic methodology, namely, the localized multiscale energy and vorticity analysis (MS-EVA), and the MS-EVA-based nonlinear instability theory, this study investigates the physical mechanism responsible for such variations with the state estimate from the Estimating the Circulation and Climate of the Ocean (ECCO), Phase II. For the first time, it is found that the decadal modulation of EKE is mainly controlled by the barotropic instability of the background flow. During the high-EKE state, violent meanderings efficiently induce strong barotropic energy transfer from mean kinetic energy (MKE) to EKE despite the rather weak jet strength. The reverse is true in the low-EKE state. Although the enhanced meander in the high-EKE state also transfers a significant portion of energy from mean available potential energy (MAPE) to eddy available potential energy (EAPE) through baroclinic instability, the EAPE is not efficiently converted to EKE as the two processes are not well correlated at low frequencies revealed in the time-varying energetics. The decadal modulation of barotropic instability is found to be in pace with the North Pacific Gyre Oscillation but with a time lag of approximately 2 years.

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Weijing Chen, Chunlin Huang, Zong-Liang Yang, and Ying Zhang

Abstract

Data assimilation provides a practical way to improve the accuracy of soil moisture simulation by integrating a land surface model and satellite data. This study establishes a multisource remote sensing data assimilation framework by incorporating a simultaneous state and parameter estimation method to acquire an accurate estimation of the soil moisture over the Tibetan Plateau. The brightness temperature of the Advanced Microwave Scanning Radiometer 2 (AMSR2) is directly assimilated into the coupled system of the Common Land Model (CoLM) and a microwave radiative transfer model (RTM) to improve the soil moisture simulation. The Moderate Resolution Imaging Spectroradiometer (MODIS) land surface temperature product and the Beijing Normal University (BNU) leaf area index product are employed to not only improve the estimation of temperature and vegetation variables from the CoLM, but they also provide more accurate background information for the RTM during the brightness temperature assimilation. In situ measurements from the Naqu network are used to evaluate the results. The model simulation showed an obvious underestimation of soil moisture and overestimation of soil temperature, which was alleviated by the assimilation experiments, particularly in the shallow soil layers. The estimated parameters also showed advantages in the soil moisture simulation when compared with the default parameters. The assimilation experiment presents promising results in the combination of model and multisource remote sensing data for estimating soil moisture over the complex mountainous region in Tibet.

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Liang Wu, Hongjie Zhang, Jau-Ming Chen, and Tao Feng

Abstract

The present study investigates the impact of various central Pacific (CP) and eastern Pacific (EP) warming on tropical cyclones (TCs) over the western North Pacific (WNP) for the period 1948–2015 based on observational and reanalysis data. Four distinctly different forms of tropical Pacific warming are identified to examine different impacts of locations and intensity of tropical Pacific warming on the WNP TCs. It is shown that WNP TC activity related to ENSO shows stronger sensitivity to the intensity of CP SST warming. The locations of TC genesis in an extreme EP El Niño featuring concurrent strong CP and EP warming (CEPW) display a notable southeastward shift that is generally similar to the CP El Niño featuring CP warming alone (CPW). These influences are clearly different from the effects of moderate EP El Niño associated with EP warming alone (EPW). The above influences of Pacific warming on TCs possibly occur via atmospheric circulation variability. Anomalous convection associated with CP SST warming drives anomalous low-level westerlies away from the equator as a result of a Gill-type Rossby wave response, leading to an enhanced broad-zone, eastward-extending monsoon trough (MT). An anomalous Walker circulation in response to EP SST warming drives an increase in anomalous equatorial westerlies over the WNP, leading to a narrow-zone, slightly equatorward shift of the eastward-extending MT. These changes in the MT coincide with a shift in large-scale environments and synoptic-scale perturbations, which favor TC genesis and development. In addition, during weaker EP SST warming (WEPW) with similar intensity to CPW, local SST forcing exhibits primary control on WNP TCs and atmospheric circulation.

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