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Kun Yang and Toshio Koike

Abstract

A scheme was proposed by Zhang et al. to estimate soil water content from soil temperature measurements by using an adaptive Kalman filter. Their scheme is based on the fact that soil heat capacity and thermal conductivity are a monotonic function of soil water content. However, thermal diffusivity, a more critical thermal parameter in such an estimation, is not a monotonic function of soil water content in most cases. This could result in multiple solutions in some cases when deriving soil water content from soil temperatures.

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Kun Yang, Nobuyuki Tamai, and Toshio Koike

Abstract

Turbulent exchange between the surface and the atmosphere strongly depends on the stability of the surface layer. If surface radiometric temperature, rather than aerodynamic temperature, is used to parameterize the surface turbulent fluxes, the solution of the stability parameter is related to the thermal roughness length z T, which is generally not identical to the aerodynamic roughness length z 0. This note derives the exact solution of the stability parameter equation for a stable surface layer and proposes an approximate analytical solution for an unstable surface layer. The solution can improve the computational efficiency of flux parameterization and is applicable in a wide range of z/z 0 (50–104) and z 0/z T (from less than 1 to greater than 104).

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Achim Stössel, Kun Yang, and Seong-Joong Kim

Abstract

An earlier estimate regarding the possible impact of sea ice on deep-ocean water mass properties and the global thermohaline circulation in a coupled sea ice–ocean general circulation model (OGCM) is updated. Compared to the earlier application, the main upgrade is a subgrid-scale plume-convection parameterization that replaces conventional grid-cell-wide convective adjustment. The different treatment of convection leads to some noticeable differences in some of the repeated sensitivity experiments. For example, in an experiment where sea ice salinity is assumed to be that of the upper ocean, thus neglecting the primary effect of sea ice formation and melting on the ocean's buoyancy forcing, Antarctic Bottom Water formation comes essentially to a halt, the global deep-ocean properties and thermohaline circulation thus being almost solely determined by North Atlantic Deep Water. The much weaker impact in the earlier estimate turns out to be mainly due to excessive open-ocean convection in the Southern Ocean, rendering that region susceptible to the open-ocean polynya mode. The associated melting in such polynyas leads to an enhancement of convection in a salty sea ice experiment, thus obscuring the effect of neglected brine release in coastal polynyas. Besides underscoring the necessity of a careful treatment of sea ice and convection in the Southern Ocean of a global OGCM, this study indicates that new-ice formation around Antarctica has a much larger effect on global deep-ocean properties and circulation than previously estimated.

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Bingyi Wu, Kun Yang, and Jennifer A. Francis

Abstract

Through both observational analyses and simulation experiments, this study investigates the intraseasonal evolution of atmospheric circulation anomalies associated with a persistent cold event in the Asian continent during late January–early February 2012, and the possible association with Arctic sea ice loss and Arctic atmospheric circulation during the preceding summer. The results suggest that the northeastern Pacific–Aleutian region and central Eurasia are two critical areas where the atmospheric circulation evolution contributed to the development of this cold event. A persistent increase in sea level pressure (SLP) over the Aleutian region was a predominant feature prior to the cold event, and then decreasing SLP over this region was concurrent with both occurrence of a polar blocking high aloft and rapid strengthening of the Siberian high, triggering outbreaks of Arctic air over the Asian continent. Consequently, the influence of the Aleutian region on this cold event (i.e., the downstream effect of the atmospheric circulation) played a critical role. Simulation experiments demonstrate that Arctic atmospheric circulation conditions in the summer of 2011 significantly enhanced a negative feedback of Arctic sea ice loss on atmospheric circulation over the Aleutian region and central Eurasia during the ensuing wintertime, which could have led to the favorable atmospheric circulation that facilitated the occurrence of cold events resembling the one in 2012. This study also implies that the Aleutian low and disturbances in the midlatitudes over the northeastern Pacific may provide precursors that could increase skill in predicting the intraseasonal evolution of extreme cold events over Eurasia.

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Xingwen Jiang, Yueqing Li, Song Yang, Kun Yang, and Junwen Chen

Abstract

The impacts of summer atmospheric heat source over the Tibetan Plateau (TP) on regional climate variation have attracted extensive attention. However, few studies have focused on possible causes of the interannual variation of atmospheric heat source over the TP. Total heat (TH) is generally composed of three components: surface sensible heat, latent heat release of condensation (LH), and radiative convergence. In this study, it is found that interannual variation of summer TH is dominated by LH in the central and eastern TP. The atmospheric circulation patterns associated with the TH over the TP in June are different from those in July and August. Large TH is accompanied by a cyclone centered over the South China Sea in June, which is replaced by an anticyclone in July and August. The interannual variation of July–August TH over the central and eastern TP is significantly affected by convection around the western Maritime Continent (WMC) that modulates the LH over the southeastern TP. Enhanced WMC convection induces an anticyclone to the south of the TP, which favors water vapor transport to the southeastern TP and thus an increase in precipitation. Enhanced convection over the southeastern TP may exert a positive feedback on local precipitation through pumping more water vapor from the southern boundary. Both observations and model simulations indicate that the enhanced WMC convection can induce the anticyclone to the south of the TP and convection–circulation is important for maintenance of the anticyclone.

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Yingying Chen, Kun Yang, Degang Zhou, Jun Qin, and Xiaofeng Guo

Abstract

Daytime land surface temperatures in arid and semiarid regions are typically not well simulated in current land surface models (LSMs). This study first evaluates the importance of parameterizing the thermal roughness length (z 0h) to model the surface temperature (T sfc) and turbulent sensible heat flux (H) in arid regions. Six schemes for z 0h are implemented into the Noah LSM, revealing the high sensitivity of the simulations to its parameterization. Comparisons are then performed between the original Noah LSM and a revised version with a novel z 0h scheme against observations at four arid or semiarid sites, including one in Arizona and three in western China. The land they cover is sparse grass or bare soil. The results indicate that the original Noah LSM significantly underestimates T sfc and overestimates H in the daytime, whereas the revised model can simulate well both T sfc and H simultaneously. The improved version benefits from the successful modeling of the diurnal variation of z 0h, which the original model cannot produce.

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Kun Yang, Xiaofeng Guo, Jie He, Jun Qin, and Toshio Koike

Abstract

Atmospheric heating over the Tibetan Plateau (TP) enhances the Asian summer monsoon. This study presents a state-of-the-art estimate of the heating components and their total over the TP, with the aid of high-accuracy experimental data, an updated land surface model, and carefully selected satellite data.

The new estimate differs from previous estimates in three aspects: 1) different seasonality—the new estimation shows the maximum total heat source occurs in July (the mature period of the monsoon), rather than in the previously reported month of May or June (around the onset of the monsoon), because previous studies greatly overestimated radiative cooling during the monsoon season [June–August (JJA)]; 2) different regional pattern—the eastern TP exhibits stronger heating than the western TP in summer, whereas previous studies gave either an opposite spatial pattern because of overestimated sensible heat flux over the western TP or an overall weaker heat source because of overestimated radiative cooling; and 3) different trend—sensible heat, radiative convergence, and the total heat source have decreased since the 1980s, but their weakening trends were overestimated in a recent study. These biases in previous studies are due to fairly empirical methods and data that were not evaluated against experimental data.

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Kun Yang, Toshio Koike, Ichirow Kaihotsu, and Jun Qin

Abstract

This study examines the capability of a new microwave land data assimilation system (LDAS) for estimating soil moisture in semiarid regions, where soil moisture is very heterogeneous. This system assimilates the Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E) 6.9- and 18.7-GHz brightness temperatures into a land surface model (LSM), with a radiative transfer model as an observation operator. To reduce errors caused by uncertainties of system parameters, the LDAS uses a dual-pass assimilation algorithm, with a calibration pass to estimate major model parameters from satellite data and an assimilation pass to estimate the near-surface soil moisture. Validation data of soil moisture were collected in a Mongolian semiarid region. Results show that (i) the LDAS-estimated soil moistures are comparable to areal averages of in situ measurements, though the measured soil moistures were highly variable from site to site; (ii) the LSM-simulated soil moistures show less biases when the LSM uses LDAS-calibrated parameter values instead of default parameter values, indicating that the satellite-based calibration does contribute to soil moisture estimations; and (iii) compared to the LSM, the LDAS produces more robust and reliable soil moisture when forcing data become worse. The lower sensitivity of the LDAS output to precipitation is particularly encouraging for applying this system to regions where precipitation data are prone to errors.

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Kun Yang, Jun Qin, Xiaofeng Guo, Degang Zhou, and Yaoming Ma

Abstract

To clarify the thermal forcing of the Tibetan Plateau, long-term coarse-temporal-resolution data from the China Meteorological Administration have been widely used to estimate surface sensible heat flux by bulk methods in many previous studies; however, these estimates have seldom been evaluated against observations. This study at first evaluates three widely used bulk schemes against Tibet instrumental flux data. The evaluation shows that large uncertainties exist in the heat flux estimated by these schemes; in particular, upward heat fluxes in winter may be significantly underestimated, because diurnal variations of atmospheric stability were not taken into account. To improve the estimate, a new method is developed to disaggregate coarse-resolution meteorological data to hourly according to statistical relationships derived from high-resolution experimental data, and then sensible heat flux is estimated from the hourly data by a well-validated flux scheme. Evaluations against heat flux observations in summer and against net radiation observations in winter indicate that the new method performs much better than previous schemes, and therefore it provides a robust basis for quantifying the Tibetan surface energy budget.

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Jun Qin, Kun Yang, Shunlin Liang, and Wenjun Tang

Abstract

Photosynthetically active radiation (PAR) is absorbed by plants to carry out photosynthesis. Its estimation is important for many applications such as ecological modeling. In this study, a broadband transmittance scheme for solar radiation at the PAR band is developed to estimate clear-sky PAR values. The influence of clouds is subsequently taken into account through sunshine-duration data. This scheme is examined without local calibration against the observed PAR values under both clear- and cloudy-sky conditions at seven widely distributed Surface Radiation Budget Network (SURFRAD) stations. The results indicate that the scheme can estimate the daily mean PAR at these seven stations under all-sky conditions with root-mean-square error and mean bias error values ranging from 6.03 to 6.83 W m−2 and from −2.86 to 1.03 W m−2, respectively. Further analyses indicate that the scheme can estimate PAR values well with globally available aerosol and ozone datasets. This suggests that the scheme can be applied to regions for which observed aerosol and ozone data are not available.

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