Influences of snow event on energy balance over temperate meadow in dormant season based on eddy covariance measurements

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Summary

Based on the eddy flux and meteorological measurements, we analyzed variation of the components of energy balance before, during and after the snow coverage in dormant season over temperate meadow. The results showed that the energy balance ratio EBR was 0.76, lowest in the fresh snow phase and positively correlated with friction velocity u. Furthermore, the energy balance closure error had a diurnal cycle. The radiation partition and energy balance changed in the presence of the snow cover. The surface albedo was high during snow coverage (maximum in the fresh snow phase) and low in the snow-free period (including pre-snow and snow-melted phases). The ratio of net radiation Rn to solar radiation Q was higher in the snow-melted phase, and lower in the fresh snow phase, so did the peaks in diurnal courses of the energy fluxes (Rn, latent heat flux LE, sensible heat flux H, and soil and storage heat flux G + S). The daily-integrated value of H increased followed by Rn in the snow-melting phase, LE and G + S increased quickly followed by Rn in the snow-melted phase. Daily average Bowen ratio β was large in the snow-melting phase and low in the snow-melted phase, indicating that more energy partitioning of Rn into H in the snow-melting phase but LE in the snow-melted phase.

Research highlights

► We analyze variation of components of energy balance with and without snow cover. ► Peaks in diurnal courses of energy fluxes differ in different phases. ► Daily-integrated H increases followed by Rn in the snow-melting phase. ► Daily-integrated LE and G + S increase followed by Rn in the snow-melted phase. ► Bowen ratio is large in the snow-melting phase and low in the snow-melted phase.

Introduction

The partitioning of available energy, latent heat flux and sensible heat flux, are important in meteorological, hydrological and ecological analyses (Chen et al., 2009). Therefore, studying energy balance in a variety of ecosystems is better to understand the interactions between energy fluxes and other earth system processes.

The Inner Mongolia grassland constitutes an important component of Chinese grasslands and is one of the largest remaining ecosystems in the world (Kawamura et al., 2005). Approximately 11% of the grassland is meadow and it is the most productive type of rangeland for supporting livestock production.

However, most investigates in meadow have focused on the ‘growing season’ (Rosset et al., 2001, Rana et al., 2007), studies in dormant season are few. Furthermore, recent studies showed that significant changes in climatic conditions occurred during dormant season (Liu and Chen, 2000, Schwartz and Reiter, 2000, Yao et al., 2000, Hodgkins et al., 2003, Wolfe et al., 2005). It seems likely that these changes such as global warming would reduce snow event, decrease snow cover depth and duration time, and affect the energy exchange between meadow ecosystem and the atmosphere (Nijssen et al., 2001, Linden, 2002, Gu et al., 2005).

Snow is a limiting factor for several surface processes (Bruland et al., 2001), its properties of a porous medium and poor heat conductor (Gray, 1970) and behavior firmly affect the climatic system especially the atmospheric energy exchange and the thermodynamic characteristics of the surfaces. The depth, duration time and purity of the snow have intensively changed after the snow event, hence, the heat fluxes between the atmosphere and the ground are strongly affected (Gustafsson et al., 2001).

Since there is great difference in energy partition before and after the snow event, comparison analysis of energy fluxes over meadow with and without snow cover is especially necessary. However, investigations generally focus on forests with snow-covered (Turnipseed et al., 2002, Yao et al., 2008) or meadows on high arctic mountains (Bruland et al., 2001, Gu et al., 2005) or meadows with snow-free (Wever et al., 2002, Hao et al., 2007, Chen et al., 2009), none is concerned with temperate snow-covered meadow in dormant season. Thus, in this paper, we will show the energy balance and illustrate the energy partition before, during and after the snow coverage in dormant season in eastern Inner Mongolia of China based on the eddy covariance and meteorological measurements.

Section snippets

Site description

The experiment was performed in the meadow of Horqin Grassland, in the territory of Horqinzuoyihou County, eastern Inner Mongolia, China. A measurement tower of 4 m height was set up in a site at 43°17′37″N, 122°16′42″E, 203 m ALT. The terrain of the field is flat and dunes are distributed several kilometers away around.

A homogeneous mixture of herbaceous vegetation stretching outwards to the surrounding dune area is mainly composed of Cyperaceae Carex inch (Carex duriuscula), Gramineae Chloris

Eddy covariance method

The open-path eddy covariance (EC) system was used to measure the energy fluxes of latent heat flux LE and sensible heat flux H. Virtual temperature and water vapor measurements were converted to true air temperature (T) and the specific humidity (q). Average turbulent fluxes were calculated online after each 30 min interval using the calculation software designed by Campbell Scientific Inc. according to the time series of vertical wind velocity, temperature, and water vapor density.

LE and H

Meteorological conditions during the measurements

Table 2 showed the mean values of some meteorological factors in the four phases during measurements, including daily total incident photosynthetic active radiation (PAR), daily averages of wind speed, air temperature at 1.5 m height (Ta), ground surface temperature (Ts−0), soil temperature (Ts−5) at 5 cm depth. It can be seen that mean daily total incident PAR in the snow-melted phases was the largest (27.0 mol m−2 d−1), while in the pre-snow phase (12.1 mol m−2 d−1) was smaller than that in the

Conclusions

The energy budget ratio EBR measured over flat temperate meadow in dormant season with 86 snow-covered days was 0.75, which was at middle compared with FLUXNET and ChinaFLUX data. Snow cover and friction velocity u* resulted in the lowest EBR in the fresh snow phase. A diurnal cycle existed in the energy balance closure. With the increase of the snow coverage, the minimum value of the diurnal curve in different phases was getting larger and its appeared time was lagging during diurnal variation.

Acknowledgments

The research was funded by National Natural Science Foundation of China (40875069) and National Key Technologies Research and Development Program (2006BAC01A12, 2006BAD03A0502).

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