The freeze/thaw process and the surface energy budget of the seasonally frozen ground in the source region of the Yellow River

Jingyuan Wang,Siqiong Luo,Shaoying Wang,Zhaoguo Li,Zhenhua Li

doi:10.1007/s00704-019-02917-6

Abstract

The freeze/thaw process and the surface energy budget of the seasonally frozen ground in the source region of the Yellow River were investigated by using observed soil temperature and moisture and the energy flux from May 12, 2014, to May 11, 2015. Compared with the Maduo site, the starting date of the freezing process was later and the freezing depth was shallower at Maqu site. The maximum frozen depth was about 320 cm at Maduo site and 90 cm at Maqu site. The soil temperature of Maqu site was higher than of Maduo site due to lower latitude and altitude. The soil was the driest under the depth of 40 cm and 80 cm at Maduo and Maqu sites, respectively. The diurnal amplitudes of soil temperature of Maduo site were larger than of Maqu site at four freeze/thaw stages; the amplitudes were the largest in the completely thawed stage (9.19 °C and 4.35 °C) and minimal in the freezing stage (1.23 °C and 0.47 °C). The diurnal amplitudes of soil moisture of Maqu site were greater than of Maduo site at all stages. The net radiation Rn’s seasonal change was hardly influenced by the freeze/thaw process. The mean ground heat flux (G0) was negative during the freezing and completely frozen stage and positive during the thawing and completely thawed stage. During the completely thawed and frozen stages, the latent heat flux (LE) and sensible heat flux (H) predominated in the surface energy distribution, respectively. Overall, the variations of fluxes were affected by both the monsoon and freeze/thaw process of the soil layer in seasonally frozen region. The freeze/thaw process had a significant effect on the diurnal change of G0 during the freezing stage. The annual energy closure status of Maduo and Maqu sites was 0.77 and 0.58, respectively. The energy closure status was the highest during the completely thawed stage at Maduo site and during the thawing stage at Maqu site and lowest during the freezing stage among the four stages, due to the snow cover’s impact. Overall, the freeze/thaw process and the high albedo caused by snow cover had effects on the energy closure status.

Highlights

The Tibetan Plateau (TP), is the largest and most extensive plateau of the world, is known as “the roof of the world” (Ma et al 2005; Qiu 2008; Wu et al 2012)
Based on the daily minimum/maximum soil temperature, the soil freeze/thaw processes in the whole year were divided into four freeze/thaw stages including the completely frozen stage, the completely thawed stage, and the freezing and thawing stages (Guo et al 2011b)
Dividing the freeze/thaw stages of Maduo and Maqu sites with this method, we found that the freezing and thawing stages of the soil layers were generally only 1–2 days, which was inconsistent with the facts

Summary

Introduction

The Tibetan Plateau (TP), is the largest and most extensive plateau of the world, is known as “the roof of the world” (Ma et al 2005; Qiu 2008; Wu et al 2012). Because of its effects on water supplies (Guo et al 2011a), energy exchanges (Gu et al 2015), and climatecryosphere interactions in the atmospheric boundary layer (Chen et al 2008), seasonally frozen soil becomes an important portion of hydrologic and climatic variables (Duguay et al 2013; Gu et al 2015). The trends of air temperature and soil temperature and the parameters of seasonally frozen ground and permafrost have been studied in the source region of the Yellow River by many recent studies (Hu et al 2012; Jin et al 2009, 2010; Li et al 2013; Ye et al 2015). During 1981–2014, the temperatures of surface soil (0 cm), shallow layer soil (5–20 cm), and deep layer soil (40–320 cm) had been risen at rates of 0.706 °C, 0.477 °C, and 0.417 °C decade−1 in three river source regions (Luo et al 2016)

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