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

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.