The role of snow in soil thermal dynamics of the arctic terrestrial ecosystems

The vast area of permanent or seasonal snow cover is an essential component of terrestrial ecosystems in northern mid-to-high latitudes (45-90°N), which has insulation effects on the soil layer beneath it. The affected soil thermal regimes will impact soil carbon dynamics. Recent observations indicate that there are substantial changes in both snow cover extent and duration due to climate change in the area. It is important to understand the insulation effect historically so as to better quantify its role in affecting ecosystem carbon dynamics under changing climate in the future. This study incorporates the snow insulation effect by introducing a snow model into an existing soil thermal model in a biogeochemistry modeling framework, the Terrestrial Ecosystem Model (TEM). The coupled model is used to evaluate the effects of snow dynamics on thermal regimes in the pan-Arctic for the period 2003-2010. Available satellite snow-cover data and site-level data are used to calibrate and evaluate the modeling system for the historical period. The study demonstrates that the revised model reproduces the top-soil layers’ thermal regime and freeze/thaw status reasonably well for the region. The study finds that the insulation effect of snow can alter soil thermal regime. The soil temperature estimations at 5cm and 20cm depths using the satellite snow data are in general 5℃ warmer in winters compared to those using the previous version of the model. There is a lag of soil cooling rate in early winter and a lag of soil warming rate in late spring. The study also finds that the insulation effect of snow can influence ground freeze/thaw status. The frozen line estimated by the revised model moves slightly southward in late spring and slightly northward in early winter. This study suggests that future analysis of soil thermal and carbon dynamics should take snow dynamics into account for the region

Long Fading Mid-Infrared Emission in Transient Coronal Line Emitters: Dust Echo of Tidal Disruption Flare

The sporadic accretion following the tidal disruption of a star by a super-massive black hole (TDE) leads to a bright UV and soft X-ray flare in the galactic nucleus. The gas and dust surrounding the black hole responses to such a flare with an echo in emission lines and infrared emission. In this paper, we report the detection of long fading mid-IR emission lasting up to 14 years after the flare in four TDE candidates with transient coronal lines using the WISE public data release. We estimate that the reprocessed mid-IR luminosities are in the range between $4\times 10^{42}$ and $2\times 10^{43}$ erg~s$^{-1}$ and dust temperature in the range of 570-800K when WISE first detected these sources three to five years after the flare. Both luminosity and dust temperature decreases with time. We interpret the mid-IR emission as the infrared echo of the tidal disruption flare. We estimate the UV luminosity at the peak flare to be 1 to 30 times $10^{44}$ erg s$^{-1}$ and for warm dust masses to be in the range of 0.05-1.3 Msun within a few parsecs. Our results suggest that the mid-infrared echo is a general signature of TDE in the gas-rich environment