. Extensive and rigorous model intercomparison is of great
importance before model application due to the uncertainties in current land
surface models (LSMs). Without considering the uncertainties in forcing data
and model parameters, this study designed an ensemble of 55 296 experiments
to evaluate the Noah LSM with multi-parameterization
(Noah-MP) for snow cover events (SCEs), soil temperature (ST) and soil
liquid water (SLW) simulation, and investigated the sensitivity of
parameterization schemes at a typical permafrost site on the Qinghai–Tibet
Plateau (QTP). The results showed that Noah-MP systematically overestimates snow
cover, which could be greatly resolved when adopting the sublimation from
wind and a semi-implicit snow/soil temperature time scheme. As a result of the
overestimated snow, Noah-MP generally underestimates ST, which is mostly
influenced by the snow process. A systematic cold bias and large uncertainties
in soil temperature remain after eliminating the effects of snow,
particularly in the deep layers and during the cold season. The combination
of roughness length for heat and under-canopy (below-canopy) aerodynamic resistance
contributes to resolving the cold bias in soil temperature. In addition,
Noah-MP generally underestimates top SLW. The runoff and groundwater (RUN) process dominates the SLW
simulation in comparison to the very limited impacts of all other physical
processes. The analysis of the model structural uncertainties and
characteristics of each scheme would be constructive to a better
understanding of the land surface processes in the permafrost regions of the
QTP as well as to further model improvements towards soil hydrothermal regime modeling
using LSMs.

/pdf/assessing-the-simulated-soil-hydrothermal-regime-of-the-3zasy90jbo.pdf

Assessing the simulated soil hydrothermal regime of the active layer from the Noah-MP land surface model (v1.1) in the permafrost regions of the Qinghai–Tibet Plateau

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2020JD032787

An enhanced MOD16 evapotranspiration model for the Tibetan Plateau during the unfrozen season

Spin‐up is essential to provide initial conditions for land surface models (LSM) when they cannot be given reliably as in the application to regional permafrost change studies. In this study, the impacts of spin‐up strategy including total spin‐up length and cycling scheme on modeling of permafrost dynamics on the Qinghai‐Tibet Plateau (QTP) were evaluated through two groups of experiments using a modified Noah LSM. The first group aims to test different total spin‐up lengths and the second group for different cycling schemes. The results show that the presence of permafrost prolongs the convergence of the model. Vertically, the slowest convergence is observed at the permafrost table. The insufficiency of total spin‐up length is prone to underestimate permafrost area and overestimate the degradation rate. Different cycling schemes considerably affect the resulting initial thermal fields and result in degradation rates with a difference of 3.37 × 103 km2/a on the QTP, which exceeds the difference (2.92 × 103 km2/a) in the degradation rates reported in existing studies. The multi‐year cycling scheme is generally preferred, but overlong cycle length should be avoided to prevent the introduction of climate change trends in the spin‐up period. We recommend a spin‐up strategy of a 500‐year cycling with the first 5‐ to 10‐year of forcing for modeling permafrost on the QTP with the Noah LSM. Our findings highlight the importance of the spin‐up strategy, which is usually neglected in present LSM‐based permafrost modeling studies.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1029/2021MS002750

On the Spin‐Up Strategy for Spatial Modeling of Permafrost Dynamics: A Case Study on the Qinghai‐Tibet Plateau

Changes in the freeze–thaw cycles of shallow soil have important consequences for surface and subsurface hydrology, land–atmosphere energy and moisture interaction, carbon exchange, and ecosystem diversity and productivity. This work examines the shallow soil freeze–thaw cycle on the Tibetan Plateau (TP) using in–situ soil temperature observations in 0–20 cm soil layer during July 1982–June 2017. The domain and layer averaged beginning frozen day is November 18 and delays by 2.2 days per decade; the ending frozen day is March 9 and advances by 3.2 days per decade; the number of frozen days is 109 and shortens by 5.2 days per decade. Altitude and latitude combined could explain the spatial patterns of annual mean freeze–thaw status well. Stations located near 0 °C contour line experienced dramatic changes in freeze–thaw cycles as seen from subtropical mountain coniferous forest in the southern TP. Soil completely freezes from surface to 20-cm depth in 15 days while completely thaws in 10 days on average. Near-surface soil displays more pronounced changes than deeper soil. Surface air temperature strongly influences the shallow soil freeze–thaw status but snow exerts limited effects. Different thresholds in freeze–thaw status definition lead to differences in the shallow soil freeze–thaw status and multiple-consecutive-day approach appears to be more robust and reliable. Gridded soil temperature products could resolve the spatial pattern of the observed shallow soil freeze–thaw status to some extent but further improvement is needed.

/pdf/on-the-freeze-thaw-cycles-of-shallow-soil-and-connections-bchd90ksam.pdf

On the freeze–thaw cycles of shallow soil and connections with environmental factors over the Tibetan Plateau

Current state and past changes in frozen ground at the Third Pole: A research synthesis

The freeze–thaw changes of seasonally frozen ground (SFG) are an important indicator of climate change. Based on observed daily freeze depth of SFG from meteorological stations on the Tibetan Plateau (TP) from 1960 to 2014, the spatial–temporal characteristics and trends in SFG were analyzed, and the relationships between them and climatic and geographical factors were explored. Freeze–thaw changes of SFG on a regional scale were assessed by multiple regression functions. Results showed multiyear mean maximum freeze depth, freeze–thaw duration, freeze start date, and thaw end date that demonstrate obvious distribution characteristics of climatic zones. A decreasing trend in maximum freeze depth and freeze–thaw duration occurred on the TP from 1960 to 2014. The freeze start date has been later, and the thaw end date has been significantly earlier. The freeze–thaw changes of SFG significantly affected by soil hydrothermal conditions on the TP could be assessed by elevation and latitude or by air temperature and precipitation, due to their high correlations. The regional average of maximum freeze depth and freeze–thaw duration caused by climatic and geographical factors were larger than those averaged using meteorological station data because most stations are located at lower altitudes. Maximum freeze depth and freeze–thaw duration have decreased sharply since 2000 on the entire TP. Warming and wetting conditions of the soil resulted in a significant decrease in maximum freeze depth and freeze–thaw duration in the most area of the TP, while drying soil results in a slight increase of them in the southeast of the TP.

/pdf/freeze-thaw-changes-of-seasonally-frozen-ground-on-the-3jl37dr2me.pdf

Shihua Lyu

Papers

Freeze–Thaw Changes of Seasonally Frozen Ground on the Tibetan Plateau from 1960 to 2014