Xiaona Chen

Observed contrast changes in snow cover phenology in northern middle and high latitudes from 2001–2014

Quantifying and attributing the phenological changes in snow cover are essential for meteorological, hydrological, ecological, and societal implications. However, snow cover phenology changes have not been well documented. Evidence from multiple satellite and reanalysis data from 2001 to 2014 points out that the snow end date (De) advanced by 5.11 (±2.20) days in northern high latitudes (52–75°N) and was delayed by 3.28 (±2.59) days in northern mid-latitudes (32–52°N) at the 90% confidence level. Dominated by changes in De, snow duration days (Dd) was shorter in duration by 5.57 (±2.55) days in high latitudes and longer by 9.74 (±2.58) days in mid-latitudes. Changes in De during the spring season were consistent with the spatiotemporal pattern of land surface albedo change. Decreased land surface temperature combined with increased precipitation in mid-latitudes and significantly increased land surface temperature in high latitudes, impacted by recent Pacific surface cooling, Arctic amplification and strengthening westerlies, result in contrasting changes in the Northern Hemisphere snow cover phenology. Changes in the snow cover phenology led to contrasting anomalies of snow radiative forcing, which is dominated by De and accounts for 51% of the total shortwave flux anomalies at the top of the atmosphere.

Assessment of Sea Ice Albedo Radiative Forcing and Feedback over the Northern Hemisphere from 1982 to 2009 Using Satellite and Reanalysis Data

AbstractThe decreasing surface albedo caused by continuously retreating sea ice over Arctic plays a critical role in Arctic warming amplification. However, the quantification of the change in radiative forcing at top of atmosphere (TOA) introduced by the decreasing sea ice albedo and its feedback to the climate remain uncertain. In this study, based on the satellite-retrieved long-term surface albedo product CLARA-A1 (Cloud, Albedo, and Radiation dataset, AVHRR-based, version 1) and the radiative kernel method, an estimated 0.20 ± 0.05 W m−2 sea ice radiative forcing (SIRF) has decreased in the Northern Hemisphere (NH) owing to the loss of sea ice from 1982 to 2009, yielding a sea ice albedo feedback (SIAF) of 0.25 W m−2 K−1 for the NH and 0.19 W m−2 K−1 for the entire globe. These results are lower than the estimate from another method directly using the Clouds and the Earth’s Radiant Energy System (CERES) broadband planetary albedo. Further data analysis indicates that kernel method is likely to underes...

Satellite observed changes in the Northern Hemisphere snow cover phenology and the associated radiative forcing and feedback between 1982 and 2013

Quantifying continental-scale changes in snow cover phenology (SCP) and evaluating their associated radiative forcing and feedback is essential for meteorological, hydrological, ecological, and societal purposes. However, the current SCP research is inadequate because few published studies have explored the long-term changes in SCP, as well as their associated radiative forcing and feedback in the context of global warming. Based on satellite-observed snow cover extent (SCE) and land surface albedo datasets, and using a radiative kernel modeling method, this study quantified changes in SCP and the associated radiative forcing and feedback over the Northern Hemisphere (NH) snow-covered landmass from 1982 to 2013. The monthly SCE anomaly over the NH displayed a significant decreasing trend from May to August (−0.89 × 106 km2 decade−1), while an increasing trend from November to February (0.65 × 106 km2 decade−1) over that period. The changes in SCE resulted in corresponding anomalies in SCP. The snow onset date (D o) moved forward slightly, but the snow end date (D e) advanced significantly at the rate of 1.91 days decade−1, with a 73% contribution from decreased SCE in Eurasia (EU). The anomalies in D e resulted in a weakened snow radiative forcing of 0.12 (±0.003) W m−2 and feedback of 0.21 (±0.005) W m−2 K−1, in melting season, over the NH, from 1982 to 2013. Compared with the SCP changes in EU, the SCP anomalies in North America were relatively stable because of the clearly contrasting D e anomalies between the mid- and high latitudes in this region.

Observed radiative cooling over the Tibetan Plateau for the past three decades driven by snow-cover-induced surface albedo anomaly

Seasonal snow cover on the Tibetan Plateau (TP) is a sensitive indicator of climate change. Unlike the decreasing snow cover extent and associated weakening of radiative cooling effects for the northern hemisphere during recent decades reported by previous studies, snow cover variability over the TP and its impact on the energy budget remain largely unknown. We defined the snow-cover-induced radiative forcing (SnRF) as the instantaneous perturbation to Earths shortwave radiation at the top of the atmosphere (TOA) induced by the presence of snow cover. Here, using satellite observations and a radiative kernel approach, we found slightly enhanced SnRF, i.e., a radiative cooling effect on the TP during the past three decades (1982–2014). However, this cooling effect weakened during 2001–2014 because of reduced snow cover at a rate of -0.61% decade-1 and land surface albedo at a rate of -0.72% decade-1. Changes in snow cover fraction (SCF) are highly correlated with anomalies in land surface albedo (as) over the TP both spatially and temporally. Moreover, the SnRF is closely related to the direct observation of TOA shortwave flux anomalies (R2 = 0.54, p = 0.004) over the TP during 2001–2014. Despite insignificant interannual variability in SnRF, its intra-annual variability has intensified driven mostly by enhanced SnRF during the snow accumulation season but weakened SnRF during the melt season, indicating greater energy release during the transition between accumulation and melt seasons. This may pose a great challenge to snow meltwater use and flood prediction for transboundary rivers originating from the TP, such as the Brahmaputra River basin.

Land Surface Albedo Estimation from Chinese HJ Satellite Data Based on the Direct Estimation Approach

Monitoring surface albedo at medium-to-fine resolution (<100 m) has become increasingly important for medium-to-fine scale applications and coarse-resolution data evaluation. This paper presents a method for estimating surface albedo directly using top-of-atmosphere reflectance. This is the first attempt to derive surface albedo for both snow-free and snow-covered conditions from medium-resolution data with a single approach. We applied this method to the multispectral data from the wide-swath Chinese HuanJing (HJ) satellites at a spatial resolution of 30 m to demonstrate the feasibility of this data for surface albedo monitoring over rapidly changing surfaces. Validation against ground measurements shows that the method is capable of accurately estimating surface albedo over both snow-free and snow-covered surfaces with an overall root mean square error (RMSE) of 0.030 and r-square (R2) of 0.947. The comparison between HJ albedo estimates and the Moderate Resolution Imaging Spectral Radiometer (MODIS) albedo product suggests that the HJ data and proposed algorithm can generate robust albedo estimates over various land cover types with an RMSE of 0.011–0.014. The accuracy of HJ albedo estimation improves with the increase in view zenith angles, which further demonstrates the unique advantage of wide-swath satellite data in albedo estimation.

Enhanced wintertime greenhouse effect reinforcing Arctic amplification and initial sea-ice melting

The speeds of both Arctic surface warming and sea-ice shrinking have accelerated over recent decades. However, the causes of this unprecedented phenomenon remain unclear and are subjects of considerable debate. In this study, we report strong observational evidence, for the first time from long-term (1984–2014) spatially complete satellite records, that increased cloudiness and atmospheric water vapor in winter and spring have caused an extraordinary downward longwave radiative flux to the ice surface, which may then amplify the Arctic wintertime ice-surface warming. In addition, we also provide observed evidence that it is quite likely the enhancement of the wintertime greenhouse effect caused by water vapor and cloudiness has advanced the time of onset of ice melting in mid-May through inhibiting sea-ice refreezing in the winter and accelerating the pre-melting process in the spring, and in turn triggered the positive sea-ice albedo feedback process and accelerated the sea ice melting in the summer.

Evaluation of Four Reanalysis Surface Albedo Data Sets in Arctic Using a Satellite Product

Surface albedo has been widely used in studying energy budgets and climate dynamics in the Arctic region. Previous efforts have focused on using reanalysis albedo data, but their uncertainties remain unknown. In this letter, we evaluated four popularly used reanalysis surface albedo products, namely, the European Centre for Medium-Range Weather Forecasts Interim Reanalysis (ERA-Interim), the Modern-Era Retrospective Analysis for Research and Applications (MERRA), the National Centers for Environmental Prediction Climate Forecast System Reanalysis (CFSR), and the Japanese 55-Year Reanalysis (JRA-55), over the Arctic Ocean using satellite-retrieved product (CLARA-SAL) from 1982 to 2009. Owing to the flawed parameterization scheme or problematic model inputs, reanalysis products are unable to capture both the interannual variation and long-term reduction of surface albedo in the Arctic. This results in a large bias in the decline of shortwave radiative forcing at both surface (from -11.74 to -38.25 W m-2) and top of atmosphere (from -5.35 to -20.19 W m-2). The most significant underestimation occurred in the melt season and after sea-ice melting acceleration started since 1996, in the central Arctic Basin north of 80° N, which is likely due to the failure in simulating the influence of thinning ice and decreasing snow depth. The JRA-55 albedo product outperformed the other three products, which is likely due to the employment of observed sea-ice concentration on the parameterization scheme. On the other hand, the other three reanalysis products, namely, ERA-Interim, MERRA, and CFSR, are unable to effectively track the interannual variation of surface albedo and significantly underestimate (from -0.016 to -0.021 relative to -0.048, by one-third to half) the decreasing surface albedo.