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The Automatic Weather Stations used in this study and their average tilt angles (β). Stations are separated into four groups based on their latitudes and altitudes.
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Surface melt and mass loss of the Greenland Ice Sheet may play crucial roles in global climate change due to their positive feedbacks and large fresh water storage. With few other regular meteorological observations available in this extreme environment, measurements from Automatic Weather Stations (AWS) are the primary data source for studying sur...
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... the diurnal phase. An offset of this amount in the diurnal phase of shortwave upwelling radiation could be caused by highly irregular topography, specular reflection or time logger error, which cannot be corrected by RIGB. The remaining number of stations is 32, of which 13 stations are from GC-Net, one from K-transect and 18 from PROMICE 5 ( Fig. 1). The radiative flux from these datasets is hourly average. We synchronize all three datasets to account the fact that the time stamp of GC-Net and K-transect is half an hour after the interval mid-point (i.e., data stamped as 8 a.m. represent the average from 7 to 8 a.m.); the one of PROMICE is half an hour before the interval ...
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... to the cloud radiative properties, CERES estimates are reasonably accurate (Minnis et al., 2011). In the Arctic, the average difference between the in situ ground-measured and CERES cloud fraction 25 is ∼ 0.15 ( Minnis et al., 2008). The effect of cloud fraction on the insolation adjustment depends on both the tilt angle and tilt direction (Fig. 10). The adjustment at local solar TCD 9,2015 noon is largest when the station tilts to the North (a w = 180 • ) or South (a w = 0 • ). The maximum of daily average turns clockwise, e.g., to a w = 30 • and a w = −150 • when the tilt angle (β) is 10 • . The cloud fraction uncertainty of 0.15 can cause an uncertainty in insolation adjustment ...
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... clockwise, e.g., to a w = 30 • and a w = −150 • when the tilt angle (β) is 10 • . The cloud fraction uncertainty of 0.15 can cause an uncertainty in insolation adjustment up to 7.5 W m −2 when cloud fraction is close to 0. The effect becomes smaller, when cloud fraction is close to 1. The tilt correction is larger as the 5 station tilts more (Fig. 10b). In 90 % of the station-months we used, the tilt angles are less than 10 • , 95 % less than 15 • . Therefore, the uncertainty in insolation adjustment caused by the uncertainty in cloud fraction should be well under 10 W m −2 , considering an average cloud fraction of 0.81 in the Arctic during summertime (Vavrus et al., 2008). ...
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Surface melt and mass loss of the Greenland Ice Sheet may play crucial roles in global climate change due to their positive feedbacks and large fresh-water storage. With few other regular meteorological observations avail-able in this extreme environment, measurements from automatic weather stations (AWS) are the primary data source for studying su...
Plain Language Summary
Much of the surface of the Greenland Ice Sheet is covered in a porous layer of old snow, known as firn. However, in some areas, surface melt has refrozen to form thick layers of solid ice in the upper few meters of this porous layer. It is generally assumed that once these ice slabs form, the firn can no longer absorb meltwat...
Plain Language Summary
The Greenland ice sheet is covered by a layer of old, porous snow called firn, which has the capacity to prevent surface melt runoff. The firn can however, form thick ice slabs or store large amounts of water underground. These change the fate of surface melt by respectively enhancing runoff and draining water to the ice shee...
Citations
... Large SZA affects the accuracy of both VSIA and PROMICE measurements. AWS sensors suffer from the intrinsic cosine response error at large solar zenith angles, which is reported to reach a maximum of 8% at a solar zenith angle of 80 • [55]. Even for solar zenith angles less than 75 • , we should expect a cosine response error around 5% [56], which can even be amplified by the riming. ...
... First, the ground measurement data does not represent the "true" value considering the big scale gap between the ground point measurement and the satellite pixel retrieval. Second, the measurements from the flux instruments contain certain uncertainty [55], due to the tilting/leveling errors and the cosine-response error of the instruments. Third, the seawater component is absent in this evaluation. ...
Ice albedo feedback amplifies climate change signals and thus affects the global climate. Global long-term records on sea-ice albedo are important to characterize the regional or global energy budget. As the successor of MODIS (Moderate Resolution Imaging Spectroradiometer), VIIRS (Visible Infrared Imaging Radiometer Suite) started its observation from October 2011 on S-NPP (Suomi National Polar-orbiting Partnership). It has improved upon the capabilities of the operational Advanced Very High Resolution Radiometer (AVHRR) and provides observation continuity with MODIS. We used a direct estimation algorithm to produce a VIIRS sea-ice albedo (VSIA) product, which will be operational in the National Oceanic and Atmospheric Administration’s (NOAA) S-NPP Data Exploration (NDE) version of the VIIRS albedo product. The algorithm is developed from the angular bin regression method to simulate the sea-ice surface bidirectional reflectance distribution function (BRDF) from physical models, which can represent different sea-ice types and vary mixing fractions among snow, ice, and seawater. We compared the VSIA with six years of ground measurements at 30 automatic weather stations from the Programme for Monitoring of the Greenland Ice Sheet (PROMICE) and the Greenland Climate Network (GC-NET) as a proxy for sea-ice albedo. The results show that the VSIA product highly agreed with the station measurements with low bias (about 0.03) and low root mean square error (RMSE) (about 0.07) considering the Joint Polar Satellite System (JPSS) requirement is 0.05 and 0.08 at 4 km scale, respectively. We also evaluated the VSIA using two datasets of field measured sea-ice albedo from previous field campaigns. The comparisons suggest that VSIA generally matches the magnitude of the ground measurements, with a bias of 0.09 between the instantaneous albedos in the central Arctic and a bias of 0.077 between the daily mean albedos near Alaska. The discrepancy is mainly due to the scale difference at both spatial and temporal dimensions and the limited sample size. The VSIA data will serve for weather prediction applications and climate model calibrations. Combined with the historical observations from MODIS, current S-NPP VIIRS, and NOAA-20 VIIRS observations, VSIA will dramatically contribute to providing high-accuracy routine sea-ice albedo products and irreplaceable records for monitoring the long-term sea-ice albedo for climate research.
... Therefore, it is often a fundamental step to correct the bias field before performing quantitative analysis of the image data. Methods of bias field correction can be divided into two groups: prospective methods [1,2] and retrospective methods [3,4]. Prospective methods aim to avoid intensity inhomogeneity by using the shim techniques, special imaging sequences, or special hardware. ...
... where |Ω| is the number of pixels in ∩ Ω . By using maximum a posteriori probability and Bayes' rule [9], the local Gaussian distribution fitting energy can be described as follows: (4) where are the positive constants and , ( ( )) is the probability density in region ∩ Ω , which is defined as ...
Image segmentation has always been a considerable challenge in image analysis and understanding due to the intensity inhomogeneity, which is also commonly known as bias field. In this paper, we present a novel region-based approach based on local entropy for segmenting images and estimating the bias field simultaneously. Firstly, a local Gaussian distribution fitting (LGDF) energy function is defined as a weighted energy integral, where the weight is local entropy derived from a grey level distribution of local image. The means of this objective function have a multiplicative factor that estimates the bias field in the transformed domain. Then, the bias field prior is fully used. Therefore, our model can estimate the bias field more accurately. Finally, minimization of this energy function with a level set regularization term, image segmentation, and bias field estimation can be achieved. Experiments on images of various modalities demonstrated the superior performance of the proposed method when compared with other state-of-the-art approaches.
... 200 m 2 . Between January and March 2011, high ablation rates caused tilting of the AWS mast which can yield errors in measured albedo due to the underestimation of SW↓ (Bogren et al., 2015;Wang et al., 2015). To account for this as well as the effect of surface slope, incoming shortwave radiation data were adjusted using the tilt and slope correction method developed by van As (2011). ...
In New Zealand, direct measurements of mass balance are sparse due to the inaccessibility of glaciers in the Southern Alps and the logistical difficulties associated with maintaining a mass balance record. In order to explore the benefit of remotely sensed imaging to monitor mass balance in the Southern Alps, this research assesses the relationship between measurements of glacier surface albedo derived from MODerate resolution Imaging Spectroradiometer (MODIS) and mass balance observations using the glaciological method on Brewster Glacier over the 2005–2013 period. We confirm that minimum glacier-wide albedo is a reliable predictor for annual mass balance in this maritime environment (R2 = 0.93). Furthermore, we show that regular monitoring of glacier-wide albedo enables a new metric of winter accumulation to be derived, namely the cumulative winter albedo, that is found to correlate strongly with winter mass balance (R2 = 0.88), thus enabling the reconstruction of separate winter and summer mass balance records. This allows the mass balance record for Brewster Glacier to be extended back to the start of MODIS observations in 2000 and to confirm that the annual balance of Brewster Glacier is largely controlled by summer balance (R2 = 92 %). An application of the extended record is proposed whereby the relationship between mass balance and the photographic record of the end-of-summer snowline altitude is assessed. This allowed the annual balance record of Brewster Glacier to be reconstructed over the period 1977–2013, thus providing the longest record of mass balance for a glacier in New Zealand. Over the 37-year period, our results show that Brewster Glacier gained significant mass of up to 14.5 ± 2.7 m w.e. by 2007. This gain was offset by a marked shift toward negative balances after 2008, yielding a loss of 5.1 ± 1.2 m w.e., or 35 % of the gain accumulated over the previous 30 years. The good correspondence between mass balance of Brewster Glacier and the phase of the Pacific (Inter-)Decadal Oscillation (PDO/IPO) associated with the fast terminus retreat observed between 1978 and 1998 strongly suggests that observed mass gain of Brewster Glacier since 1977 is only offsetting a longer sequence of dominantly negative balances.
... Some of these studies are listed in Table 1. The tilt correction methods include correction for level errors during site visits (Stroeve et al., 2001); averaging over 24 h (Stroeve et al., 2013); correction of direct beam contribution to the irradiance (Van As, 2011); model for tilt correction based on independent leveled measurements (Weiser et al., 2015); retrospective, iterative, geometry-based (RIGB) analysis to retrieve tilt and rotation by comparing simulations and measurements for temporal shifts (Wang et al., 2015). ...
... Using inclinometer and compass information Van As (2011) tilt-corrected the direct component of the downwelling shortwave radiation. Wang et al. (2015) recently presented a retrospective iterative geometry-based tilt correction method. For cloudless sky measurements the tilt and rotation angles are estimated by finding the modeled insolation for various tilt and rotation angles that best agrees with the measured insolation. ...
... The estimated tilt and rotation angles may subsequently be used to correct both cloudless and cloudy measurement data. Wang et al. (2015) show that this tilt correction method gives lower biases both for unadjusted measurements and also for measurements tilt-adjusted using inclinometer information. This method requires no extra measurements of tilt and orientation to be made. ...
We have evaluated the magnitude and makeup of error in cryospheric radiation observations due to small sensor misalignment in in-situ measurements of solar irradiance. This error is examined through simulation of diffuse and direct irradiance arriving at a detector with a cosine-response foreoptic. Emphasis is placed on assessing total error over the solar shortwave spectrum from 250 to 4500 nm, as well as supporting investigation over other relevant shortwave spectral ranges. The total measurement error introduced by sensor tilt is dominated by the direct component. For a typical high latitude albedo measurement with a solar zenith angle of 60°, a sensor tilted by 1, 3, and 5° can respectively introduce up to 2.6, 7.7, and 12.8 % error into the measured irradiance and similar errors in the derived albedo. Depending on the daily range of solar azimuth and zenith angles, significant measurement error can persist also in integrated daily irradiance and albedo.
