Near-surface temperature inversion during summer at Summit, Greenland, and its relation to MODIS-derived surface temperatures

Abstract

Abstract. As rapid warming of the Arctic occurs, it is imperative that climate indicators such as temperature be monitored over large areas to understand and predict the effects of climate changes. Temperatures are traditionally tracked using in situ 2 m air temperatures and can also be assessed using remote sensing techniques. Remote sensing is especially valuable over the Greenland Ice Sheet, where few ground-based air temperature measurements exist. Because of the presence of surface-based temperature inversions in ice-covered areas, differences between 2 m air temperature and the temperature of the actual snow surface (referred to as "skin" temperature) can be significant and are particularly relevant when considering validation and application of remote sensing temperature data. We present results from a field campaign extending from 8 June to 18 July 2015, near Summit Station in Greenland, to study surface temperature using the following measurements: skin temperature measured by an infrared (IR) sensor, 2 m air temperature measured by a National Oceanic and Atmospheric Administration (NOAA) meteorological station, and a Moderate Resolution Imaging Spectroradiometer (MODIS) surface temperature product. Our data indicate that 2 m air temperature is often significantly higher than snow skin temperature measured in situ, and this finding may account for apparent biases in previous studies of MODIS products that used 2 m air temperature for validation. This inversion is present during our study period when incoming solar radiation and wind speed are both low. As compared to our in situ IR skin temperature measurements, after additional cloud masking, the MOD/MYD11 Collection 6 surface temperature standard product has an RMSE of 1.0 • C and a mean bias of −0.4 • C, spanning a range of temperatures from −35 to −5 • C (RMSE = 1.6 • C and mean bias = −0.7 • C prior to cloud masking). For our study area and time series, MODIS surface temperature products agree with skin surface temperatures better than previous studies indicated, especially at temperatures below −20 • C, where other studies found a significant cold bias. We show that the apparent cold bias present in other comparisons of 2 m air temperature and MODIS surface temperature may be a result of the near-surface temperature inversion. Further investigation of how in situ IR skin temperatures compare to MODIS surface temperature at lower temperatures (below −35 • C) is warranted to determine whether a cold bias exists for those temperatures.

Deriving Arctic 2 m air temperatures over snow and ice from satellite surface temperature measurements

“…Contrary, the maximum T2m insitu -IST skin differences over the GrIS occur at wind speeds of about 5 m s -1 . This is also seen by Adolph et al (2018) at Summit, GrIS and by Hudson and Brandt (2005) at the South Pole, and the feature is related to the pronounced katabatic winds in these regions. Furthermore, Nielsen-Englyst et al 2019found that the T2m insitu -IST skin difference tends to decrease linearly as a function of the cloud cover fraction for all seasons and all regions.…”

“…However, as expected the biases and root mean squared differences (RMS) are larger for the IST skin_L3 -T2m insitu differences than for the IST skin_L3 -IST skin_insitu differences. The reason is that the radiometric surface skin temperature can be significant different from the surface air temperature measurements (Adolph et al, 2018;Hall et al, 2008;Hudson and Brandt, 2005;Nielsen-Englyst et al, 2019;Vihma et al, 2008). On average, the skin temperature is colder than the air temperature, with the largest differences during clear-sky conditions and when the skin temperature is constrained by the melting point (melting snow has a maximum temperature of 0°C) (Nielsen-Englyst et al, 2019).…”

“…Several studies have compared satellite retrieved IST skin and T2m from AWSs located on the Greenland Ice Sheet (GrIS; Dybkjaer et al, 2012a;Hall et al, 2008Hall et al, , 2012Koenig and Hall, 2010;Shuman et al, 2014) and over the Arctic sea ice (Dybkjaer et al, 2012) and found temperature differences of which a significant part could be attributed to the temperature difference between T2m and IST skin . Previously, work has been done to investigate the relationship between the surface and near-surface air temperature over ice using in situ observations (Adolph et al, 2018;Hall et al, 2008Hall et al, , 2004Hudson and Brandt, 2005;Nielsen-Englyst et al, 2019;Vihma et al, 2008). Nielsen-Englyst et al (2019) found that on average T2m is 0.65-2.65°C warmer than IST skin with variations depending on location of the measurement i.e.…”

Ultralow Surface Temperatures in East Antarctica From Satellite Thermal Infrared Mapping: The Coldest Places on Earth

“…Validation studies of MODIS c5 and c6 LST for nonpolar regions, while limited, suggest that the surface temperature data are generally within 1°C of the in situ measured thermal emission temperature (Wan, 2014), although the validation did not extend to this temperature range or surface type. A recent assessment of the accuracy of MODIS LST c5 and c6 at the summit of the Greenland ice sheet (Adolph et al, 2018) found a very small bias in c6 data (À0.4 ± 0.9°C for cloud-filtered data) in the À5 to À35°C surface temperature range using an in situ Table 1). The reprocessing sought to retain the performance of c5 for lower temperatures (Wan, 2014).…”

“…The reprocessing sought to retain the performance of c5 for lower temperatures (Wan, 2014). A recent assessment of the accuracy of MODIS LST c5 and c6 at the summit of the Greenland ice sheet (Adolph et al, 2018) found a very small bias in c6 data (À0.4 ± 0.9°C for cloud-filtered data) in the À5 to À35°C surface temperature range using an in situ Table 1). We also determined the near-surface vertical air temperature gradients at the Dome A AWS from 4, 2, and 1 m (nominal height) air temperatures ( Table S1 in the supporting information).…”