# GNSS Profile from the Greenland Korth Expeditions in the Context of Satellite Data
## 5 Greenland North Expeditions and Satellite Gravimetry  
For comparing the GNSS profile differences over the period of 2002-2015, we will make use of satellite gravity data from the GRACE mission (Gravity Recovery And Climate Experiment; [35]). During its 15-year flight (2002-2017), GRACE provided a time series of monthly gravity solutions with a resolution of 250-500 km at the Earth's surface. These observations of global time-variable gravity have substantially contributed to various geoscientific domains, such as continental hydrological cycle, terrestrial water storage, solid earth changes due to large earthquakes, sea level monitoring, mass changes in polar ice sheets and large glaciers, and many others (for a review, see e.g., [36, 37, 38]). In our analyses, we will use one of the standard monthly GRACE gravity field solutions in the form of the spherical harmonic coefficients (UT CSR release 6, [39]). In order to study the variations in the mass distribution of the water (snow, ice) storage near the Earth surface, satellite gravity measurements are expressed by means of the so-called _equivalent water height_, which is directly connected to surface mass density (i.e., mass/area) [9; 40; 41]. To suppress the errors in monthly gravity solutions that grow with harmonic degree (short scales, high-frequency noise), a Gaussian smoothing filter is usually applied; for Greenland we chose its radius to be 200 km (cf., [42]). In Figure 8a-c, there are maps of the linear trend in the time series of GRACE monthly fields; the trend is expressed as the change in equivalent water height (cm/yr). They illustrate that the location of the maximum mass loss varied over this time period. Over the first Greenland North Expeditions period of 2002-2006, Greenland's mass loss was concentrated in the south-east part of the island (Figure 8a). During the second Greenland North Expeditions period of 2006-2010, the maximum mass loss migrated to the central west coast of Greenland (Figure 8b). Finally, the dominant mass loss location shifted southward during the third Greenland North Expeditions period of 2010-2015 (Figure 8c). This agrees with other observations [6]. Within the currently available record of Greenland's satellite observations in 1992-2019, satellite gravimetry estimates the maximum mass loss rate in Greenland to have taken place in 2011, followed by an extreme summertime surface melting in 2012 [6]. Next, the GNSS observations reported here are compared to GRACE along the Greenland North Expeditions profile. As mentioned in the Introduction, there are several differences between local and satellite observations. The GNSS data are obtained locally, but GRACE integrates the mass variations over an area of about 300 km in diameter. The GNSS vertical component measures the variation in height, while GRACE observes the mass changes. Finally, one must decide on the use of glacial isostatic adjustment model to get the net change in height or mass, corrected for the Earth's crust uplift. In Figure 8d we show the correction due to glacial isostatic adjustment (GIA) for Greenland. It is one of the standard GIA models, giving constant time rates for compressible Earth, computed using the ICE-5G ice history and the VM2 viscosity profile [43; 44]. The figure shows that there is only a small change in the altitudes due to GIA over the Greenland North Expeditions profile; in Greenland the crust is modeled to rise somewhat only in the north. We will use this GIA correction for net mass balance estimates in the next section, while in this section we will not correct GRACE for GIA. Another point to consider is different spatial averaging of ground-based GNSS and GRACE observations (e.g., [45]). Here we applied the Gaussian smoothing filter of the same radius of 200 km to the GNSS data [40; 41]. In Figure 8e, on the yearly averaged time series of GRACE monthly solutions we superimposed the GNSS elevation differences obtained at one of the Greenland North Expeditions sites (shown as a red point in Figure 8a-c). This particular site, No.