Lake Geoid and gravity from altimetry
Event: 2023 Ocean Surface Topography Science Team Meeting
Session: The Geoid, Mean Sea Surfaces and Mean Dynamic Topography (ROUND TABLE)
Presentation type: Type Poster
Contribution: PDF file
The data from NASA’s Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2) mission offer a unique opportunity to map rivers and lakes with an unprecedented number of observations and precision in areas where previous radar missions have failed to provide valuable water level estimates. ICESat-2 provides an along-track resolution of the ATL03 product better than 1 meter typically illuminating a circular region with a diameter of 17 meters. With its three pairs of beams located at nadir and 3.3 km to each side, the mission provides exceptional opportunities for inland water studies in areas with mountainous topography. In this study, we evaluate the first attempt to extract gravity anomalies from altimetry over several medium (100-1000 〖km〗^2) and large (>1000 〖km〗^2) lakes and compare them with conventional radar altimetry to investigate the performance of ICESat-2 for gravity determination. Aerial gravimetry from the GRAV-D project over the United States are utilized as the best estimate of the gravity field over the lakes. We use radar altimetry data from the CryoSat-2 satellite as it has a similar inclination to ICESat-2 giving coverage to within 2 degrees of the poles. We also use radar altimetry measurements from the SARAL satellite for additional comparison. We evaluate the quality of ICESat-2, CryoSat-2, and SARAL for gravity determination by computing gravity from each dataset and compare with data from the GRAV-D project over lakes. Gravity determination from altimetry is done using Fast Fourier Techniques (FFT) within a remove-restore geoid-to-gravity approach. The resulting altimetry derived gravity anomalies are then compared to the EGM2008 geoid over each lake with respect to GRAV-D. 18 lakes with area ranging from 108 〖km〗^2 to 82,220 〖km〗^2 across the United States were considered. Overall, gravity determination from ICESat-2 provides more reliable estimates than the other two radar altimetry missions. For all considered lakes, the performance of ICESat-2, measured by the standard deviation of the difference between ICESat-2 and GRAV-D, is comparable or better than the EGM2008 estimates over the same lake. Lake Tustumena is the best performing case, in which the standard deviation of the ICESat-2 derived gravity anomaly field is 1.598 standard deviations lower than that of EGM2008, with respect to GRAV-D. Over lake Tahoe, which is surrounded by mountainous terrain, ICESat-2 performs comparably to EGM2008 and captures the clear signal of the gravity field as expected by the lake's bathymetry, whereas CryoSat-2 produces very unstable results. In few cases, CryoSat-2 or SARAL seem to outperform both ICESat-2 and GRAVD-D. While this is seen for lakes Ontario, Huron, and Salton, it should not be taken to be entirely true. This is because for these lakes, CryoSat-2 and SARAL often have groundtracks covering only a part of the lake, resulting in seemingly lower standard deviations. Additionally, it is important to consider that for many of the medium lakes, GRAV-D coverage is sparse, thus it is hard to assert that EGM2008 truly performs better or worse than ICESat-2. Despite this, the method presented here for deriving gravity anomalies from altimetry applied to ICESat-2 laser altimetry data produces results comparable in trend and magnitude to the GRAV-D project.