Lake – Radiometry of Snow on Lake Ice
Introduction
Microwave radiometry provides good penetration into dry snow and, hence, dry snow characteristics including snow water equivalent can be measured from space. Regional algorithms have been developed and validated for this purpose. Accuracy of retrieved snow water equivalent is decreased e.g. by the existence of several land cover types within a radiometer pixel. The two main problems related to land use categories in microwave radiometry of snow are vegetation (forest) and water areas (lakes). On top of lake ice, snow depth, density and grain size may be substantially different from those on land and, additionally, the emission behavior of ice is different from that of frozen/thawed soil.
Test site
The lakes over which radiometer measurements have been carried out are located in southern Finland not far from the Helsinki Airport. Lake Bodom is 5 km long and 1.4 km wide with a maximum /average depth of 12.7/4.3 m. The adjacent Lake Matalajärvi, which is separated from Lake Bodominjärvi by a narrow isthmus, is shallow and somewhat smaller. The flight path over these lakes consists of 1.7 km over Lake Bodominjärvi, 0.3 km over the isthmus, and 1.0 km over Lake Matalajärvi. The land areas before (forested) and after (farmland) the lake flight line provide a reference for comparison of results for snow-covered lake ice with those for snow-covered terrain.
Airborne instrumentation
We have used our airborne microwave radiometer system HUTRAD to measure microwave emission signature of lake ice during the winters of 2004, 2007, 2011, and 2012. HUTRAD operates at 6.8, 10.7, 18.7, 23.8, 36.5, and 94 GHz, providing data for both vertical and horizontal polarization at an incidence angle of 50 degrees off nadir with the 36.5 GHz receiver operating in a fully polarimetric mode. The antenna beam-width decreases from 5 degrees at 6.8 GHz to 3 degrees at 36.5 GHz; hence, results at various frequencies can be reasonably well compared with each other. The HUTRAD system is accommodated on our research aircraft Skyvan and looks to the rear along the flight track; the rear cargo door is removed before taking off. Accurate position and attitude information provide antenna footprint track on the ground.
Our airborne L-band interferometric radiometer HUT-2D was used for lake ice/snow measurements in winter 2011 along with the HUTRAD system. HUT-2D consists of 36 antenna/receiver pairs in U shape geometry and operates at 1.4 GHz. It is located below the fuselage of our research aircraft. Due to its operation mode, HUT-2D provides data for incidence angles between nadir and 25 degrees off nadir without mechanical or electronic scanning of antennas. The synthesized antenna beam-width is about 7 degrees and the system provides an image of the target area with a swath equal to the flight altitude. Hence, the HUT-2D system provides an emission map over a wide area, whereas the non-scanning HUTRAD system provides data along the flight track.
Surface measurements
Surface data were collected including ice thickness, snow depth, snow and ice surface temperature, snow density and wetness, snow water equivalent and, in selected locations, snow grain size by photography. Additionally, the presence of water and slush on top of ice (below the snow layer) was checked carefully along the flight line.
Figure 1. Surface data collection in progress on Lake Bodom.
Data collection
Airborne and surface data have been acquired on the following dates:
2004: April 7
2007: January 30, March 20, March 26, April 2
2011: January 27, February 28, March 31 (AM and PM), April 14
2012: February 16, February 29.
A wide variety of snow and ice conditions have been covered ranging from dry snow conditions to those with occasional water / slush on top of ice, and to melting period conditions.
Recent publication
Hallikainen, M., Seppänen, J., Rouhe, E., Lemmetyinen, J., “Microwave emission signature of snow-covered lake ice,” Proc. IEEE 2011 International Geoscience and Remote Sensing Symposium (IGARSS’11), pp. 3507–3509, Vancouver, Canada, July 25-29, 2011.