President Franklin D. Roosevelt took a strong personal interest in Greenland’s fate. On 9 April 1941, the anniversary of the German occupation, the Danish envoy Kauffmann, against the instructions of his government, signed an executive agreement with Secretary of State Cordell Hull, allowing the presence of American troops and making Greenland a de facto United States protectorate. The cryolite mine in Ivigtut was a unique asset that made it possible for Greenland to manage fairly well economically during the war. The United States supplied the island and sent patrol boats to survey the east coast of Greenland although this activity was limited by seasonal ice. The Coast Guard, in coordination with Eske Brun, created the Northeast Greenland Sledge Patrol, consisting of 15 men, many of them former hunters in the area. Their task was to patrol the coast to discover any German activity. However, in 1941 there was no such landing, although the Norwegian resupply trawler Buskø was encountered in September. Among its otherwise innocuous crew and passengers was a civilian Norwegian who intended to provide weather reports for German contacts in Norway. This ship was seized and brought to Boston by the Coast Guard. In 1941, Royal Navy ships continued to interfere with weather stations on the east coast. A couple of reconnaissance aircraft from Norway flew over Scoresbysund.
During WWII a German weather station was installed on Greenland, in August of 1942.
A Greenland patrol found the station, engaged in a gun battle and retreated. Two members of the patrol were captured and one was killed.
Later the two members escaped, captured the German Commandant, and made their way back to Scoresbysund. They reported in and the German weather station was destroyed by American air power on 25 May of 1943.
There is a book detailing the events.
D-day was successful, in part, because the Allies had better meteorological information to make forecasts with.