Career
He joined the Royal Air Force (Royal Air Force) as an Aircraft Apprentice in 1921. In 1925 he became a Flight Cadet Sergeant. In 1927 he was on the staff of Royal Air Force Calshot with the rank of Pilot Officer.
During the 1930s he held a number of different positions in the Royal Air Force as Armament and Engineering Officer.
On 1 January 1941 he worked on the Air Staff in the Directorate of Plans. At a press briefing two days after the Dresden raid on 14 February 1945, Grierson told journalists:
First of all they are the centres to which evacuees are being moved.
They are centres of communications through which traffic is moving across to the Russian Front, and from the Western Front to the East, and they are sufficiently close to the Russian Front for the Russians to continue the successful prosecution of their battle. I think these three reasons probably cover the bombing.
One of the journalists asked whether the principal aim of bombing of Dresden would be to cause confusion among the refugees or to blast communications carrying military supplies.
Grierson answered that the primary aim was communications to prevent them moving military supplies, and to stop movement in all directions if possible. He then added in an offhand remark that the raid also helped destroying "what is left of German morale" Howard Cowan, an Associated Press war correspondent, subsequently filed a story saying that the Allies had resorted to terror bombing. This set in train a number of embarrassing questions on both sides of the Atlantic on the morality of this form of attack.
He retired from the Royal Air Force on 31 August 1950 with the rank of Air Commodore.