'merciless sun' is a correct and usable term in written English.
For example, you could use it in a sentence like, "The merciless sun beat down on the desert, leaving nothing untouched."
But today, all is calm under the merciless sun.
Spring's merciless sun reveals the cracks and flaws, the window streaks.
A merciless sun caught the golden rim of the cop's dark glasses.
Pro football summer training camps were once filled with two practices a day, grueling sessions that featured helmeted players clashing gladiator-style under a merciless sun.
It was high season: hordes of German, Japanese and American tourists were struggling up the white-hot rocks in the merciless sun.
His reality, however, a smouldering summer evening under a merciless sun in a Nevada desert carpark, was more nightmarish incubus than fulfilling dream.
1936 Drought Loss Up to 1/4 Billion Drought embattled farmers of the Northwest prairies tonight [July 7] abandoned hope of saving their crops from a merciless, white-hot sun.
Japanese and Brazilian news crews filmed live shots under a merciless late-morning sun as the temperature climbed past 103 degrees. .
On that Saturday in 1997, the course was without shade and the sun was merciless.
Unlike all the other adults, he never turned down my appeals to go out to the garden, even after lunch, when the sun was merciless and the household napped.
If you are caught using an improperly placed modifier on U.S. soil, I will nail your hands and feet to a large wooden cross and mount it in the desert where you will suffer with no water under the merciless glare of the sun.
