Carb Ice
Typical LIGHT carb ice, happens here in our PA28s on early morning start ups even in Spain - and we don't park on grass.
Carb Ice because it's early in the morning, temp and dewpoint still close together even though it might be quite warm, and a low power setting. The engine has some natural resistance to carb ice if the exhaust manifold is located very close to the carburretor. In the PA28 they as good as touch. However it takes a while for the heat to conduct through which is why it typically happens within two or three minutes after starting.
LIGHT carb ice because that's the symptom of light carb ice when you apply carb heat. It clears quickly without much fuss, allowing more mixture through, in this case enough to counteract the effect of adding the hot, less dense air, which isn't actually all that hot and less dense yet, and hence increase rpm.
Larger quantities of ice though on application of carb heat, dumping water into the fuel air mixture will cause the more commonly quoted 'further drop, rough running then increase in rpm'