Does the ground temperature reach the freezing point quicker, slower, or the same if it is wet from rain, or dry? (assuming all other factors equal).
In other words, if it is raining today and 40 degrees, then we get a winter storm with 25 degrees and snow tonight, is it more likely that we will have more accumulation, less accumulation, or the same accumulation of snow if it did NOT rain today?
You mean you have two surfaces, one dry and one wet, and dump the same amount of snow on each? I think the dry surface will cool down faster. Water will increase the heat capacity of the ground. That is to say, you need to cool down the water absorbed in the soil, in addition to the soil itself.
But if you have the two surfaces and expose it to cold air instead of snow, I think the result may be different because the wet surface will cool down by evaporative cooling.
I would think the wet ground would reach the freezing point more slowly than dry ground, simply due to the heat that the water contains. Of course, there could also be a cooling effect due to the latent heat if some of that moisture evaporates, and that would depend on the air conditions (temperature and humidity). Wind would also probably play a role. I assume that you mean ground surface temperature, not the ground temperature at some depth.
Citrus growers often flood the orchards to protect against freeze damage. Water can hold quite a bit of heat, so I 'd say in general that wet ground stays warmer longer than dry ground.