Yes, on the ground ice could form at temperatures well above zero. It requires a particular set of circumstances and would usually be associated with "tankering" fuel. This is means that the company require the aircraft to arrive with as much fuel on board as possible in order to avoid or minimise uplift for the return sector. "Tankering" occurs for various reasons, the most common being fuel price differential but other reasons might include industrial action, lack of fuel bowsers or low fuel stocks. The result is that the aircraft arrives with a relatively large quantity of cold-soaked fuel in the tanks and if temperatures at cruise altitude are low (N. Europe winter) the fuel can be even colder. Now land at an airfield with highish humidity and condensation very quickly forms on the wings that contain a large amount of very cold fuel, result a layer of ice. Now 8 degrees C is a bit on the high side, but add a few knots of wind and the effective temperature could easily be below 5 degrees. I am sure that you have seen how long it can take for frozen ground to thaw, even in sunlight with positive temperature and this is really the problem here. The solutions; wait for it to melt, it will take time but it will happen eventually, de-ice the affected areas (wings) if facilities are available (remember de-icing fluid has to be heated up first) or add some nice warm fuel to raise the temperature of the fuel already in the tanks.
I hope that this answers your question.
YS