As ambient temperature drops, the cooling tower’s capacity for heat rejection actually skyrockets. A tower designed to cool 100°F water down to 85°F on a 95°F summer day can easily overcool that same water to 40°F or lower on a 20°F winter night. While this sounds like a performance gain, it leads to the "Ice Paradox": The better the tower performs thermally, the faster it self-destructs structurally.
If you see ice, do not shut down. Increase heat load. Increase water flow. Do not stop the fan unless you intend to scrap the cell. cooling tower handbook
From the Cooling Tower Handbook, 4th Edition As ambient temperature drops, the cooling tower’s capacity
When ice forms, panic leads to silence. Silence leads to stagnation. Stagnation leads to a tower that looks less like a heat exchanger and more like a frozen waterfall. A frozen cooling tower cannot be thawed with steam hoses; it must be rebuilt in April. If you see ice, do not shut down