Search radars operate at frequencies above 1 gHz, where atmospheric refraction is less of a factor. The Butterworth radar track data photo that was posted some time ago went out to 200 miles, with a gap from around 100 to 130 miles. Using an online radar horizon calculator it looks like flying at 5000', then climbing thru 10000' at 130 miles would produce a gap like that. That would jibe with the claim that MH370 flew "as low as 4500'". However, the radar horizon at 200 nautical miles is about 23000', which does not jibe with the reported last altitude data of 29500'. In my experience, a huge target like an airliner is easily detectable at max range if it is above the radar horizon. Max range of a search radar is typically around 250 nautical miles, and it is a hard limit, determined by the pulse repetition frequency.