Climatological Beam Propagation Conditions for China's Weather Radars Network

The accuracy of spatial location of weather radar echoes depends on radar electromagnetic wave propagation paths, and the vertical refractivity gradient (VRG) of the atmosphere is a critical factor that determines ground-based weather radar wave beam paths. In practice, the standard atmosphere is often assumed when calculating the wave beam path.

China’s operational weather radar network covers a vast area, across which the geography and climatic conditions change dramatically. To better document propagation conditions of radar electromagnetic wave, which is essential for hydrology and numerical weather forecast model to benefit from weather radar observations more fully, 6-year continuous sounding data at 00:00 and 12:00 UTC were applied to investigate VRG characteristics within 1 km of the surface and beam elevation errors for the operational weather radar network in China. Note that radars put into operating by the end of 2015 are considered in this work. Major conclusions are as follows.

Under the effects of both regional climatic conditions and topography, VRG values were larger in dry regions and seasons, and smaller in moist regions and rainy seasons. Values generally increased with surface elevation by about 6 km−1 per km. In most regions, values were the smallest in July and August, and the largest in January and February. However, the time that the largest or smallest value appears is not quite the same in different regions. In most inland regions, morning values were smaller by about 5−10 km−1 than evening ones, but there were slight differences near coasts. Compared with the surrounding areas, smaller values were frequent in basins, lakes and river valleys, and larger values were common in hilly or mountainous areas.

During the rainy season when weather radar observations are very important, for most operational weather radars, VRG values were generally smaller than those of the standard atmosphere. Only a few radars had VRG equal to or slightly larger than standard atmospheric values, except on the Tibetan Plateau. VRGs smaller than standard atmospheric value are more prominent in China’s operational weather radar network. Compared to actual conditions, radar beam elevations estimated using standard atmospheric conditions tend to be overestimated for most radars, especially in the rainy season. Underestimation is much less and of much shorter duration in contrast, overestimation is dominant throughout the year for many radars.