PNNL

Abstract

Despite the importance of the diurnal cycle of precipitable water vapor (PWV), its knowledge is very limited due to the lack of data with sufficient temporal resolution. Currently, from GPS receivers, PWV can be obtained with high temporal resolution in all weather conditions for all hours of the day. In this study we have calculated the diurnal cycle of PWV for ten GPS stations over Spain. The minimum value is reached approximately at the same time at all the stations, ~0400-0500 UTC, whereas the maximum is reached in the second half of the day, but with a larger dispersion of its occurrence between stations. The amplitude of the cycle ranges between 0.72 mm and 1.78 mm. The highest values are recorded at the stations on the Mediterranean coast, with a doubling of the values of the stations on the Atlantic coast or inland. The amplitude of the PWV cycle, relative to the annual mean value, ranges between 8.8 % on the Mediterranean coast and 3.6 % on the Atlantic coast. Two distinctly different seasonal diurnal cycles have been identified, one in winter and other in summer, with spring and autumn being only transition states. The winter cycle is quite similar at all locations, whereas in summer, local effects are felt strongly, making the diurnal cycle quite different between stations. The amplitude of the summer cycle is 1.69 mm, it is almost double the winter one (0.93 mm). Analogous to the annual cycles, the seasonal cycles of the different stations are more similar during the night and early morning hours than during the afternoon. The observed features of the PWV diurnal cycle are explained in a qualitative way on the basis of the air temperature, the transport of moisture by local winds, and the turbulent vertical mixing.