Characteristics and Cause of Diurnal Variation of Precipitable Water Vapor Derived from Ground-Based GPS in Qilian Mountains in Summer

Based on the precipitable water(PW) derived from ground-based GPS and the observations from radiosondes and automatic weather stations in Qilian mountains of Northwestern China during the period of July～August 2007,the diurnal variations of PW are systematically investigated.The results show that PW exhibited a pronounced diurnal variation in Qilian Mountains in summer.The diurnal variations of PW without precipitation days are more prominent than with precipitation days.The diurnal(24 h) and semidiurnal(12 h) cycles of the PW are fitted using the harmonic analysis method.The results show that the diurnal and semidiurnal cycles together can represent the diurnal variations very well.During the without precipitation days,diurnal cycle,which explains over 76% of the subdaily variance,has an amplitude of 0.8～1.6 mm and peaks appear at 18:00～21:00.The semidiurnal cycle is generally weaker,with an amplitude of 0.6～0.7 mm,and peaks appear in 05:00～06:00 and 17:00～18:00 respectively.However,during the with precipitation days,the characteristics of diurnal variation of PW at different stations are different.At some stations,the diurnal cycle explains more the subdaily variance than semidiurnal cycle.At other stations the semidiurnal cycle is more significant than diurnal cycle.The causes of diurnal variation of PW are preliminarily discussed.It can be seen that the diurnal variations of PW occur prior to the changes of hourly accumulated precipitation frequency.The former phase is two and half hours earlier than that of the latter.The diurnal variations of PW can be affected by surface air temperature,specific humidity and mountain-valley breeze.