Spatio-temporal changes in free-air freezing level heights in Northwest China, 1960-2012

Based on the NCEP/NCAR reanalysis, the long-term changes in free-air freezing level height (FLH) in Northwest China during 1960-2012 were investigated. The results indicated that FLH of Northwest China has increased at a rate of approximate 1.2 gpm (geopotential meters) per year, which is statistically significant at the 0.01 level. Most grid boxes in Northwest China show consistent increasing trends, except the northern Tibetan Plateau. Periodic analyses demonstrate that primary period of FLH in Northwest China is approximately 15 years, and the different climate zones (eastern monsoon region, northwest arid region and Tibetan cold region) show similar periods. FLH is significantly related to surface air temperature during the study period (r = 0.90), especially in northwest arid region. The correlation coefficients are relatively higher in the colder months and lower in the hotter months. Altitude also correlates with mean FLH positively (r = 0.64) and trend magnitude in FLH negatively (r = −0.63).