Spatial variabilities and their relationships of the trends of temperature, water vapor, and precipitation in the North American Regional Reanalysis

Spatial variabilities and their relationships of the trends of temperature, water vapor, and precipitation in the North American Regional Reanalysis are examined for each season from March 1979 to February 2007. Results show that warming dominates the domain in the troposphere from the surface to 300 hPa. Water vapor increases at lower levels but does not change much at mid‐upper levels. Because of the large increase of water vapor holding capacity of the air at all levels due to the warming, relative humidity has a decreasing trend at all levels. The decrease is small at the surface and largest at midlevels. Precipitation, which corresponds well to ascending motion in trends, both increases and decreases in about half of the domain. Statistical analysis from the very large spatial samples indicates that the precipitation trend positively relates to both specific humidity trend and relative humidity trend. However, temperature trend positively relates to specific humidity trend but negatively relates to relative humidity trend. So, in strong warming places, whether precipitation increases or not depends on whether the decrease of relative humidity becomes a limiting factor; small decrease of relative humidity may still allow precipitation to increase, but large decrease of relative humidity may make precipitation decrease. The uncertain relationship between the trends of precipitation and temperature can also be understood from the nonlinear characteristics of the atmospheric processes.