Lower-Tropospheric Humidity–Temperature Relationships in Radiosonde Observations and Atmospheric General Circulation Models

Annual and seasonal correlations between temperature and both specific and relative humidity are presented based on radiosonde station data over the tropical Pacific Ocean and North America. Results are presented for the surface and the 850-, 700-, and 500-hPa levels. The correlations between anomalies of temperature and relative humidity are generally negative, and those between temperature and specific humidity are generally positive. Longitudinal differences in the pattern of correlations are found both in low latitudes and over midlatitude North America. In particular, near-zero or negative temperature–specific humidity correlations are found in the western United States at and below 700 hPa (especially in summer) and over the western tropical Pacific at 700 and 500 hPa (especially in winter). The observed correlation patterns are compared with those of 12 atmospheric general circulation model (AGCM) simulations. Simulated high-latitude correlation patterns qualitatively agree with observations, but a sizable fraction of the correlations are higher than observed. The models show varying degrees of success in simulating the longitudinal differences in the temperature–specific humidity relationship in midlatitudes. At low latitudes, the models are generally unsuccessful at simulating the observed longitudinal differences. The simulated mean humidity fields are also compared with observations. The models show 10%– 20% higher relative humidity than is observed at midand high latitudes and over the Tropics. However, over the Pacific region, the relative humidity bias is of opposite sign (models drier than observed) and is confined to the 850-hPa level.