Physically Consistent Atmospheric Variational Objective Analysis Model and Its Applications over the Tibetan Plateau. Part II: Characteristics of Cloud–Precipitation, Heat, and Moisture in the Naqu Region

This study sets up a long-term (2013–2017) dynamically and thermodynamically consistent atmospheric dataset over the Tibetan Plateau-Naqu analysis region that is derived by a constrained variational objective analysis with ground-based, sounding, and satellite measurements as well as ERA-Interim reanalysis data. Annual evolutions of atmospheric basic environments, cloud precipitation, and large-scale dynamic and thermal structures in the Naqu analysis region are analyzed using averaged results from the five-year dataset. Results show that: (1) The seasonal variation of the wind speed above 350 hPa is significant with a maximum (＞50 m s⁻¹) from November to February in the next year. The vertical variation of the wind speed is the weakest, while that of the temperature is the strongest from July to August. The high-humidity area is located at 350–550 hPa in summer and autumn but at 300–400 hPa in winter and spring. (2) There is rich precipitation in the analysis region from June to early July. The 300–400 hPa layer (as the junction of atmospheric ascending and descending motion) is the cloud concentration area in spring, autumn, and winter. However, the enhanced atmospheric ascending convection and water vapor in summer lead to an increase of high clouds and total clouds and a decrease of medium and low clouds. (3) The surface latent heat flux and the total air-column latent heat are the strongest, whereas the air-column net radiative cooling is the weakest in summer. The strong surface sensible heating in the plateau results in the horizontal warm advection below 500 hPa, while the strong westerly and radiative cooling cause the cold advection above 500 hPa. In addition, the analysis region is characterized by dry advection in the whole year. However, there is a weak moist advection in summer. (4) The apparent heat source Q₁ has obvious vertical stratification characteristics, i.e., showing diabatic cooling below 500 hPa and diabatic heating in 300–500 hPa and 100–150 hPa in the whole year. Meanwhile, the 150–300 hPa layer has diabatic cooling in the dry seasons (winter and spring) and diabatic heating in the wet seasons (from the end of spring to autumn). In summer, the entire air column is almost dominated by the diabatic heating because of the enhanced ascending motion, net latent heating, transport of sensible heat by rising turbulence, and existence of high clouds.