Dynamical Impedances of Diurnal and Seasonal Horizontal Convections

The atmosphere of the earth is stable density-stratified fluid. Because the thermal response of land is different from that of sea, the differential heating causes horizontal convection. Horizontal convection consists of convergent and divergent fields. Typical examples of horizontal convection are land- and sea-breeze (diurnal cycle) and seasonal wind (annual cycle). There aren't any studies that examine the difference of dynamical feature of diurnal-cycle- and annual-cycle-horizontal convections. The purpose of this study is to examine the difference of dynamical feature of diurnal-cycle- and annual-cycle-horizontal convections using observational data. We characterize horizontal convection by two features: thermal skin depth and dynamical impedance. We define the ratio of pressure to temperature as thermal skin depth. The ratio of pressure to wind speed is dynamical impedance. To analyze observational data and extract the phenomenon that we are interested in, it is important that we should eliminate a basic flow. To eliminate the basic flow, we calculate flow convergence and divergence along closed circle because horizontal convection consists of convergent and divergent fields. The study area includes the four main islands of Japan. The Japanese Islands have 156 meteorological observation stations. Using the wind, pressure, and temperature data, we obtain the values of the thermal skin depths and the dynamical impedances. In diurnal-cycle-horizontal convection thermal skin depth is 600m in winter and 1500m in summer. In annual-cycle-horizontal convection thermal skin depth is 600m. In diurnal-cycle- and annual-cycle- horizontal convection dynamical impedance are 0.7 and 2 respectively. Stommel studied a steady convective motion under the condition that a gravitationally stable fluid was heated non-uniformly at the bottom (Stommel et al. 1957). This study and our results indicate that either diurnal-cycle- horizontal convection or annual-cycle-horizontal convection is not a steady flow at least. From the relationships between the diurnal variation of convergent and divergent flow and diurnal variations of solar radiation, temperature, pressure, and radiative cooling, it indicates that daily-cycle-horizontal convection is non-steady flow.