The Asymptotic Frontgenesis Convergence Line and Its Mechanism of Wet-Thermodynamics on Heavy Rainfall in Low Latitude Zone

The asymptotic frontogenetic convergence line (AFCL) is a key system of non-typhoon and non-tropical cyclone heavy rainfall in tropical low latitudes. The system has not been introduced in any previous meteorological textbooks. Usually associated with the multi-year heavy rainfall centers in low latitude zone, AFCL is a kind of shallow circulation system mainly occurring in coastal areas and landing from the sea to the shore. The convergence and upward movement of the system is accompanied by significant front area and moist thermal dynamic instability. Affected by the combination of seasonal circulation background and underlying surface topography, AFCL can provide an effective indicator for the location of the heavy rainfall area. To be more specific, the temperature front causes the thermodynamic ascending, and the humidity front forms the wet dynamic unstable stratification with the upper water vapor transport. The vertical ascending layer cooperates with the latent heat center in the middle layer to form a deep moist thermal dynamic instability, which corresponds to the heavy precipitation area on the left side of the asymptotic frontogenetic convergence axis. The objective tracking of the AFCL and the establishment of its coordinate system are helpful for the characteristic extraction of the system structure and the distribution of the system variables and for predicting the center of heavy precipitation. The comparative experiments of WRF numerical model show that diabatic heating can show the role of water vapor transport and moisture heat, and enhance the temperature stratification instability and convective instability in the rainfall area; the diabatic heating also maintains the low value system strength at the low layer when enhancing the strength and thickness of the vertical rising layer. The advection sensible heating can enhance the temperature front and its dynamic uplift, and the condensation latent heating can affect the convergence position and intensity of the AFCL, and then affect the activity of the system. The latent heating in the middle layer inhibits the cooling of advection sensible heat into the warm air mass, and therefore maintains the moisture thermal instability, the rainfall intensity and the rainfall amount at the precipitation area. This is the moisture-thermal dynamic mechanism of the AFCL on heavy precipitation. The topographic dynamic mechanism will be discussed in another paper.