Nonhydrostatic Simulation of Frontogenesis in a Moist Atmosphere. Part I: General Description and Narrow Rainbands

Abstract

A series of experiments using a two-dimensional, nonhydrostatic, numerical cloud model with fine horizontal and vertical resolution is performed with the Hoskins-Bretherton solution to the Eady problem as initial condition. Dry and wet simulations are presented with 5-, 10-, and 40-km horizontal resolutions and vertical resolution from 160 m at the ground to 330 m at the domain top. Sensitivity experiments on the initial Brunt-Väisälä frequency and vertical shear are also discussed. Two classes of narrow bands are identified: 1) A narrow cold-frontal rainband at the surface cold front, consisting of a line of shallow convection triggered by the frictionally induced instability in the boundary layer at the surface front. The associated precipitation is organized in a narrow line with a large rainfall rate. Latent heating due to condensation contributes in large part to the tilting of isentropes and to the increasing of the vertical jet strength. Sensitivity experiments show that both friction and condensation processes are important to simulate this jet. 2) Narrow free-atmosphere rainbands above the narrow cold-frontal band. A succession of updrafts and downdrafts are generated in the stable free atmosphere above the narrow cold-frontal rainband along the frontal surface. Weak precipitation is associated with these bands. The conditions for conditional convective instability and conditional symmetric instability are not met. Detailed analyses show that the linear theory of stationary and hydrostatic gravity waves gives a reasonable explanation of these bands. Simulations with different horizontal resolutions indicate that the horizontal wavelength is related to the width of the vertical jet. Two classes of wide rainbands are also obtained in particular regions of the frontal system. 1) Wide cold-frontal rainbands consisting of bands periodic in the frontal zone, with a 75–100 km scale and a lifetime of 6–9 hours. 2) A single warm-sector wide rainband, located in the warm sector 300–400 km ahead of the surface cold front. The width of the vertical ascent varies during simulation time from 80 km up to 250 km. This band leads to widespread precipitation on an 80–120-km scale. These are discussed in detail in Part II.