A NUMERICAL STUDY ON THE MECHANISM OF CLOUD-STREET FORMATION IN THE LEE OF AN ISOLATED MOUNTAIN NEAR A COAST

Cloud streets are frequently observed when a cold air mass advects ove r a warm sea surface in the winter season; thick and long cloud street s can be sometimes seen in the lee of some islands or mountains near a coast. According to the conventional theory, the vertical shear and s tatic instability play an important role in the formation of the thin cloud streets. For the thick cloud streets in the lee of the isolated mountain, however, another mechanism seems to be important in addition to the conventional theory. The effects of the two factors, strong st atic instability and topographically induced mechanical disturbance, a re investigated by use of the CSU RAMS (Regional Atmospheric Modeling System) with high horizontal and vertical resolution. In the calculati on, uniform stratification and wind velocity (low Froude number) are a ssumed at the inflow boundary. To clarify the importance of both the s trong static instability and the topographically induced mechanical di sturbance on the formation and maintenance of cloud streets, three kin ds of numerical experiments with different sea-surface temperature wer e carried out, including a numerical experiment without a mountain. Cl oud streets were successfully simulated in the lee of an isolated moun tain near a coast, with the addition of a large sensible heat flux at the sea surface. Well developed cloud streets occur in the simulation between a pair of convective rolls below a height of 1 km over the sea . The following five results were obtained: 1) For the formation of th e pair of convective rolls, both strong static instability and a topog raphically induced mechanical disturbance are required at the same tim e. 2) Strong sensible heat flux from the sea surface is the main sourc e of the convective rolls (the buoyancy caused by the condensation pro cess in the cloud is negligibly small). 3) The pair of convective roll s contain two sub-rolls. One is the outer roll, which has a large radi us but a weak circulation, and the other is the inner Sell, which has a small radius but a strong circulation. The former gathers a large am ount of moisture by convergence in the lower marine boundary layer, an d the latter transfers the convergent moisture to the upper boundary l ayer by the strong vertical motion between the rolls. 4) The pair of i nner rolls form the line-shaped cloud streets, and keep them on a narr ow line along the center-line of the domain. 5) Cloud streets can be s imulated both by non-hydrostatic and by hydrostatic models, which impl ies that vertical inertia is not always important for cloud-street for mation. The horizontal scale of the topography does not seem to be res tricted to within the small scale, where non-hydrostatic effects are i mportant.