H-4. Cloud resolving simulation – winter marine stratocumulus

In winter, a continental cold air mass sometimes flows out over a warmer sea, supplying a great amount of heat and water vapor. As a consequence of this air-mass transformation, a maritime mixed layer develops, and stratocumulus is generated in its upper layer. In the Japanese Cloud and Climate Study　(JACCS), aircraft observations of cloud physics and radiation were performed for lower clouds around the Japanese Islands during FY1996 and FY1997 winter seasons. The FY1996 experiment was conducted for marine stratocumulus in the cloud streets west of Kyushu in January 1997. In this study, we tried to reproduce observed features of stratocumulus by using a 3-D non-hydrostatic model and investigated the heat balance in the mixed layer.

H-4-1. Outline of numerical experiment

The elastic version of MRI-NHM (Ikawa and Saito, 1991; Saito and Kato, 1996) with a horizontal grid size of 1km was used. The calculation domain has a 300x300x38 horizontal and vertical grid. In this study, the cloud physics in the model contains the cold rain scheme, and the atmospheric radiation scheme for the cloud-resolving model (G-5-2) is used. The initial and boundary conditions for MRI-NHM are provided from the output produced by RSM. MRI-NHM is one-way nested within the RSM forecast with an initial time of 2100 JST, 21 January 1997.

H-4-2. Results

Figure H4-2-1 shows the horizontal map of the liquid water path (LWP) simulated by MRI-NHM at 0300 UTC on 22 January 1997 (nine-hour forecast). Several cloud streets are obtained and roughly correspond with satellite observations (Fig. H4-2-2). They are about 10km wide and extend north and south. Figure H4-2-3 presents the vertical profile of liquid water content (LWC) along the cloud street. Aircraft observations (Fig. H4-2-3a) show that clouds contain LWC of about 0.7 gm^-3 and extend from 1 to 2km in height. The vertical distribution and the magnitude of LWC simulated by MRI-NHM are similar to observations (Fig. H-4-2-3b).

Figure H4-2-4 shows the vertical profiles of the components of heating rate averaged in the box in Fig. 1. The heating is seen in the whole layer. In the sub-cloud layer, the heating due to convergence of the upward sensible heat flux is greater than the cooling due to large-scale advection. In the under part of the cloud layer, the sum of the heating due to the convergence of the upward sensible heat flux, condensation and radiation is greater than the cooling due to large-scale advection. In the upper part of the cloud layer, the sum of the cooling due to the divergence of the upward sensible heat flux, evaporation and radiation is almost balanced by the heating due to large-scale advection. Above the cloud, the heating due to large-scale advection, especially subsidence, is dominant. Furthermore, the radiative heating and cooling is not small compared with other terms.

Fig. H4-2-1. Horizontal distribution of LWP at 0300 UTC on 22 January 1997 (nine-hour forecast). Solid line (box) is used in Fig. H-4-2-3 (Fig. H-4-2-4).

Fig. H4-2-2. Visible satellite image at 0300 UTC on 22 January 1997. Solid line denotes flight path of the aircraft.

Fig. H4-2-3. Vertical profile of LWC of (a) aircraft observations along the solid line in Fig. H4-2-2 and (b) MRI-NHM simulation along the solid line in Fig. H4-2-1. Bar graphs (error bars) in (b) denote mean value (standard deviation).

Fig. H4-2-4. Vertical profiles of heating rate components, averaged in solid box area in Fig.H-4-2-1, at 0300 UTC on 22 January 1997. Local time tendency is indicated by a thick solid line, area-scale advection by a thin solid line, convergence of the convective heat flux by a dotted line, the effect of condensation and evaporation by a dashed line, and the effect of radiation by a dash-dotted line. The hatched area denotes a cloud layer.