Wind and air temperature distribution night

At 00:00 LST, August 15, as nighttime radiative cooling of the ground surface due to no solar radiation increases, synoptic south-westerly wind over the top of Mt. Taeguallyung moves down along the eastern slope toward the coast and is associated with mountain wind generated by air temperature difference between the mountain and the plain surfaces, resulting in down-slope wind. Then, the downslope wind is further intensified by land breeze due to horizontal air temperature difference between the coastal inland and the sea surfaces, becoming strong downslope wind storm (Figs. 3(a)-(c)).

As the downslope wind storm reaching the bottom of the eastern slope is too strong, a hydraulic jump motion of air occurs near the ground surface of Kangnung downtown, bounding up to the 1 km height over the coastal sea and causes the generation of lee-side internal gravity waves. Maximum speed of wind storm on the lee slope of the mountain reaches 15m/s, but nighttime surface wind speed of 2 m/s near Kangnung city beneath of the internal gravity waves is much weaker than daytime one of 5 m/s, under the sea breeze.

Turbulent diffusion coefficients for heat of 1 m2/s exists near the ground surface and indicates occurrence of a thin shallow nocturnal surface inversion layer (NSIL). Although its thickness is within the height of about 150 m over the ground in the west of the mountain, no inversion layer due to the destruction of the inversion by strong downslope windstorm is found along the eastern slope and shallow surface inversion layer exists in the coast (Fig. 3(d)). Over the sea surface, marine inversion layer (MIL) forms with a thickness of about 250 m.

The thickness of the marine inversion layer is slightly larger than one of the inland NSIL, because the cooling of sea surface is much smaller than that of the ground surface. At 00:00 LST, surface air temperature at Kangnung city is 29° C, which is over than 25° C and this temperature continues to be until next day morning. If air temperature over 25°C persists for

Fig. 3. (a) Wind (m/s) in a coarse-mesh domain at 00:00 LST, August 15, 1995. Thin dash line and box denote topography and (b) a fine-mesh domain near Kangnung city. (c) Vertical distribution of wind (m/s) in a fine-mesh domain near Kangnung city. Kan denotes Kangnung city. (d) Vertical diffusion coefficient for turbulent heat (m2/s).

Fig. 3. (a) Wind (m/s) in a coarse-mesh domain at 00:00 LST, August 15, 1995. Thin dash line and box denote topography and (b) a fine-mesh domain near Kangnung city. (c) Vertical distribution of wind (m/s) in a fine-mesh domain near Kangnung city. Kan denotes Kangnung city. (d) Vertical diffusion coefficient for turbulent heat (m2/s).

whole night, it is called tropical night in the northeastern Asian countries (Fig. 4(a)).

Sensible heat fluxes at the surface of the mountain top and in lower atmosphere with —70 and —10 W/m2 produce sensible heat flux divergence, indicating heat loss at the ground surface and cooling of ground surface (Fig. 4(b)). Sensible heat fluxes of —140W/m2 on the eastern slope to — 10 W/m2 in the lower atmosphere also produces a great sensible heat flux divergence along the slope, resulting in great change of air temperature

Fig. 4. (a) As shown in Fig. 3(c), except for air temperature (°C) and sensible heat flux (W/m2).

in the narrow atmospheric boundary layer along the slope, as shown in Fig. 4(a). On the other hand, sensible heat fluxes at the inland coastal surface (or coastal sea) and in the lower atmosphere are just —20 W/m2 (or — 10 W/m2) and —20 W/m2 (or —10 W/m2), very small sensible heat flux divergence (or no flux divergence) occurs in the coastal area.

So, sensible heat flux divergence is much greater at the top of mountain and along the eastern slope than at the coastal inland surface near Kangnung city and the sea surface. It appears that the mountain surface more cools down than the coastal inland surface and the sea surface, and the nighttime air temperatures near the coastal inland and sea surfaces are not much changed from daytime one, resulting in the persistence of nocturnal warming over the coast near Kangnung city and the sea surface and the formation of nocturnal thermal high (tropical night). One of possible mechanism on the formation of tropical night is some amount of heat transfer from warm pool of 34° C into the coastal surface, penetrating through very shallow nocturnal surface inversion layer less than 100 m depth.

3.3. Comparison of observed results with calculated ones by a model

Through 48 h numerical simulation with G-ANAL data sets, numerical simulation results of air temperature on heat budget such as sensible heat in

Table 1. Comparison of calculated air temperature (°C) to observed one at Kang-nung city from August 14 through 15, 1995.

Date

Comparison

1200

500

800

2100

0000

0300

0600

8/15

Observed Calculated

35.0 35.0

34.6 34.0

33.4 32.0

29.0

26.5 29.0

29.9 32.0

the coarse-mesh and fine-mesh domains were compared with hourly air temperature measured by Kangnung Local Meteorological Administration. The air temperature calculated by the model well represents observed ones with a discrepancy of 5-10% error.

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