The Effects of Terrain Slope and Orientation on Different Weather Processes in China under Different Model Resolutions

NO.5 HUANG Danqing and QIAN Yongfu 617 The Effects of Terrain Slope and Orientation on Different Weather Processes in China under Different Model Resolutions HUANG Danqing ( ) and QIAN Yongfu ( ) School of Atmospheric Sciences, Nanjing University, Nanjing 200030 (Received September 11, 2008) ABSTRACT Currently, short wave radiation at the ground surface (GSW) is calculated under the assumption of a horizontal surface. This method of estimating the GSW may lead to considerable errors when the model resolution becomes higher and the model terrain becomes steeper. In this paper, to improve the short wave solar radiation simulations, a terrain slope and orientation parameterization has been implemented into the non-hydrostatic mesoscale model GRAPES (Global/Regional Assimilation and Prediction System). The effects of the terrain slope and orientation on different short range weather processes in China under different model resolutions are simulated and discussed. In the simulations, topography height is taken from NCEP (National Centers for Environmental Prediction) with a resolution of 1 km, and the slope and orientation of terrain are calculated using different staggering schemes and under different weather conditions. The results show that when the model resolution is low (30 and 60 km) and the slope of terrain is not large, the influence of the slope and orientation of terrain on the GSW is not evident; otherwise, however, it is not negligible. Under high model resolutions (3 and 6 km), the increase (decrease) of simulated precipitation corresponds to the decrease (increase) of the GSW induced by the slope effect, and the variations of precipitation are usually ranged between 5 and 5 mm. Under the high resolution, the surface temperature and heat fluxes are strongly correlated to each other and the high correlation exists mostly in the complex terrain regions. The changes of the GSW, precipitation, surface temperature, and heat fluxes induced by the effects of the terrain slope and orientation are more obvious in mountainous regions, due to the alternations in the atmospheric circulation. It is found as well that under the weather condition of less cloud and less precipitation, the effects of the terrain slope and orientation can be more realistically seen. Therefore, the terrain slope and orientation can usually be neglected in numerical models when the horizontal model resolution is low and the slopes are moderate, but should be taken into account when the model resolution becomes high and the terrain is steep and undulating. Key words: mesoscale model GRAPES, slope irradiation, terrain slope and orientation, weather processes Citation: Huang Danqing and Qian Yongfu, 2009: The effects of terrain slope and orientation on different weather processes in China under different model resolutions. Acta Meteor. Sinica, 23(5), 617 628. 1. Introduction The effect of terrain has been one of the important considerations in numerical simulations. Nowadays, the atmospheric numerical models can well describe the dynamic effect caused by the large-scale terrain, such as the mechanical forcing, dynamic obstruction and friction, and also models can well simulate the spatial distribution and seasonal change of weather and climate due to the thermal effect of different terrains of different elevations (Qian et al., 1998; Wang et al., 2004; Wu Guoxiong et al., 2005; Zhang et al., 1999, 2006; Shen et al., 2004). Some effective parameterization schemes for depicting the dynamic effect of the subgrid scale topography, such as the envelope orography (Wang et al., 2004; Wallace et al., 1983; Li et al., 1990) or orographic gravity-wave-drag parameterization scheme (Palmer et al., 1986; Li et al., 2004), have been proposed. In addition, solar radiation is the source of heating in the atmosphere, and how to parameterize solar radiation over terrains has become a challenging issue in numerical simulations. Previous scholars have paid a lot of attention to the radiative process related to Supported by the Chinese Academy of Meteorological Sciences 10.5 Key Project under Grant No. 2001BA607B, the National Key Development Program for Basic Sciences under Project No. 2004CB418300, and the Key Project of the National Natural Science Foundation of China under Grant No. 40233037. Corresponding author: qianzh2@mail.nju.edu.cn.

618 ACTA METEOROLOGICA SINICA VOL.23 terrain slope. Fu (1958) discussed the impacts of mountain slope on solar radiation. His result shows that different slopes, seasons, and latitudes have different impacts on short wave radiation, and he theoretically explains why different terrain slopes and orientations have different effects on the diurnal cycle of solar radiation. Lqbal (1984) and Weng et al. (1981) proposed a formula to calculate surface short wave radiation (GSW) with the consideration of the impacts of terrain slope and orientation, but this formula was not then incorporated into numerical weather prediction models. Recently, some scholars have considered the slope related radiation in three-dimensional atmospheric models. Mahrer and Pielke (1977) firstly studied airflows over irregular terrain by numerical simulations. Zhang et al. (2002) studied the effects of the Qinghai- Tibetan Plateau on direct solar radiation and surface temperature. Zhu and Zhang (2005) attempted to depict the effect of topography using three major parameters, including the terrain height, slope, and orientation, and they also considered the short wave solar radiation at the ground and long wave radiation in a regional climate model with p-σ vertical coordinate. If the micro-terrain parameters (such as the slope and orientation) with thermal effects can be parameterized in numerical models, variances in the surface heat balance can be better calculated. It is then possible to make the interaction process in the regional earth-atmosphere system involving complex terrains closer to reality to some extent, and thus, weather and climate systems modeling and prediction can be improved. This will also help us have a better understanding of the regional weather and climate change mechanism. Considering the terrain slope and orientation parameterization, changes of different terrain elements under different weather conditions are not the same. For example, in the clear sky, precipitation is little, thus, the effect of slope and orientation can be truly strong. However, in precipitating weather, since cloud cover and surface humidity may affect radiation through the change of albedo, the effect of slope and orientation on radiation is insignificant. Meanwhile, in the same weather condition, under different model resolutions, the results may vary. Usually with a lower horizontal resolution and gentle terrain slopes, the slope effect on radiation is ignored, but when higher horizontal resolution and steep terrain present, the slope effect on radiation is indispensable. In this paper, in order to improve short wave radiation simulations, a terrain slope and orientation parameterization scheme has been implemented into the non-hydrostatic mesoscale model GRAPES (Global/Regional Assimilation and Prediction System). The effects of terrain slope and orientation on different short range weather processes in China under different model resolutions are simulated and discussed. 2. Model description and characteristics of different weather processes 2.1 GRAPES model description and experiment design The GRAPES model has been developed by the Numerical Prediction Innovative Center of China Meteorological Administration. It is optionally a global or limited area, hydrostatic or non-hydrostatic multiscale numerical prediction model. Descriptions of the parameterization schemes used in this model can be found in the references (Huang et al., 2005; Wu Xiangjun et al., 2005). The short wave radiation parameterization scheme with terrain slope and orientation has been described in detail by Shen and Hu (2006). In this paper, we only discuss the effects of terrain slope and orientation on the surface downward short wave radiative flux. Four model resolutions, i.e., 60, 30, 6, and 3 km, have been chosen. The model domain covers areas of 15 60 N, 70 145 E (East Asia) for 60- and 30-km resolutions; 25 35 N, 105 125 E for 6-km resolution; and 25 35 N, 113.5 122 E (the Yangtze- Huaihe River Valley Meiyu Front District, hereinafter referred to as Meiyu Front District ) for 3-km resolution. From east to west, grid points are 133, 241, 358, and 287 for the four resolutions, respectively, and from north to south, grid points are 81, 151, 179, and 211. Under each weather condition, two numerical experiments have been performed: the control

NO.5 HUANG Danqing and QIAN Yongfu 619 experiment (CTRL) without the slope and orientation parameterization, and the sensitivity experiment (SLOPE) inclusive of the slope and orientation scheme. The two experiments have been integrated for 48 h, and results are output at every 2 h. 2.2 Topographic height and characteristics of different weather processes Figure 1 shows topographic distributions of the model domains at 60-, 30-, 6-, and 3-km resolutions. It clearly presents that under a higher resolution, more detailed topographic structure could be disclosed, especially in areas of Yunnan-Guizhou Plateau, Qinling and Daba Mountains, Dalou Mountains, Dabie Mountains, Wuyi Mountain, and Nanling Mountains, and features of sub-grid terrains can be more accurately described. Four cases have been chosen in 2005, named as 20050621, 20050716, 20050827, and 20050901. The number represents a combination of year, month, and day, during which the case happens. For the first two cases, analysis is focused on the simulation results of 6- and 60-km resolutions, and results of all resolutions are discussed for other two cases. The characteristics of the four weather cases are as follows: (1) Case 20050621. From 17 to 25 June, because of the interaction of warm humid air and weak cold air, the strongest precipitation emerged in most parts of South China and in the middle and lower reaches of the Yangtze River. (2) Case 20050716. Typhoon Begonia (No. 0505) landed at Taiwan during dawn to noon on 18 July. From 17 to 24 July, some parts of Taiwan, Fujian, Zhejiang, Jiangxi, and Anhui provinces suffered strong winds and heavy rainfall. Therefore, the case on 16 July represents a typicall before typhoon landing situation. (3) Case 20050827. This is a fair weather case. Fig. 1. Topographic height (m) under different model resolutions of (a) 60 km, (b) 30 km, (c) 6 km, and (d) 3 km.

620 ACTA METEOROLOGICA SINICA VOL.23 (4) Case 20050901. No. 0513 typhoon Talim at 0730 BT (Beijing Time) 1 September landed at Taiwan, and at 1430 BT landed at Fujian. The weather process on September bears a typical typhoon landing case. According to the weather characteristics, the four cases can be divided into three kinds: before and after typhoon landing cases (cases 20050716 and 20050901), the precipitation case (case 20050621), and the fair weather case (case 20050827). 3. Comparative analysis of the effects of slope and orientation on different weather processes Because the short wave radiation at the ground surface (GSW) is affected by both terrain distribution and weather conditions, simulations of the precipitation case 20050621 with two experiments CTRL and SLOPE under two resolutions are analyzed and compared in detail. For other cases, focus is only placed on the differences compared with the precipitation case. 3.1 Precipitation case (20050621) 3.1.1 The effect on GSW In order to analyze GSW changes affected by terrain slope and orientation, the percentage of the short wave radiation changes is defined as: C = A B B 100 (B 0), C = A B 100 (A 0), or C =0 (A = A 0 and B = 0), where A and B are values of GSW derived from simulations with and without the effects of terrain slope and orientation. Figure 2 gives percentages of GSW changes under a low resolution (60 km). The initial time is at 0000 BT 21 June 2005. Figure 2a is the average of the first 24-h forecasts, Fig. 2b is the forecast result at 26 h, and Fig. 2c is the average of the 48-h forecasts. Comparing Figs. 2a and 2b, the GSW difference Fig. 2. Percentages of GSW changes (unit: %) under a lower model resolution (60 km). The model initial time is 0000 BT 21 June 2005. (a) Average of the first 24-h forecasts; (b) the forecast at 26 h (1000 BT); (c) average of the 48-h forecasts.

NO.5 HUANG Danqing and QIAN Yongfu 621 at 26 h is obviously larger in magnitude than the first 24-h forecast average, and the area of the GSW difference is much wider at 26 h. As shown in Figs. 2a and 2c, the GSW difference of the first 24-h forecast average is slightly larger than that of the 48-h forecast average, but for the spatial distribution, the latter is much broader than the former. In conclusion, with the growth of the integration time, the effects of terrain slope and orientation become more remarkable. However, in a certain period of time, the values of the GSW difference become stable, while the affected area is expanding. Figure 3 gives the percentage of GSW changes in Meiyu front areas under a higher model resolution (6 km). It indicates that the spatial distribution and numerical values of GSW differences under the high resolution are almost similar to those under the low resolution (60 km). However, under the high resolution, the larger GSW differences between CTRL and SLOPE are concentrated in the complex mountainous terrain, which means the effects of terrain slope and orientation on radiation occur significantly over the local topography under the high resolution. 3.1.2 The effect on precipitation Figure 4 shows percentages of GSW changes and difference of precipitation in the Meiyu front areas under different resolutions. As shown in Fig. 4, the difference of precipitation is less than 20 mm between SLOPE and CTRL experiments. Under the high resolution, the increase and decrease of simulated precipitation (contoured) correspond respectively to the decrease and increase of the GSW (shaded) induced by the slope effect. The increments of precipitation are usually ranged between 5 and 5 mm. However, under the low resolution (terrain distribution is shown in Fig. 1), the relationship cannot be discerned. Under the high resolution (6 km), for both 24- and 48-h forecasts, the areas of increased or decreased precipitation lie not only in the regions of larger GSW differences, but also in the mountainous region with complex topography. Figure 4 also points out that under both high and low (60 km) resolutions, the GSW difference of 48-h forecast is larger than that of 24-h forecast. Thus, with the integration time growing, the effects of terrain slope and orientation become more significant. As mentioned above, the relationship between precipitation changes and GSW differences due to terrain slope and orientation is complex. In the SLOPE experiment, since precipitation scheme has not changed, the variance of precipitation is only induced by slope effects. Moreover, the increase or decrease in precipitation, in turn, affects the slope radiation. Therefore, the interaction between precipitation and slope radiation determines the spatial distribution of GSW and precipitation. Nevertheless, while under the higher resolution and steep terrain, the terrain slope and orientation become the primary factor affecting radiation, while precipitation reaction is relatively weak. Thus, under the high resolution (6 km), the relationship between precipitation changes and GSW differences in the mountainous regions of Fig. 3. Percentages of GSW changes (unit: %) in the Meiyu front areas under the higher model resolution (6 km). The model initial time is 0000 BT 21 June 2005. (a) Average of the first 24-h forecasts; (b) the forecast at 24 h (0800 BT).

622 ACTA METEOROLOGICA SINICA VOL.23 Fig. 4. Percentages of GSW changes (unit: %) and differences of precipitation (unit: mm) in the Meiyu front areas for (a, c) the 24-h forecast, (b, d) the 48-h forecast, (a, b) under the lower model resolution (60 km), and (c, d) under the higher model resolution (6 km). complex topography is more remarkable. In the following, the vertical transition of water vapor flux under the effects of terrain slope and orientation is discussed. It is assumed that no water vapor exists in the atmosphere above 300 hpa. The vertical water vapor transport flux of a unit of atmosphere in the whole column is calculated as follows: Q = 1 g 850 300 wqdp, (1) where w is vertical velocity, and q is specific humidity in each layer of the air column. Figure 5 shows the distribution of the difference of average vertical vapor transport flux in the 48-h forecast under the high resolution (6 km). Regions with larger average GSW difference are chosen, i.e., 25 30 N, 110 120 E. Figure 5 indicates that considering the effects of terrain slope and orientation, most regions with GSW differences are negative, thus water vapor upward transport is weakened, just corresponding to the negative variances of precipitation shown Fig. 5. Difference (SLOPE CTRL) of the average vertical water vapor flux (unit: 10 2 Pa s) in the Meiyu front areas under the higher model resolution (6 km). in Fig. 4. 3.1.3 Effects on surface heat flux and surface temperature Changing the short wave radiation scheme has

NO.5 HUANG Danqing and QIAN Yongfu affected the entire surface energy budget. Considering that one of the model output is the surface heat flux component, and the surface heat balance equation can be expressed as: F1 + F 2 = F 3 + F 4 + F 5, (2) where F1 is upward long wave radiation; F2 is upward heat flux (including sensitive heat and latent heat fluxes); F3 is surface heat flux from soil to the ground surface, all of which have a direct relationship with surface temperature; F4 is long wave radiation at the ground surface; and F5 is the GSW. Generally speaking, if the short wave radiation scheme (then GSW) is changed, both surface heat flux and surface temperature will be affected. As Sections 3.1.1 and 3.1.2 presented, under the low resolution (60 km), it is difficult to find the relationship between the effects of terrain slope and orientation on changes in GSW and precipitation over the complex mountainous terrain. The impact of surface heat flux and the surface tem- 623 perature is the same (figure omitted). Therefore, only results under the high resolution (6 km) are given as follows. Figure 6 shows the difference (SLOPE-CTRL) of the surface heat fluxes and surface temperature of the 24-h average forecast in the Meiyu front areas, under the higher model resolution (6 km). It clearly indicates that the variances of surface temperature and surface heat flux are consistent. Corresponding to the high-resolution topographic distribution shown in Fig. 1c, the large changes of surface heat flux and surface temperature caused by various slope and radiation occur over the most complex terrains, such as the Qinling Mountains, Daba Mountains, Lou Hill, Dabie Mountains, Wuyi Mountains, and other mountainous regions. Regions with differences of heat flux and surface temperature in 24-h forecast are in accordance with those in 48-h forecast, but the latter are more obvious. The largest differences rise to 15 W m 2 Fig. 6. Difference (SLOPE-CTRL) of the surface heat fluxes (W m 2 ; a, c) and the surface temperatures (K; b, d) in the Meiyu front areas under the high model resolution (6 km) for (a, b) the 24-h forecast, and (c, d) the 48-h forecast.

624 ACTA METEOROLOGICA SINICA VOL.23 and 0.4 C, respectively. For both 24-h and 48-h forecast results, differences are consistent in surface temperature and surface heat flux. For the 24-h forecast result, regions of the surface temperature changes of more than 0.2 C correspond to where the surface heat flux changes are more than 5 W m 2. For the 48-h forecast result, regions with the surface temperature changes of more than 0.3 C correspond to the surface heat flux changes of more than 10 W m 2. As shown in Fig. 6, over the ocean, differences of heat flux and surface temperature are not in accordance with each other. There are subtle differences of heat flux over the ocean, but surface temperature does not change. This is because the sea surface temperature that drives the GRAPES model is prescribed, thus, there is no difference over the ocean in the two experiments. The surface heat flux alters because the effects of slope and orientation are incorporated into the model, and the atmospheric circulation has been changed, and then the ocean flux is impacted. With the growth of the integration time, the difference of ocean heat flux becomes more remarkable with large values and wider areas. Figure 7 depicts the difference of surface heat flux of 2-h forecast. There is a slight difference in the ocean between the two experiments. It well explains that in a short period of time, the effects of terrain slope and orientation do not influence the circulation. Therefore, there is no surface heat flux difference over the ocean. On the other hand, regions with appreciable differences mainly concentrate in mountainous areas of complex topography. Fig. 7. Differences (SLOPE CTRL) of the surface heat fluxes (unit: W m 2 ) for the forecast at 2 h. 3.1.4 Effects on 500-hPa geopotential height and 500- hpa temperature To further explain why heat flux and surface temperature also change over the gentle terrain after considering the effects of terrain slope and orientation, the circulation changes impacted by slope radiation are analyzed. Analyses of 500-hPa geopotential height and temperature in the 24- and 48-h forecasts (figure omitted) suggest that they have changed due to the effects of terrain slope and orientation, thus the effects extend from mountainous regions to the sea. The geopotential height difference is in the range of 0.8 to 0.8 gpm, while the temperature difference is in the range of 0.4 C to 0.2 C. For both 24- and 48-h forecast results, regions of 500-hPa geopotential height differences are relatively consistent with the 500-hPa temperature differences. Especially, for the 48-h forecast, distributions of their differences match better. 3.1.5 Relationship between precipitation in the first and the second 24-h forecasts In order to compare the results in the first and second 24-h forecasts, considering the characteristics of the precipitation case, the relationship between GSW and precipitation has been analyzed. Figure 8 gives percentages of GSW differences and changes (SLOPE CTRL) of precipitation in the first and second 24-h forecasts. As shown in Fig. 8, the GSW differences in the second 24-h forecast are larger than the first 24-h forecast, and regions with differences of GSW and precipitation in the first and latter 24-h forecast average well correspond, and mainly concentrate in the Wuyi Mountain areas. From the analysis above, in this case, not only short wave radiation but also the precipitation are affected by terrain slope and orientation. With the growth of the integration time, effects of terrain slope and orientation gradually enlarge the differences of some physical elements. Therefore, differences in the second 24-h forecast are larger than those in the first 24 h. 3.2 A comparative analysis of the fair weather case, the before-typhoon-landing case and the after-typhoon-landing case 3.2.1 The effect on GSW Comparing the fair weather case (20050716),

NO.5 HUANG Danqing and QIAN Yongfu 625 Fig. 8. Percentages of averaged GSW changes (%; a, b) and differences (SLOPE CTRL) of precipitation (mm; c, d) in the Meiyu front areas under the high model resolution (6 km) for (a) average of the first 24-h forecasts, (b) average of the latter 24-h forecasts, (c) the first 24-h forecast, and (d) the latter 24-h forecast. the before-typhoon-landing case (20050827), and the after-typhoon-landing case (20050901), the effects of terrain slope and orientation on GSW are almost the same as case 20050621. That is, the GSW differences of instantaneous forecast are larger than the average forecast, basically by an order of magnitude, and regions of differences are in a wider range. At the same forecasting time, changes of GSW differences in the high-resolution (6 or 3 km) simulations are more evident and the spatial distribution is more detailed (figure omitted). However, comparing case 20050827 and case 20050621, GSW differences of case 20050621 are more obvious than those of case 20050827. Though GSW differences of case 20050827 are not significant, the FY-2 satellite image (figure omitted) indicates that for case 20050827, for the 24-h forecast, the cloud cover extends to a larger range across the sky, affecting the short wave radiation, which may be one of the reasons why the GSW differences are not so large. GSW differences in case 20050716 are more significant than those in case 20050621, and the FY-2 satellite image (figure omitted) shows that under the 3-km resolution, particular in the mountainous areas of complex topography, little cloud cover results in slight precipitation, so that the case can relatively truly reflect the changes of each physical element in the model after considering the effects of terrain slope and orientation. 3.2.2 Effects on precipitation, surface temperature, 500-hPa geopotential height, and 500-hPa temperature As mentioned above, under the low resolution (60 km), it is difficult to find the effects of terrain slope and orientation on GSW and precipitation and their relationship with the complex topography of the mountainous areas, but under the high resolution (6 or 3 km), for both the 24- and 48-h forecasts, differences of precipitation and GSW are evident, and regions of

626 ACTA METEOROLOGICA SINICA VOL.23 the differences are almost the same. Especially, the differences of precipitation in the 48-h forecast are obviously larger than those in the 24-h forecast and the regions are broader. In addition, larger changes of surface heat flux and surface temperature due to the effects of terrain slope and orientation mainly occur in complex terrain regions. Moreover, because of the effects of terrain slope and orientation, differences of 500-hPa geopotential height and 500-hPa temperature extend gradually from mountains to the ocean with the growth of integration time. For both the 24- and 48-h forecasts, regions where the geopotential height differs with temperature at 500 hpa are consistent, especially in the 48-h forecast. 3.2.3 Comparing the results before and after the typhoon landing At comparing different weather conditions before and after the typhoon landing with the effects of terrain slope and orientation, it is known that circulation will vary with the different weather conditions and short wave radiation will also change after typhoon landing. Figure 9 gives percentages of GSW changes in forecasts before and after typhoon landing. Comparing Figs. 9a with 9b, the GSW difference in the 6-h instantaneous forecast is much larger than the 6-h average forecast. The different region becomes more extensive and the former difference is larger. Regions with larger differences lie in accordance with the mountains. Comparing Figs. 9b with 9c, it is clearly indicated that differences of the latter are slightly larger than the former. The latter differences are more uniform, but do not show the corresponding relationship between the regions of larger differences and the mountain locations. A possible reason is that the typhoon landed at 1430 BT in Putian, Fujian Province, brought abundant rainfall and other adverse weathers, which covered up the effects of Fig. 9. Percentages of averaged GSW changes (unit: %) before and after the typhoon landing. (a) At 1400 BT 1 September, (b) average of the first 6-h forecasts, and (c) average of the 1400 BT 1 and the 0800 BT 2 September forecasts.

NO.5 HUANG Danqing and QIAN Yongfu 627 terrain slope and orientation on the short wave radiation. 4. Conclusions and discussion In this paper, a terrain slope and orientation parameterization affecting short wave radiation at the ground surface has been implemented into the nonhydrostatic mesoscale model GRAPES. The effects of terrain slope and orientation on different short range weather processes in China under different model resolutions are simulated and discussed. For four cases with different weather conditions, i.e., 20050621, 20050716, 20050827, and 20050901, the simulation results are compared and analyzed. Conclusions are as follows: (1) Under a high model resolution (3 or 6 km), whatever the weather processes are, regions with larger GSW differences between the two experiments with and without slope parameterization mostly concentrate in the mountainous areas of complex topography. The increase and decrease of simulated precipitation respectively correspond to the decrease and increase of GSW induced by the terrain slope effects, and the changes of precipitation are usually ranged between 5 and 5 mm. Changes of surface temperature and GSW have a better relevance, and they mostly occur in the mountainous regions of complex topography. The effects of terrain slope and orientation on short wave radiation can be more truly reflected (e.g., in case 20050716) when precipitation is slight and cloud cover is little. However, under a low model resolution (30 or 60 km), no matter what the weather is, it is difficult to find the relationship mentioned above. Therefore, when under a low model resolution and gentle slopes, the terrain slope effects on radiation are usually ignored. But when the model resolution is high and the terrain is steep, the effects of terrain slope and orientation on GSW have to be considered. (2) Differences of GSW, precipitation, surface temperature, surface heat flux, and other elements, are more obvious in the mountainous regions of complex topography than in other regions. This is because the effects of terrain slope and orientation in the model cause the atmospheric circulation to change, and other physical elements to vary as well. (3) In the case 20050601, GSW differences in the 6-h instantaneous forecast are much larger and extensive than in the 6-h average forecast. Regions with larger differences are usually oriented along the mountain range, especially for the 6-h forecast results. Comparisons of the conditions before and after typhoon landing clearly show that the differences of the latter are slightly larger than the former. The latter differences are averagely distributed, not showing the consistency of larger differences along the mountain range as in the former. This is possibly because the typhoon landed in Putian, Fujian Province at 1430 BT, brought heavy rainfall and other adverse weathers, and decreased the short wave radiation affected by the terrain slope and orientation. Conclusions of this paper are based on analyses of only four selected cases with different weather conditions. There are still many issues worthy deep investigations, such as variations of additional physical elements other than those examined in this paper and identification of the most significant change under the terrain slope and orientation effects and associated reasons. The terrain slope and orientation parameterization could also be considered for long wave radiation, so that the effects of complex topography on radiation can be more truly represented in numerical models. REFERENCES Fu Baopu, 1958: The effect of slope on sunlight and solar radiation. J. Nanjing University (Natural Sciences), 2(1), 22 45. (in Chinese) Huang Liping, Wu Xiangjun, and Jin Zhiyan, 2005: Schemes and applications of GRAPES model standard initialization. J. Appl. Meteor. Sci., 16(3), 374 384. (in Chinese) Li, L., and Zhu B. Z., 1990: The modified envelop topography and the air flow over and around mountains. Adv. Atmos. Sci., 7, 249 260. Li Qingquan, Ding Yihui, and Zhang Peiqun, 2004: Primary verification and assessment on the extra-seasonally predictive capability of a global

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