A total of eight sensitivity experiments have been conducted to find the impact of microphysical schemes on surface wind forecasting in the WRF-ARW model in complex terrain at a location of a wind turbine cluster. Here, we analyzed only the inner domain (2 km resolution) produced simulations. Figure 2 shows wind at 120-meter height time series of all experiments from 5th June 12 UTC to 8th June 12 UTC, 2022. SCADA data, which is our observation, is indicated in a black dotted line. The 3-day simulation performed reasonably well as compared to the observation. Although, two abnormal peaks in simulation at around the 26th and the 50th hour lead-time can be observed. These overestimations may be accompanied by high sensitivity to a slight fluctuation, while the WSM5 scheme simulated comparatively well with fewer fluctuations.
From Fig. 2, we see that WSM5 performed better than other microphysical schemes. Apart from windspeed, we also investigate other important physical variables which have direct/indirect impact on windspeed. To examine the performance of microphysical schemes on other physical variables, domain averaged LST, RH and PBL height have been investigated and compared against the ERA5 dataset, shown in Fig. 3. Windspeed is clearly linked with PBL height, RH, and LST. From Figs. 2 and 3, windspeed is almost proportional (inversely proportional) to RH (LST and PBL).
Interestingly, it can be seen that very high wind (an anomaly compared to SCADA data) is observed when PBL and LST are high, and RH is low. Generally, it is observed that lower LST, PBL height and high RH resulted in higher wind speed. As per the physical variables, high simulated wind speed at around 24th hours lead-time can be considered as the right response by the model. Where we can see SCADA observation showed a much lower windspeed, but the model showed a large overestimation. The model’s internal dynamics may play a significant role in this case. Overall, as per our model’s response to other physical variables like LST, RH, and PBL height, wind speed outcome can be regarded as a correct response. But a comparatively large underestimation can be observed in LST and PBL at around 30th and 53rd hours lead-time (Fig. 3), where we can also see a large overestimation in windspeed by the model.
Thus, model response was appropriate, but from 30th to 53rd lead-times LST and RH simulation show noticeable difference from the observation except PBL, where PBL height is correctly simulated in between this lead time. Therefore, it is seen that windspeed is highly related with the PBL height.
On the other hand, microphysical schemes represent the hydrometeor distribution of cloud, domain averaged reflectivity is also plotted to observe the hydrometeor distribution pattern (Fig. 4). Here, we see the THOMPSON, GODDARD, WSM5 and WSM6 schemes produced higher concentration of hydrometeor distribution, resulting in higher reflectivity. While FERRIER and KESSLER schemes did not produce much concentration of hydrometeors like others. This figure is the purpose for representation of hydrometeors by all the schemes. Here, we can see the response of WSM5 as compared to the THOMPSON in terms of hydrometeor distribution. Although, hydrometeor distribution directly impacts the cloud-forming process, but for windspeed it has an indirect effect, via cloud-PBL-temperature feedback. A study by D. Choudhury et al. (2017) also reported that for high-resolution WRF modeling, Thompson and Goddard schemes are suitable for tropical cyclone intensity forecasting; they can capture well the cloud-hydrometeor structure of a system.
Finally, to get a quantitative assessment, Table 2 documents the MAE and MAPE of all experiments. From Table 2, it can clearly see that WSM5 performs the best among all other microphysical schemes with 1.42 m/s MAE. Where 1.73, 1.5. 1.5. 1.67, 1.63, 1.72, and 1.65 m/s MAEs are observed for FERRIER, GODDARD, THOMPSON, KESSLER, LIN, WSM3, and WSM6, respectively. In terms of MAPE too, WSM5 produced the least error with 25.64%, while the maximum MAPE is found for Ferrier with 32.8%. Poor representation of hydrometeor in Ferrier and Kessler schemes, as seen in Fig. 4, can lead to higher MAE than other schemes.
Table 2
Model performance based on Mean Absolute Error (MAE) and Mean Absolute Percentage Error (MAPE)
EXPTS | FERRIER | GODDARD | THOMPSON | KESSLER | LIN | WSM3 | WSM5 | WSM6 |
MAE (m/s) | 1.73 | 1.5 | 1.5 | 1.67 | 1.63 | 1.72 | 1.42 | 1.65 |
MAPE (%) | 32.8 | 27.96 | 27.96 | 31.93 | 31.59 | 31.99 | 25.64 | 29.82 |
Summaries
Microphysical sensitivity experiments were conducted for complex terrain wind forecasting over a wind-turbine cluster site in Maharashtra in India. 6 km outer domain and 2 km inner domain resolution in two-way nesting mode have been used for WRF-ARW model simulation. After conducting some PBL sensitivity experiments, YSU is considered the PBL parameterization for both domains, KF is taken as the cumulus scheme for the outer domain, and none for the inner domain. It is well established that the PBL plays a very significant role in high-resolution wind forecasts by NWP models. After finding a suitable combination of dynamics and physics options, we design microphysical sensitivity experiments to choose the best microphysical scheme for such complex terrain wind forecasting. A total of eight sensitivity experiments have conducted to find the most suitable microphysical scheme for complex terrain wind modeling using WRF-ARW.
The experimental results showed that the WSM5 scheme produced the least MAE for such complex mountainous terrain, while FERRIER performed the worst, and produced the largest error in wind speed at 120-meter height. Moreover, for complex terrain wind forecast modeling, PBL and topography representation plays a vital role. The performance of other physical variables for different microphysical schemes remains almost similar with minor fluctuations. We know that large differences exist between the schemes for total water vapour, cloud water, and accumulated precipitation. The different distributions of atmospheric moisture strongly impact on both the shortwave and longwave downward fluxes at the ground.
From our experiments, we can say that the high-resolution WRF-ARW model with proper dynamical and physical scheme combinations significantly improves in wind forecasting over complex terrains wind turbine sites. However, this study has few potential limitations. Due to lack of availability of SCADA data in other weather variables (LST, RH, etc.), coarser-resolution ERA5 data have been compared to the high-resolution simulated parameters. And the sensitivity experiments conducted in the study focused on a narrow temporal and spatial scope, consisting of only a 3-day simulation and a small geographical region. Further studies will be required for longer-period simulation in multiple windfarm sites, especially over the intra-seasonal periods and with several other physical combinations of the WRF-ARW model to get more robust combinations.