Data and Analysis
A numerical weather prediction with a convection permitting regional atmospheric model was used for the simulation of ten (10) severe weather events. They were simulated based on two simulations. On the event that happened on November 25-26, 2009 the accumulated rainfall was 140mm between 0600 and 1200 UTC. Convection-permitting simulations were performed, particularly using the WRF or Weather Research and Forecast Model. This was used to produce a short-term numerical weather forecast-type simulations of 36-h duration from 1200 UTC of the previous day to 0000 UTC of the following day. The first 12h of the simulation was not included in calculating the total precipation because it was considered the spin-up period. Therefore, they produced ten 36-h of simulations for each event. The physical parameters used in the WRF model include the particle property scheme for microphysics (Morrison and Milbrandt 2015), the Mellor-Yamada-Janjic´ planetary boundary layer scheme (Janjic´ 1994), the RRTMG shortwave and longwave schemes (Iacono et. al. 2008), the Unified Noah Land Surface Model (LSM) (Tewari et. al 2004), and the eta similarity scheme for the surface layer (Janjic´ 2002, 1994; Monin and Obukhov 1954). They also used the urban canopy model within the LSM which was used in the URBAN experiment. The urban scheme was then replaced with the natural land cover for the desert simulation.
The MODIS land-use data of the WRF Model was used in identifying urban grid points, and the urban morphology parameters are specified. Based on the previous studies that used WRF model and the urban parameters, it suggests that the model performs well at simulating urban rainfalls. The simulations are then analyzed to evaluate land coverage effects on precipitation and the nine other cases are added for the assessment. Changes in meteorological conditions at the surface and in the atmosphere with regards to precipitation are explained. Based on the figures shown in the article, it was found out that the model agrees with the actual observations. Focusing on the effects of urbanization in precipitation, it shows that precipitation is greater in the URBAN scheme than in DESERT. The recorded amount of rainfall for DESERT experiment was about 50 to 140mm per day, while in the URBAN, the longest range was 70mm per day indicating a significant increase in rainfall in the urban area.
The area if interest now is the physical mechanisms behind the differences in rainfall pattern in the two experiments. Change in land use was investigated next to find any connections with the said differences in precipitation. It was observed that the sensible and latent heat flux of the two simulations were barely indistinguishable. Given that the planetary boundary layer was deeper even before the rainfall event happen, this deeper PBL intensified atmospheric instability in the urban, compared to the desert. This implies the impact of urbanization, and it was also shown that urban areas have higher surface temperature relative to the desert. The influence of urbanization in the intensification of rainfall due to land use and UCM was about 30% of the total average.
The dynamics behind the effects of urbanization to precipitation was investigated using the vorticity equation to evaluate the terms that contributes to the physical processes. The first term on the right hand side represents the horizontal advection, the second term represents vertical advection, followed by the third term which indicates the divergence acting on vorticity. The fourth term represents tilting of the vertical sheared flow and the last term represents gradients in friction force. The terms for both experiment were calculated hourly based in the simulations. The analysis of different terms in the vorticity equation suggests that the horizontal and vertical advection, and the divergence are the main factors that contribute to the dynamical mechanisms. A moisture analysis was also conducted which showed that additional moisture from the Red Sea provided development for the storm.
Instrumentation
The instruments used in the study were the Weather Research and Forecasting (WRF) Model which incorporated the urban canopy model and land surface model. An article was published regarding the implementation and verification of LSM in WRF Model. It was because of the strong interaction of the LSM with the atmosphere at all scales which shows its significance in this model (Tewari et al. 2004) Existing studies about the urban canopy model also shows its effectivity when incorporated in the WRF model (Chen et al. 2011). Results of extensive simulations conducted in previous studies using the same scheme and parameters suggest that the WRF Model performs well when it comes to simulating urban rainfall (Dasari et al. 2017, 2019; Viswanadhapalli et al. 2017; Yesubabu et al. 2016). The Climate Prediction Center (CMORPH) and the Tropical Rainfall Measuring Mission were also used in analyzing the spatial distribution of the rainfall events that happened in Jeddah over the past decades. Existing studies about CMORPH shows that this method yields spatially complete precipitation analyses which is a great help for this study (Joyce et al. 2004). Same goes for the Tropical Rainfall Measuring Mission. (Huffman et al. 2007). The analyses from the modeling and observational data are therefore in agreement with regards to the increase in intensity of precipitation over Jeddah due to urbanization.
Conclusion and Recommendation
It has been claimed that urbanized surfaces reduce albedo and absorb more incoming solar radiation . This caused increase in heat storage of urban areas. There is daytime cooling and during nighttime, this stored heat are released causing cities like Jeddah to have increased planetary boundary layer height.
The ten heavy rainfall events with the similar synoptic features are studied to analyze the effect of urbanization in Jeddah City's rainfall. The simulations from two experiments suggested to have a 26% increase on rainfall in the urban. Observations from different datasets also indicated increase in rainfall over the city for the past years. The conclusion from this experiments is that the urban area of Jeddah exhibits a nocturnal heat-island effects which leads to a deeper PBL, stronger baroclinicity, and more unstable atmosphere. These factors enhanced convective activities that eventually leads to thunderstorm formation and increase in rainfall.
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