Abstract
Abstract. We investigate whether explicit representation of the urban land surface improves the simulation of the record-breaking 24-h heavy rain event that occurred over Mumbai, India on 26 July 2005 as the event has been poorly simulated by operational weather forecasting models. We conducted experiments using the Regional Atmosphere modeling system (RAMS 4.3), coupled with and without explicit urban energy balance model-town energy budget (TEB) to study the role of urban land – atmosphere interactions in modulating the heavy rain event over the Indian monsoon region. The impact of including an explicit urban energy balance on surface thermodynamic, boundary layer, and circulation changes are analyzed. The results indicate that even for this synoptically active rainfall event, the vertical wind and precipitation are significantly influenced by heterogeneity in surface temperatures due to urbanization, and the effect is more significant during the storm initiation. Interestingly, precipitation in the upwind region of Mumbai city is increased in the simulation, possibly as a feedback from the sea breeze – urban landscape convergence. We find that even with the active monsoon, the representation of urbanization contributes to local heavy precipitation and mesoscale precipitation distribution over the Indian monsoon region. Additional experiments within a statistical dynamical framework show that an urban model by itself is not the dominant factor for the enhanced rainfall for a Mumbai heavy rain event; the combination of updated SST fields using Tropical Rainfall Measurement Mission (TRMM) data with the detailed representation of urban effects simulated by the TEB model created realistic gradients that successfully maintained the convergence zone over Mumbai. Further research will require more detailed morphology data for simulating weather events in such urban regions. The results suggest that urbanization can significantly contribute to extremes in monsoonal rain events that have been reported to be on the rise.
Highlights
On 26 July 2005, an unexpected heavy precipitation event occurred over the Mumbai urban region and adjacent areas
The results suggest that urbanization can significantly contribute to extremes in monsoonal rain events that have been reported to be on the rise
With the town energy budget (TEB) run, the results indicated that the low-level convergence increased up to 3000 m and decreased thereafter
Summary
On 26 July 2005, an unexpected heavy precipitation event occurred over the Mumbai urban region and adjacent areas. Shyamala and Bhadram (2006) extended the analysis using observational data for synoptic, thermodynamic, radar, and satellite analyses They concluded that a cloud band over the Arabian Sea on 25 July corresponded to a strong low-level wind over Mumbai, and that mid-troposphere dryness might have contributed to the heavy rain. Kishtawal et al (2008) analyzed the long-term rainfall data and found urban regions have more statistically significant precipitation occurrence as compared to the rural regions Studies such as Braham and Wilson (1978), Shepherd and Burian (2003), and Niyogi et al (2006) have shown urban land surface can influence local storm structure causing enhanced convection and increased precipitation. By using an explicit 3-D urban surface energy balance model – Town Energy Budget (TEB), we can better represent the land – atmospheric interactions and improve the simulation of the heavy rain event over Mumbai city in a mesoscale model; 2. The effect of urbanization on heavy precipitation occurs on a relatively local scale near the urban region, and the large-scale precipitation will be influenced by the sea surface temperature (SST) fields
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