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A building-resolving simulation of sea breeze over Sendai downtown
with a parallelized CFD model
Guixing Chen, Weiming Sha, and Toshiki Iwasaki
Department of Geophysics, Graduate School of Science, Tohoku University, Sendai, Japan

Hiromu Seko and Kazuo Saito
Meteorological Research Institute, Tsukuba, Japan

In the study, we develop a parallelized CFD model to realize the building-resolving
simulation for one whole city. The test run on Sendai downtown at a high resolutionof 3 m is conducted by the NEC SX-9 supercomputer at the Cyberscience Center ofTohoku University. The experiment is designed to simulate the winds and temperatureduring a typical sea breeze event. The general features of turbulent flows in the urban
canopy layer are well reproduced. The results suggest that this parallelized CFD
model is promising for the high-resolution urban weather forecast in the future.

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1. Introduction
In recent years, a research project of high performance numerical weather
prediction, as one part of the Strategic Programs for Innovative Research of Japan, isinitiated to achieve the high-precision mesoscale forecast [1]. As a part of this project,
the building-resolving CFD model is developed to realize the super high-resolution
mesoscale modeling of the urban weather [2]. The purpose is to simulate the winds
and temperature for big city with every buildings resolved.
One of the major difficulties we faced is the huge demand of calculation amount,
and another is the memory limit at a single node. To solve these problems, we convert
the source code of CFD model to support the high-performance calculation with theMessage Passing Interface (MPI). This parallelized CFD model has been recently
tested on the K computer at RIKEN Advanced Institute for Computational Science,
for a large domain of 25 km with a resolution of 10 m. In this study, we transfer thisCFD model to the NEC SX-9 supercomputer at the Cyberscience Center of Tohoku
University. The test run on SX-9 is implemented to understand the performance of theCFD model at an even higher resolution of 3 m for the realistic complex building.

2. System configuration and experiment design
The CFD model used here is a local meteorological model based on large-eddy
simulation [3]. The model is based on the Cartesian coordinate, with the block-off
technique to treat the buildings and steep topography. The model equations are solved
by an algorithm of the semi-implicit method for pressure-linked equation, and a fulltime-implicit scheme is applied. This CFD model has been applied to simulate thelocal flows and temperature on the small-scale buildings and street blocks [4]. The
CFD code has been revised to support the MPI for parallel calculation. The parallel
calculation is made at both X and Y directions, while the auto parallel split is applied
at Z direction.
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! $()  The sea breeze is a typical weather phenomenon during the summer season that
greatly influences the local winds and temperature at the coastal areas [5]. At 13 JST
June 19, 2007, sea breeze blows over the Sendai City. In this study, the experiment isdesigned to study this sea breeze penetration over the Sendai downtown. The model
domain is centered at Sendai station with a scale of about 3 km (Figure 1). The spatial
resolution is 3 m for X, Y, and Z directions. The model top is 300 m, with 100 verticallevels. Therefore, the total grid points are one hundred millions. The forecast time isabout 120 seconds. The initial conditions of atmosphere and ground temperature areprovided the short-range forecast of non-hydrostatic model [6, 7] from the assimilated
data [8]. On SX-9 supercomputer, we use 16 nodes to perform the parallel calculation.
It takes about 14 minutes to complete the forecast.

(a) Full domain

3 km

3 km
(b) Sendai station

Sendai Station

Figure 1. The buildings of Sendai downtown for the CFD model. (a) Full domain; (b)Sendai station

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3. Results
Figure 2 shows the general pattern of zonal wind over the Sendai downtown atforecast time T=120 seconds. The tails of low wind speed are elongated at the lees ofthe buildings, with an horizontal extension of about 2-3 times of the buildings height,
which are often interrupted by the downstream buildings. As a whole, the buildings
drag is clearly shown to slow down the wind speed. In the vicinity of some tallbuildings, the local maximum of high wind speed can be easily identified. Broadly
speaking, in the urban canopy, the air flow exhibits strong turbulent characteristics
due to the complex buildings structures. Compared to the low wind speed within theurban canopy, the sea breeze exhibits a relatively fast penetration at the high layerfrom the southeastern regions.
Figure 3 shows the spatial distribution of temperature near Sendai station. It isclearly shown that the air mass trapped in the urban canopy exhibits a strong turbulent
feature near the major buildings. There are some warm bubbles that grow at the leesof the buildings, which are generally stretched downstream by sea breeze. Although
the wind speed is some low in the urban canopy, there is an active vertical mixing thatstrongly transports warm air upward and cool air downward. Considering a relatively
fast penetration of cool marine air above urban canopy, sea breeze helps to decrease
the temperature near surface from top in the presence of strong vertical mixing. Thissuggests that sea breeze can bring an effective cooling to the urban areas, despite of
the low wind speed by building drag. The cooling effect may be estimated by thewind speed of marine air above urban canopy and the vertical mixing within, which isusually related to the urban ventilation.

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m/sFigure 2. Spatial pattern of the zonal wind speed over Sendai downtown.

Figure 3. Spatial distribution of the temperature near Sendai station.

4. Summary
In this study, we have conducted the building-resolving CFD simulation on thesea breeze over the Sendai downtown. The general features of turbulent flows in theurban canopy are well simulated by the CFD model. The results suggest that theparallelized CFD model has a good performance at the very high resolution of 3 m. It
should be emphasized that this is a realistic simulation of the weather conditions.
With high-precision mesoscale forecast available to drive the CFD model, it seems
reasonable to realize the high-resolution forecast of urban weathers for street corners
in the coming years [1, 2]. In the ongoing works, we continue to perform moreexperiments and verify the system performance with intense observation network.

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Acknowledgment
This study was partly supported by the Strategic Programs for Innovative Research
funded by the Ministry of Education, Culture, Sports, Science and Technology(MEXT) of Japan. Numerical calculations had been performed using the K computerat RIKEN Advanced Institute for Computational Science (Proposal number hp120282)
and the supercomputing resource at the Cyberscience Center, Tohoku University.
Technical supports were given through the collaborative works with NEC company,
JAMSTEC Earth Simulator Center, and Tohoku University Cyberscience Center.

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