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Relative role of domain size, grid size, and initial conditions in the
simulation of high impact weather events
Himesh S 1 , Goswami P 1 , and Goud BS 2
1. CSIR Centre for Mathematical Modeling and Computer Simulation, NAL Belur Campus, Wind Tunnel Road, Bangalore-
560037, India. Email:himesh@cmmacs.ernet.in
2. UVCE, Department of Civil Engineering, Bangalore University, Bangalore-560056, India
1. Summary
The limited domain, and the associated artificial lateral
boundaries, introduces several uncertainties and errors into
simulation of Limited Area Models (LAM). The size of the
model domain with its implicit geographical coverage also
implicitly determines the large-scale dynamics and terrain
effects. Thus, domain size and resolution together will
determine the spectrum of resolved scale and the nature of
scale interaction in the model dynamics. A comparative,
comprehensive and quantitative estimation of the relative
role of domain size, horizontal grid size (horizontal grid
spacing) and initial condition in the simulation of mesoscale
events is however, lacking. We investigate this issue using a
mesoscale model (MM5V3) with respect to heavy rainfall
event with a series of ensemble (5 initial conditions)
simulations. We first use a high-resolution (10-km) bench
mark simulation to show the model’s performance in
simulating extreme rainfall events. The sensitivity
simulations (varying domain size and grid distance) are
carried out at coarser resolutions in view of the large number
of simulations involved, and as our emphasis is on relative
roles and not on precise forecast. Our results show that along
with initial conditions and grid distance the size of the
domain also significantly affects simulated quantities like
total rain, maximum rain and other dynamical fields. While
the quantitative aspects of this conclusion are likely to
change based on type and location of the event, the results
show that domain size play as much an important role as that
of horizontal resolution and initial condition in the
simulation of high impact weather events like heavy rainfall.
2. Introduction
Genesis and evolution of mesoscale events are strongly
determined by local inhomogenities of boundary forcings in
addition to non-hydrostatic dynamics that is characterized by
small spatial scales. Limited area or mesoscale models
(LAM) thus continue to be the most popular and effective
tools for simulating and forecasting mesoscale events.
Numerous studies have demonstrated the ability of high
resolution mesoscale models to successfully simulate high
impact weather events like extreme rainfall and their
associated synoptic features. A necessary price for such high
horizontal resolution, however, is a small numerical domain
of integration due to computational constraints. In reality,
both resolution and the size of the mesoscale domains play
critical role in the quality of the simulation. The size of the
domain implicitly determines the large-scale dynamics and
terrain effects, while the horizontal resolution determines the
smallest resolvable scale. Thus the size of the domain and
the resolution together determine the spectrum of resolved
scales and the associated scale interaction in the model
dynamics. Warner et al (1997) described the Lateral
Boundary Conditions (LBC) as potential limitation and an
inevitable problem of regional weather prediction. Giorgi et
al (1999) discussed the importance of the choice of model
domain in mesoscale simulation and concluded that it was
not feasible to evolve general criteria for the choice of
model domain but to depend largely on trial and error
approach. One of the necessary requirements in mesoscale
simulations to minimize the adverse impact of lateral
boundary condition on the model solution is to keep the
lateral boundaries away from the region of interest (Pielke,
2002). The question of relative sensitivity of domain size,
horizontal grid distance and initial conditions on the
quality of mesoscale simulation, however, has not yet been
studied in depth in the context of high impact weather
events. The objective of the present study is to investigate
this issue in the context of high impact weather event
occurred in Indian Monsoon region.
3. Brief Synoptic Description of the Event
The extreme rainfall event that flooded the metropolis of
Mumbai on the west coast of India (72.52E and 18.52N)
was an intense and highly localized meso-scale (meso-β)
convective system, with a spatial scale of only about 30
Km. It was also a short-lived system, with recorded
rainfall of around 94 cm in 24 hours between 26 -27 th July
2005, with most of the rainfall occurred in the afternoon
of July 26 th , 2005. Satellite pictures had shown the event
as a centre of intense precipitation along with a few other
precipitation centers in the west-east direction. Satellite
images also showed deep cloud system (one form east and
the other from Arabian Sea) over the region. The overall
synoptic situation was that of intense convective activity
with strong continuous convergence of offshore winds
(westerlies).The other features were; the presence of low
pressure system (22N, 85.5E) and its associated cyclonic
circulation up to 300mb, offshore trough was also seen at
10N-20N.
4. Design of Experiments
In our study we have considered Extreme Rainfall Event
occurred over Mumbai (west coast of India, July 26-27,
2005). Fifth generation NCAR /Penn State mesoscale
model (MM5V3) was used in this study. This is a nonhydrostatic
model [Dudhia, 2004] with multiple choices
for parameterization of various processes like cumulus
convection, Planetary Boundary Layer (PBL) and nesting
capability. All simulations in this study were carried out
with a single domain and with same physics options
(cumulus parameterization-Anthes-Kuo, Boundary
Condition-Relaxation., Microphysics-Simple Ice,
Radiation Scheme- Dudhia, Vertical levels-23). We have
considered three grid distances (90, 60 30 and 10-km) in
our study to examine the relative role of domain size and
horizontal grid distance in the simulation of extreme
rainfall event. While it could be argued that proper
simulation of mesoscale events require much higher grid
distance, the focus here is on quantitative assessment of
relative roles of domain size versus grid distance through
large number of sensitivity studies. Seven domains (D1 to
D7, Fig.1.) of varying longitudinal and latitudinal extent
were chosen for this study, out of which five domains (D2
to D6) were common to all the three grid distances (90, 60