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METEOROLOGICAL FLOW CHARACTERISTICS OF DIFFERENT REGIONS OF NEPAL Ram P. Regmi & Sangeeta Maharjan
Atmospheric Resource and Environmental Research Laboratory
Central Department of Physics, Tribhuvan University, Kathmandu, Nepal
Figure 7: Near surface wind vectors (left) and cross sectional plot of potential temperature, wind
vectors, vertical wind (right ) along the northwest-southeast line.
5. RATIONALIES
Figure 8: Comparison of calculation and model predictions of near surface wind speed,
temperature and relative humidity over Kathmandu.
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Regional scale spatial and temporal distribution of meteorological flows over different regions of
Nepal have been studied with the application of Weather Research and Forecast Model (WRF)
incorporating field observations to identify major roots of air mass transport towards Himalayas
from the Ganges Plain. The calculation domain consists of triply nested two-way interacting
mesh with 9km, 3km and 1km grid resolution for coarse, fine and finest domains. The study
includes selected areas located in the Eastern, Central, Western, Mid-Western and Far-Western
Development Regions of Nepal. The study has identified three major routes of air mass transport
from Ganges Plain to the Tibetan Plateau from Western, Central and Eastern regions of Nepal.
The air mass transport route from Western Region of Nepal appears to be highly important for
aerosols/suspected particulates transport from Ganges Plain to Nepal and then to the Tibetan
Plateau.
ABSTRACT
Atmospheric transport processes over
the complex terrain of Nepal Himalaya
are yet to be studied extensively. Very
few studies appears from this region.
Present study, thus, has been carried out
to understand the meteorological flow
characteristics over different parts of
middle hills of Nepal Himalaya and to
assess possible roles on the transport of
aerosols from low lands of Ganges Plain
up into the Himalayas.
Past studies appear largely confined
within Kathmandu valley in conjunction
with air pollution and meteorological flow
characteristics over couple of Himalayan
valleys in the of Western Development
Region of Nepal. As the prevailing
meteorological flow characteristics over
the middle hills of Nepal Himalaya can
have significant role in pumping up the
lowland air pollutants and suspected
particulates of the Ganges Plain to the
Himalayas and Tibetan plateau, present
study has been focused in
understanding the local flows of the
middle hill regions of Nepal.
1. INTRODUCTION
Figure 1: Bird Eye view of study areas in
the middle hills of Nepal Himalaya. The
figures show the terrain structure of the
finest domain of WRF calculation
domains of 51km x 51km areas.
0
10
20
30
40
50
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1000150020002500
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Terrain elevation in meters above mean sea level
2. THE STUDY AREA The study has been conducted in several
regions of Nepal. Out of several areas
studied, so far, we would like to present
the preliminary findings over six different
areas, namely, Dadeldhura, Surkhet,
Palpa, Kathmandu, Makawanpur, and
Remechhap areas located in the Middle
Hills of Nepal Himalaya in the Far-
Western, Mid-Western, Western, Central,
and Eastern Development Regions. The
calculation domains of the study areas
were centered, respectively, at Amargadi
(29.31N, 80.59E), Birendranagar (28.57N,
81.59), Tansen (27.7N, 85.3E),
Kathmandu (27.7N, 85.3E), Hetaunda
(27.38N, 84.99E) and Manthali (27.33N,
86.08E). These areas capture several
municipalities, small towns and densely
populated villages. An impression of
topographic complexities of the study
areas may be gained from the
Figure 1. The prevailing local meteorological flow
systems over the study areas for dry
season have been numerically simulated
with the application of meso-scale
meteorological model Weather Research
and Forecast (WRF) model default
physics options. The model was initialized
with 1°x1° NCEP/FNL meteorological and
25 categories land use and 30 second
USGS terrain elevation data. The domain
system consists of a triply nested two-way
interacting mesh. Each domain includes
51 X 51 X 34 grid points, and horizontal
grid sizes are, and for the coarse, fine,
and the finest domains, respectively.
3. METHODOLOGY
The simulated results were
compared with observations, as
far as, the data were available.
Simulation results show that the regional scale mountain-plain wind system
over the Middle Hills of Nepal Himalayan develops in combination of
several types of local flows such as valley, upslope, and plain-to-plateau
winds. The local flows over different region of Nepal appear to possess
remarkable differences in nature and in their diurnal and spatial
distribution. One of the important common features is the little day-to-day
variation in diurnal and spatial distribution of the local flow in all the regions
despite having characteristics differences in their flow patterns. It is
expected that the air mass circulation characteristics discussed in this
paper should largely capture the typical nature of local flows that prevails
over the region during the dry season.
4. RESULTS
a. Local Flow Pattern in the Far-Western Region The local flow structure over the Far-Western Region of Nepal appears to
be significantly different from the flow prevailing over other parts of the
region. The most remarkable feature of the local flows over this region is
that there prevails a deep, laminar, strong and nearly unidirectional wind
almost 24 hours. However, deep valleys and river basins of this region may
remain calm for only couple of hours during the night time. No such local
flow structure were predicted in other parts of the region studied, so far.
The reasons behind this particular feature of local flows over this region is
still under investigation.
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b. Local Flow Pattern in the Mid-Western Region
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Figure 2: Near surface wind vectors (left) and cross sectional plot of potential temperature, wind
vectors and vertical wind (right ) along the northwest-southeast line.
The local flows over the this region appear to be remarkably different from
the far-western region. There is no persistency in the speed, direction and
vertical structure in the local flows prevailing over this region. Valleys
become largely calm through out the night and morning times with
occasional intrusion of near surface wind from southeast. Valley
atmosphere appears to be strongly stratified. The local flows over this
region may be characterized as normal mountain valley wind system.
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West-East Distance (km)
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Figure 3: Near surface wind vectors (left) and cross sectional plot of potential temperature, wind
vectors, and vertical wind speed (right) along the southwest-northeast line.
The local flow structure over the Western Region middle hills of Nepal
appears to be typically shallow. The local flows over major wind corridors
over this region appear to switch just 180 ° during day and nighttime. The
straight wind corridor (Lumbani-Palpa-Gulmi) over this region is expected
to be playing a significant role in transporting low land pollutants up into
the Himalayas. The visible transport of black aerosol clouds along this
wind corridor from lowland towards the Himalayas may qualify this
assertion.
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c. Local Flow Pattern in the Western Region
d. Local Flow Pattern in the Western part of Central Region
The western part of the Central Region appears to be significantly
influenced by frequent subsidence flows from about 4 km above the mean
sea level during day and night. The subsidence appears to be relatively
strong during the late afternoon time. Such a flow structure have not been
predicted in the other parts of the region studied. The subsidence may
bring the upper level aerosols (Brown Clouds) to affect the heath of
endangered species protected in the Chitwan National Park.
Figure 4: Near surface wind vectors (left) and cross sectional plot of potential temperature, wind
vectors, and vertical wind (right ) along the southwest-northeast line.
West-East Distance (km)
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km
) 1745 LST 1745 LST
West-East Distance (km)
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km
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Figure 5: Near surface wind vectors (left) and cross sectional plot of potential temperature, wind
vectors, and vertical wind (right ) along the south-north line.
West-East Distance (km)
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e. Local Flow Pattern in the Central Region
Central region of Nepal appears to experience pronounced upslope wind
during the daytime whereas it appears largely calm during the nighttime.
Kathmandu valley over this region typically executes a plateau/basin dual
nature. The major wind corridors over this region appears to be southern
plain-Kathmandu-Sunkoshi River valley and to the Himalayas and the
southern plain-Trishuli River valley-Himalayas. Unlike the Western Region
wind corridor, the flow appears to be rather one-way traffic, that is, it
largely experiences daytime upslope and very weak downslope wind.
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f. Local Flow Pattern in the Eastern Region
The local flows over the Eastern Region appears to be quite different from
other part of the middle hills of Nepal Himalaya. Very pronounced
downslope wind appears over the region during the nighttime and tend to
do the same during the daytime as well except flow over the southern plain
area tends to be southeasterly. The downslope wind appears rather
shallow. Interestingly, valleys over this region appear mostly calm during
the daytime which was not the case in other parts of the middle hills.
Figure 6: Near surface wind vectors (left) and cross sectional plot of potential temperature, wind
vectors and vertical wind (right ) along the northwest-southeast line.
West-East Distance (km)
South
-Nort
h D
ista
nce (
km
)
1745 LST 1745 LST
West-East Distance (km)
South
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h D
ista
nce (
km
)
1745 LST 1745 LST
6. CONCLUSION
• Middle Hills of Nepal Himalaya possesses different local flow
characteristics in different regions during the dry season.
• The local flow over Far-Western Region appears to be deep, persistent
and strong compared to other parts of the country.
• The local flows over Western Region of Nepal may be playing an
important role in the transport of lowland pollutants up into the Himalayas
and to the Tibetan Plateau.
• Strong subsidence appears over the western part of Central Nepal.
• Central Nepal possesses two major routes of air mass transport from
southern plain to Himalayas via Kathmandu valley and along Trishuli
River valley west to Kathmandu.
• The daytime upslope wind over flows over Eastern Nepal is rather weak
compared to the nighttime downslope wind in contrast to the western and
central regions.
Acknowledgements:
The research was partly supported by University Grants Commission, Nepal.
Contact: [email protected]
