Mechanism of Intrusion and Channeling of Regional Air Mass Over Kathmandu Valley
Sangeeta Maharjan & Ram P. Regmi
Atmospheric Resource and Environmental Research Laboratory,
Central Department of Physics, Tribhuvan University, Kathmandu, Nepal
ABSTRACT
3. METHODOLOGY
Numerical experiments were performed to understand the
mechanism of intrusion and channeling of regional aerosols over
the Kathmandu valley with the applications of regional scale
atmospheric transport model by releasing tracers inside and
outside the valley. The study reveals that the regional aerosol
intruding into the valley with the westerly/southwesterly wind
contributes 20 to 80 percent of the total concentration over the
valley during the night and daytime, respectively. The valley’s air
mass, particularly, over the central area of the valley appears to
be significantly decoupled from the regional air mass transport
processes during the nighttime whereas in the afternoon regional
air mass regularly sweeps the valley floor with little day-to day
variation.
• Meteorological fields were generated with the application of
PSU-NCAR Meso-scale Meteorological Model MM5.
• Hypothetical emission fields of identical non-reacting species
were developed for inside and outside the valley separately at
1km x 1km grid resolution.
• Atmospheric transport processes over the valley were
performed using the Chemical Transport Model developed at
Kitada Laboratory, TUT, Japan.
• Calculations were performed for a week during late winter
time in Kathmandu.
Distributions of tracer released outside the Kathmandu valley floor .
1. INTRODUCTION
c. Coupling strength of local
and regional flows
The Kathmandu valley is a broad bowl-shaped valley located in
the middle hills of Central Nepal Himalaya. It is completely
surrounded by steeply rising mountains and hills except that it
has a narrow river gorge in the southwest edge and couple of low
mountain passes in the west and east edges connecting
surrounding lowland valleys. The valley floor is relatively
leveled with an average elevation of 1350 meters above the mean
sea level (MSL) and is interspersed with shallow and low flow
riverbeds. The valley possesses a warm temperate semi-tropical
and seasonable climate. The dry seasons over the valley persists
for October to June with only 10 to 20 % of the annual rainfall
during the season. Spring months are showery, windy with higher
humidity and pre-monsoon thunderstorm and occasional hails.
The spatial and temporal
distribution of the ratio of the
tracer concentration released
inside the valley floor to the
total tracer concentration, that
is, X2/(X1+X2) appears to
vary from 0.8 to 0.2. The ratio
appears highest during the
night and early morning times
and attains a minimum value
in the afternoon at around
1400 LST. After 1800 LST, it
Winter skies over the valley
remains clear leading to the
formation of early morning
dense fog that may linger up
to late morning. In the
afternoon, the valley often
appears hazy. This may
indicate that the valley
possesses a poor dispersive
power and there could be an
appreciable decoupling of
valley air mass with the
regional flows.
However, detail mechanism
of decoupling is yet to be
understood. Present study,
thus, attempts to quantify the
magnitude of decoupling of
valley air mass with the
regional flows.
Kg km-2 day-1
Kg km-2 day-1
Hypothetical Emission Fields. Tracer X1 released outside the valley (left) and
Tracers X2 released inside the valley (right).
abruptly
increases
in
conjunction with the ceasing of
the prominent southwesterly
and northwesterly local flows
in the valley. This may imply
that the air mass over the
Kathmandu valley floor largely
remains decoupled from the
regional flows during the night
and morning times. However,
during the late morning until
late afternoon time, the local air
mass of the valley floor is well
coupled with the regional
flows.
4. RESULTS
a. Distributions of tracer
released inside the valley
Google Earth view of Kathmandu and its
surroundings.
Formation of dense fog in the morning and
haze in the afternoon.
2. RESEARCH QUESTIONS
Present study seeks answer to the following questions:
• What spatial and temporal patterns does the local air mass
assumes over the valley?
• What are the major pathways of intrusion and channeling of
regional air masses?
• How strongly the valley air mass is decoupled from regional
flows?
In order the understand the
mechanism of flushing out the
pollutants released within the
valley boundary, the tracer X2
was
released
uniformly
throughout the valley floor
keeping zero concentration
outside its boundary. The tracer
X2 appears to be stagnated
within valley from midnight to
early morning time. The
stagnated tracer starts to
disperse in the late morning
moving slowly towards the
east. The valley finally flushes
the tracers into the eastern
neighboring valley close to the
noontime. The flushing of
tracers continues till late
afternoon. In the evening valley
again starts to accumulate
tracers within its boundaries
until the mixed layer activities
develops next day. Very little
day-to-day variation appears in
the dispersion processes over
the valley.
Distributions of tracer released within Kathmandu valley floor.
b. Distribution of tracer
released outside the valley
The intrusion of outside
pollutants into the valley was
explored by releasing the
tracer X1 uniformly outside
the valley floor and zero
emissions inside. Calculation
suggests
that
there
is
appreciable influx of outside
tracer into the valley that gets
accumulate
during
the
nighttime within the valley.
Contact: sangeetamaharjan@gmail.com & regmi_rp@hotmail.com
Spatial and temporal distribution of the ratio of tracer released inside the valley
to the total tracer released
5. CONCLUSIONS
1.
The Kathmandu valley located in the middle hill region of
Central Nepal Himalaya typically executes plateau/basin
dual nature.
2.
Depending upon the time and area, regional aerosols can
have 20 to 80% contribution in the overall concentration in
the valley.
3.
The valley air mass appears to well coupled with the
regional flows at least during the daytime.
4.
The valley brings regional air masses into the valley from
northwest and southwest mountain gaps and channels it to
the eastern valley that finally moves towards the Himalayas
in the north.