On the Space-Time Patterns of Precipitation in the Himalayan Range: a Synthesis
Ana P. Barros
Pratt School of Engineering, Duke University, Box 90287, Durham NC 27708,
e-mail: barros@duke.edu
Abstract
Through diagnostic studies combining space-time scaling analysis of ground-based hydrometeorologicalobservations, radiosonde profiles, METEOSAT and TRMM satellite data, as well as simulations using a Cloud Resolving
Model, we were able to identify and characterize the dominant weather systems and associated precipitation processes in theCentral Himalayas: a) monsoon depressions; b) wintertime storms; c) stationary orographic gravity waves; and d)ridge-locked convection. Our analysis shows that while the first two regimes are associated mainly with large-scale
circulations, and exhibit strong inter-annual variability in frequency, intensity and spatial track; the second two regimes
control the diurnal cycle during the monsoon and the spatial distribution of precipitation year round (Barros et al. 2004a-b,
Lang and Barros 2004, Magagi and Barros 2004, Barros and Lang 2003, Lang and Barros 2002, Barros et al. 2000).
Therefore, a synthesis of Himalayan hydrometeorology is proposed that relies on three principal modes of space-time
variability: 1) an inter-seasonal mode linked to large-scale dynamics that explains infrequent events producing significant
amounts of precipitation over one-three day periods (wintertime storms and monsoon depressions); 2) a regional mode linked
to ocean-land-atmosphere interactions over Northern-India and the Bay of Bengal at time-scales of days to weeks consistent
with the succession of rainy and dry episodes during the break and active phases of the monsoon; and 3) an orographic modethat explains the spatial variability of the diurnal cycle on the Himalayan range during the monsoon.
Keyword: Himalaya, Precipitation, Monsoon, Climate

1. Introduction
This manuscript consists of a review of recent
publications that constitute the intellectual foundation of thesynthesis proposed in the abstract.

2. Ground Observations
A hydrometeorological network was installed in the
Marsyandi river basin in Central Nepal to obtain high-frequency(1/2 hourly) measurements of rainfall and snow accumulation,
temperature, relative humidity, wind direction and speed along
upwind ridges and adjacent valleys in central Nepal up to 5,500 m,
and then into the rainshadow.
The first phase of thehydrometeorological network was installed during the pre- andpost-monsoon seasons in 1999 to augment the existing operational
network operated by The Nepalese Department of Hydrology andMeteorology. This network has been enlarged by five stations
and improved in the pre-monsoon season of 2000 with NSF
funding.
Currently the network consists of 20 operational
stations, including six towers, distributed by three ridges, twovalleys and the rainshadow plateau behind the Annapurna Range
into the Tibetan Plateau (Barros et al. 2000). In the Spring of
2001, one of the network towers (Telbrung specifically, seediagram to the right) was equipped with radiation sensors (solar,
longwave, PAR), soil temperature and soil moisture probes at threedifferent depths, as well as relative humidity, surface pressure andair temperature and wind speed and direction sensors at two
different levels.
The Marsyandi network provided new insights into thewater cycle of the Himalayan region. For the first time, it waspossible to document the widespread and long-lasting rainfall
events associated with the monsoon onset in the region, showingthat although precipitation mechanisms may differ, the monsoon
onset is equally strong up to 5,000 m. Along altitudinal gradientsin the Middle Himalayas, the annual precipitation total amounts to
4 m of water equivalent per unit area, and we do not detect
substantial changes with elevation up to 5,000m. However, thetiming and phase of precipitation does change for elevations above
3,000m with increasing snow component that reaches about 40%
at 5,000m. The strong rainshadow effect in the monsoon season
is not apparent during the winter. We find similar accumulations ofsnow from our highest station on the upwind slopes and up thevalleys in the north-facing slopes of the Annapurnas into the
Tibetan Plateau. This can be explained in part by the fact thatsnow is lighter than rain, and therefore can be transported anddistributed over large distances.

Fig. 1: Geographic location and layout of the network.
Analysis of METEOSAT data suggests however that theorigin and pathways of storm systems during the winter season arevery different from those during the monsoon and thus should bestudied separately. Beside the aforementioned rainfall totals, both
Barros et al. (2000) and Lang and Barros (2002a) noted significant
spatial variability in precipitation (factor of ~4 over ~10 km
distance), which did not show any specific dependence on
elevation, particularly at the seasonal scale.
In addition, Barros et al. (2000) noted an interesting
nocturnal peak in rainfall, just past midnight (See Fig.2 below).

This is unusual for mountainous regions, which often show anafternoon peak associated with diurnally forced upslope flow.
While the afternoon peak is visible in the data (Barros et al. 2000),
it is secondary, particularly at low elevations. One of the moreinteresting facets of the nocturnal rainfall peak is that it reaches amaximum during the summer monsoon, while during other timesof the year the afternoon peak is predominant.

Fig.2: Average diurnal cycle of monsoon rainfall using stations
marked in Fig.1 above.

3. Hydroclimatic Scenarios - Monsoon Onset
The local and large-scale aspects of the 1999, 2000 and2001 monsoon onsets in this region have been described andexplained through a variety of observations (Lang and Barros 2002,
Barros and Lang 2003). The onsets manifested themselves in theform of heavy, multi-day rain events, which then heralded thearrival of subsequent daily/near-daily rainfall for the rest of thesummer season. The rain events were synchronized with aregional wind shift to moist, conditionally unstable, southeasterly
upslope flow. The rain systems consisted of broad areas ofstratiform rain with embedded convection. The large variability
in rain totals was the result of variability in convective activity;
rain gauges with larger fractions of convective rainfall generally
received the most rain, and vice-versa. Large-scale circulations
likely set the stage for high precipitation accumulations. But
small-scale and mesoscale circulations, affected by the local
topography and unresolved by the ECMWF analyses used here,
probably played key roles in actually realizing the heavy rainsfrom convection, and determining which gauges received the mostrain.
Through an examination of the large-scale conditions,
it was determined that Bay of Bengal monsoon depressions were
the proximate cause of the onsets in central Nepal. These
depressions occurred very early in the season, as the Indian
monsoon was strengthening and spreading northward. The most
relevant features of these depressions were the enhanced winds
on their northeastern flanks, in response to storm motion andblocking by the Himalayas. These winds provided moist
upslope flow in central Nepal, and the convergence caused bytheir eventual deceleration led to the development of organized
convection in this region.
In summary, onset monsoon
depressions tend to form over the easterly notch of the monsoon
trough over the Bay of Bengal, and propagate westward on thenorthern flank of the westerly jet over the Indian subcontinent:
the more southward the jet, the weaker the interaction with theeasterly vertical shear along the Himalayas, keeping thedepressions away from the Himalayan range, and favoring
disorganized convection over northern India. For example, the2001 onset depression moved mostly west from the Bay, and did
not interact as strongly with the mountains as did the onset
depressions from 1999 and 2000 (Lang and Barros 2002).
Therefore, although the atmospheric structure exhibited definitechanges, rainfall totals were not as high as in 1999 and 2000

because convection associated with the depression did not reachthe mountains. That is, local processes facilitated by thesynoptic SE monsoon flow were more important for rainfall
during the 2001 onset.

4. MOHPREX – Monsoon Himalaya Precipitation
Experiment
In June of 2001, with funding from the National Science
Foundation, MOHPREX (Monsoon Himalaya Precipitation
Experiment) took place. The experiment consisted of launching
GPS radiosondes at 3-hour intervals from a location within the
network region at the foothills of the Himalayan range during the
25 days at the beginning of the monsoon (Barros and Lang 2003).
Although this experiment proper took place without NASA funds,
several participants were supported by TRMM and the Marsyandi
network provided the essential infrastructure.
The
comprehensive data sets obtained during MOHPREX are essential
to evaluate the skill of our modeling efforts vis-à-vis the
reproduction of the regional thermodynamic structure of the lower
atmosphere during the monsoon, and to elucidate the role ofsurface hydrological processes on the diurnal cycle of rainfall
which will be instrumental in establishing confidence toextrapolate our simulations of the space-time variability of rainfall
from Central Nepal to the entire Himalayan range.

5. ModelingA three-dimensional, anelastic, and non-hydrostatic
cloud-resolving model (CSM) originally introduced by Clark(1977, 1979) and Clark and Hall (1991, 1996) was used in ourmodeling experiments. The focus of our initial modeling effort
was to investigate the daily cycle of orographic effects on thespatial variability of winds and rainfall under monsoon conditions(deep moist flow) in the Himalayas (Barros and Lang 2003,
Barros et al. 2004). The results indicate that orographic gravity
waves forced by the underlying terrain may be responsible formuch of the observed spatial variability. Vertical motion cells
formed band-like structures that remained largely stationary intime, with updrafts on the slopes and downdrafts over ridges.
Predominant wind direction was a critical factor in the spatial
pattern of meteorological fields, reflecting the change in effective
topography presented as an obstacle to flow. Upvalley flow in
response to diurnal solar forcing was strongest in open valleys
and weakest in enclosed ones. This flow weakened but did notreverse itself during the evening. The lack of a coupled landmodel to incorporate the diurnal cycle of surface fluxes asboundary conditions prevents the development of surface
convergence zones along the Himalayan range, which may be
critical to produce the observed nocturnal rainfall, a distinctive
feature of the diurnal cycle of precipitation during the monsoon.
For illustrative purposes, we discuss here a simulation of the 1999
monsoon onset. The model was initialized with ECMWF
analysis data, with three nested grids at 24, 4, and 2 km resolution
centered with a sub-domain that encompasses the central
Nepalese Himalayas.
Model simulations of the spatial
distribution of cloud fields were compared to IR satellite imagery
for the same times (see above). Note the rotational cloud
structure at approximately 0600 UTC June 12, 1999, which isreadily apparent in the IR imagery. The actual rain rates
obtained from simulation results have a maximum near 70 mm/hr
and their locations and spatial structure concur with the TRMM
estimates of maximum values near 70 - 80 mm/hr.

(Magagi and Barros, 2004).

8. Orographic Land-Atmosphere Interactions in
the Himalayas During the Monsoon and Spatial
Variability of Precipitation

Fig. 3: Model simulations of onset depression in 1999.

6.

Winter Hydrometeorology and Snow Storms

Snowfall contributes up to 25-35% of annual
precipitation at high elevations in the central Himalayas. This
percentage increases with altitude and tends to mitigate differencesin annual precipitation between drier very high altitude stations (>4000 m MSL) and wetter lower elevation ridge stations(3000-4000 m MSL). The snow contribution can be up to 100 cm
or more, liquid water equivalent. However, these amounts arestrongly modulated by inter-annual variability, which can exceed
20-30% of annual totals. Low-elevation stations (< 3000 m MSL)
stay above 0 °C and receive little rain during the winter months.
The bulk of winter precipitation in the central Himalayas is causedby so-called Western Disturbances, which are westerly waves
trapped and intensified by the unique large-scale topographic
features, most notably the notch formed by the Himalayas andHindu-Kush mountains. The inter-annual variability of thesesystems is large, but difficult to predict based on popular climate
indices (ENSO, AO, etc.). There is some evidence suggestingthat the overall strength of the circumpolar westerly flow and theplacement of the wave with respect to the Tibetan Plateau (north or
south) are controlling factors in the frequency of snowstorms.
Modeling simulations suggest that orographic forcing is thedominant factor in wintertime precipitation in the central
Himalayas (Lang and Barros 2004, Barros et al. 2004).
Significant precipitation in this region only occurs when thelarge-scale flow evolves to a favorable geometry with respect tothe mountains. In addition, fast-moving precipitation features
likely associated with along-boundary waves play an importantrole in bringing precipitation to lower elevations.

7. Energy and Moisture Fluxes - Latent Heating
RetrievalBased on a cumulus convection parameterization andthe thermodynamic equilibrium equation, we developed a simple
algorithm to extract the vertical profile of the latent heating froma combination of radiosondes and TRMM-PR data, mainly theradar reflectivity and the rain rate estimates. The methodology
was evaluated through an application to the Himalayan region[70-95E, 15-32N]. As a first step we determine the spatial andtemporal criteria that provided a convenient match between the
TRMM-PR data and the radiosondes. Then, cloud parameters andthe rain rates at the top of the moist layer were extracted from the
TRMM-PR 2A25 products. For the months of June 1999, June
2000, and June 2001, average values of the maximum latentheating are 7.5 K hr-1, 10 K hr-1, and 9.50 K hr-1, respectively,
with a high variability probably related to the intrinsic variabilityof cloud and atmospheric parameters. Mean values of 4.06 km,
3.43 km, and 4.44 km were obtained, respectively, on June 1999,
June 2000, and June 2001, for the height of the maximum latentheating. These results place the peak production of latent energy
at the average height of the great Himalayan range, which isconsistent with the observed distribution of rainfall in the region

The linkages between the space-time variability ofobserved clouds, rainfall, large-circulation patterns andtopography in northern India and the Himalayas were investigatedusing remote sensing data. The research purpose was toinvestigate the hypothesis that cloudiness patterns are dynamic
tracers of rainstorms, and therefore their temporal and spatial
evolution can be used as a proxy of the spatial and temporalorganization of precipitation and precipitation processes in the
Himalayan range during the monsoon. The results suggest thatthe space-time distribution of precipitation, the spatial variabilityof the diurnal cycle of convective activity, and the terrain(landform and altitudinal gradients) are intertwined at spatial
scales ranging from the order of a few kms (1-5 km) up to thecontinental-scale. Furthermore, this relationship is equallystrong in the time domain with respect to the onset andintra-seasonal variability of the monsoon.
Infrared and microwave imagery of cloud fields wereanalyzed to characterize the spatial and temporal evolution ofmesoscale convective weather systems and short-lived convectionin Northern India, the Himalayan range, and in the Tibetan
Plateau during three monsoon seasons (1999, 2000 and 2001).
The life cycle of convective systems suggests landform andorographic controls consistent with a convergence zone
constrained to the valley of the Ganges and the Himalayan range,
bounded in the west by the Aravalli range and the Garhwal
mountains and in the East by the Khasi Hills and the Bay of
Bengal, which we call the Northern India Convergence Zone
(NICZ). The NICZ exhibits strong night-time activity along thesouth-facing slopes of the Himalayan range, which ischaracterized by the development of short-lived convection (1-3
hours) aligned with protruding ridges between 1 and 3 AM. The
intra-annual and inter-annual variability of convective activity in
the NICZ were assessed with respect to large-scale synoptic
conditions, monsoon activity in the Bay of Bengal, and themodulating role of orography. Scaling analysis of cloudinesssuggests three different scaling regimes of orographic
land-atmosphere interactions: 1) a synoptic-scale regime [70-80
km]; 2) an orographic meso- regime [30-70 km] associated withthe succession of wide valleys and bulky terrain features; and 3)
an orographic meso- regime [ 30 km] associated with the complex
succession of protruding south-facing ridges and narrow valleys
that characterize the Himalayan foothills between 3,000 and 5,000
m. In addition, empirical orthogonal function (EOF) andcanonical correlation (CC) analysis suggest that joint modes ofvariability of monsoon weather and topography, which we callorographic land-atmosphere interactions, modulate the space-time
variability of cloudiness in the region.

References
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Lang, T.J. and Barros, A.P., 2004: Winter Storms in theCentral Himalayas. J. Japanese Meteo. Soc., Vol.
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