| The linkages between the space-time variability of observed clouds, rainfall,
large-circulation patterns and topography in northern India and the Himalayas
were investigated using remote sensing data. The research purpose was to test
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 temporal organization of precipitation and precipitation processes
in the Himalayan range during the monsoon. The results suggest that the
space-time distribution of precipitation, the spatial variability of 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 (1–5km) up to the continental-scale. Furthermore, this relationship
is equally strong in the time domain with respect to the onset and intra-seasonal
variability of the monsoon.
Infrared and microwave imagery of cloud fields were analyzed to characterize
the spatial and temporal evolution of mesoscale convective weather systems and
short-lived convection in 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 and orographic 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 the
south-facing slopes of the Himalayan range, which is characterized by the
development of short-lived convection (1–3h) aligned with protruding ridges
between 1:00 and 3:00 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 the modulating role
of orography. Empirical orthogonal function (EOF) and canonical correlation (CC)
analysis suggest that joint modes of variability of monsoon weather and topography,
which we call orographic land-atmosphere interactions, modulate the space-time
variability of cloudiness in the region. Finally, scaling analysis of cloudiness
suggests three different scaling regimes of orographic land-atmosphere
interactions:
1) a synoptic-scale regime (≥70-80km); 2) an orographic meso–β regime
(30–70km) associated with the succession of wide valleys and bulky terrain
features; and 3) an orographic meso–α regime (≤30km) associated with
the complex succession of protruding south-facing ridges and narrow valleys that
characterize the Himalayan foothills between altitudes of 3000 and 5000m elevations. |