An Investigation of the Onsets of the 1999 and 2000 Monsoons in Central Nepal

Abstract

The Marsyandi River basin in the central Nepalese Himalayas is a topographically complex region, with strong spatial gradients of precipitation over various timescales. A meteorological network consisting of 20 stations was installed at a variety of elevations (528-4435 m) in this region, and measurements of rainfall were made during the 1999 and 2000 summer monsoons. The onsets of the 1999 and 2000 monsoons in central Nepal were examined at different spatial scales by using a combination of rain gauge, Meteosat-5, Tropical Rainfall Measuring Mission (TRMM), ECMWF analysis, and Indian radiosonde data. At the network, the onsets manifested themselves as multiday rain events, which included a mixture of stratiform and convective precipitation. Moist and unstable upslope flow was associated with the occurrence of heavy rainfall. During each onset, 2day rainfall reached as high as 462 mm, corresponding to 10%-20% of the monsoon rainfall. Differences among rain gauges were up to a factor of 8, reflecting the role of small-scale terrain features in modulating rainfall amounts. At the larger scale, the onsets were associated with monsoon depressions from the Bay of Bengal that moved close enough to the Himalayas to cause the observed upslope flow from the winds on their eastern flank. During the 1999 onset, convection in this eastern flank collided with the mountains in the vicinity of the network. In 2000 no major collision occurred, and 33%-50% less rain than 1999 fell. Analysis of observations for a 5yr period (1997-2001) suggests that the interannual variability of the monsoon onset along the Himalayan range is linked to the trajectories and strength of these depressions.

Probing orographic controls in the Himalayas during the monsoon using satellite imagery

“…These spatial features are consistent with previous studies of synoptic-scale monsoon circulations in the region: 1) the role of the Aravalli range in separating western, drier and more unstable air masses from moist and relatively stable air masses from the Bay of Bengal ( Fig. 4b; Murakami et al, 1999); and 2) the detection of shallow embedded convection only over the central Himalayas during TRMM PR (Precipitation Radar) overpasses (Lang and Barros, 2002;Barros and Lang, 2003a).…”

“…7a and 7b). Lang and Barros (2002) pointed out that precipitation in this region mostly was associated with shallow convection embedded in stratiform precipitation systems during onsets, consistent with the characteristically low frequency of lightning flashes. Analysis of TRMM data, surface observations and model simulations (Barros et al, 2000;Barros and Lang, 2003a, b) indicate that the spatial variability of rainfall is consistent with the spatial variability of orography in the frontal ridge-valley region (5-10 km), and that the high nocturnal rainfall on the Himalayan ridges results from the synergistic interaction of orographic gravity waves and atmospheric instability.…”

“…He found that the diurnal convection over the Tibetan Plateau is enhanced in the afternoon and is suppressed in the early morning during the summer monsoon, while on the eastern foothills of the Himalayas heavy convection prevailed in the early morning and was suppressed in the afternoon. Regions of strong low-level shear and high atmospheric instability develop on the N-NW flanks of monsoon vortices along the Himalayas and against the smaller Aravalli and Maikala mountain ranges in the Madhya Pradesh (Lang and Barros, 2002;Figs. 1 and 4c).…”

“…During the monsoon onset, heavy precipitation events in the Himalayas are associated with relatively organized shallow convection at the foothills of southfacing slopes and valleys brought by mesoscale depressions originating in the Bay of Bengal (Lang and Barros, 2002). Along the Himalayan range, convective cells appear embedded in large areas of persistent stratiform rainfall, which occur over elevations of 5000-6000 m and exhibit a leadingline-trailing stratiform MCS-type structure.…”

Chemical composition and aerosol size distribution of the middle mountain range in the Nepal Himalayas during the 2009 pre-monsoon season

“…It is also possible that there may be some localized influence at Site B during traditional cooking hours due to the relatively higher density of the residential housing near the sampling station, which coincide with the timing of peak concentrations. Nevertheless, because of the narrow width and orientation of the Marsyangdi valley at Besisahar (Site B), the diurnal cycle of mountain-valley circulations is very consistent at this location independently of large-scale conditions (Barros and Lang, 2002), suggesting that Site B is suitable for monitoring the influence of large-scale aerosol transport from the IGP to the inner regions of the Himalayas.…”