Monitoring the Monsoon in the Himalayas: Observations in Central Nepal, June 2001

Abstract

The Monsoon Himalayan Precipitation Experiment (MOHPREX) occurred during June 2001 along the south slopes of the Himalayas in central Nepal. Radiosondes were launched around the clock from two sites, one in the Marsyandi River basin on the eastern footslopes of the Annapurna range, and one farther to the southwest near the border with India. The flights supported rainfall and other hydrometeorological observations (including surface winds) from the Marsyandi network that has been operated in this region since the spring of 1999. The thermodynamic profiles obtained from the soundings support the observed nocturnal maximum in rainfall during the monsoon, with total column moisture and instability maximized just before rainfall peaks. Coinciding with the appearance of a monsoon depression over central India, the onset of the monsoon in this region was characterized by a weeklong weakening of the upper-level westerlies, and an increase in moisture and convective instability. The vertical structure of convection during the project was intense at times, and frequent thunder and lightning were observed. This is suggestive of monsoon break convection, which is expected to be predominant since the monsoon had not fully matured by the end of the month. Comparisons of the MOHPREX data with the NCEP-NCAR reanalysis data reveal that upper-level winds are characterized relatively well by the reanalysis, taking into account the coarse model topography. However, moisture is severely underestimated, leading to significant underestimation of rainfall by the reanalysis. The interaction of the ambient monsoon flow with the south slopes of the Himalayas, modulated by the diurnal variability of atmospheric state, is suggested as the primary cause of the nocturnal peak in rainfall.

Rising Precipitation Extremes across Nepal

“…Similar results were obtained from station-based observations in different regions of Nepal where precipitation peaks around 2500-3600 mm and decreases further with increase in altitude in high mountain regions [48][49][50][51]. In addition to the horizontal and altitudinal precipitation gradients, a large seasonal precipitation gradient (~factor of 4) has also been observed over a short horizontal distance of ~10 km in MRB [52].…”

“…In the DHM database, the longest precipitation record available since 1946 is from only three stations, although there are records from 23 stations since 1947, and from around 40 stations since 1950. Until 1956, the precipitation observations were available only from the stations located In addition to the horizontal and altitudinal precipitation gradients, a large seasonal precipitation gradient (~factor of 4) has also been observed over a short horizontal distance of~10 km in MRB [52].…”

The neglected Indo‐Gangetic Plains low‐level jet and its importance for moisture transport and precipitation during the peak summer monsoon

“…To show that the cyclonic flow over the Ganges Basin and IG LLJ does not reflect the mean of many transient events and a mean flow persists, we decompose the total moisture flux over the region for the JRA55 during July and August (S3) and show the temporal monthly distribution of zonal winds over the IGP transect (blue line in Figure 1a) from May to September (S4). We briefly evaluate the possibility that the easterly flow is induced by changes in diurnal heating over topographic slopes [Barros and Lang, 2003] and show four-times daily output of the zonally averaged winds for JRA55 (S5).…”

Effect of Spatial Resolution and Cumulus Parameterization on Simulated Precipitation over South Asia

“…Typically, there is also a well-defined diurnal variation in this region. Precipitation occurs during the afternoon and early morning over the southern plain in Bangladesh (Ohsawa et al 2001;Islam et al 2004;Terao et al 2008), whereas the southerly monsoon flow and low-level jet tend to generate nocturnal precipitation over the northern mountainous areas (Barros and Lang 2003;Kataoka and Satomura 2005;Terao et al 2006;Murata et al 2008;Sato 2013). These seasonal differences in precipitation over South Asia, together with the remarkable diurnal variations, allow us to evaluate the impact of CPs on precipitation simulations.…”