Role of Orography, Diurnal Cycle, and Intraseasonal Oscillation in Summer Monsoon Rainfall over the Western Ghats and Myanmar Coast

Abstract

Rainfall over the coastal regions of western India [Western Ghats (WG)] and Myanmar [Arakan Yoma (AY)], two regions experiencing the heaviest rainfall during the Asian summer monsoon, is examined using a Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) dataset spanning 16 years. Rainfall maxima are identified on the upslope of the WG and the coastline of AY, in contrast to the offshore locations observed in previous studies. Continuous rain with slight nocturnal and afternoon–evening maxima occurs over the upslope of the WG, while an afternoon peak over the upslope and a morning peak just off the coast are found in AY, resulting in different locations of the rainfall maxima for the WG (upslope) and AY (coastline). Large rainfall amounts with small diurnal amplitudes are observed over the WG and AY under strong environmental flow perpendicular to the coastal mountains, and vice versa. Composite analysis of the boreal summer intraseasonal oscillation (BSISO) shows that the rain anomaly over the WG slopes lags behind the northward-propagating major rainband. The cyclonic systems associated with the BSISO introduces a southwest wind anomaly behind the major rainband, enhancing the orographic rainfall over the WG, and resulting in the phase lag. This lag is not observed in the AY region where more closed cyclonic circulations occur. Diurnal variations in rainfall over the WG regions are smallest during the strongest BSISO rainfall anomaly phase.

Differences in Northward Propagation of Convection Over the Arabian Sea and Bay of Bengal During Boreal Summer

“…The model is able to capture these basic rainfall and wind structure, albeit with slightly weaker than observation rainfall over the northern BoB. It may however be pointed that TRMM rainfall 3B42 has some bias of showing excess rainfall near the mountains, especially in southeast Asia (Shige et al, ).…”

Observed Vertical Structure of Convection During Dry and Wet Summer Monsoon Epochs Over the Western Ghats

“…Figures c and d show the frequency of cross‐shore component of upstream horizontal wind (m/s) at 850 hPa for dry and wet composites of 2014, respectively. Following Shige et al (), three regimes are defined: weak ( U ≤ − σ ), medium ( − σ < U < + σ ), and strong ( U ≥ + σ ) regimes, where U is the cross‐shore component of upstream horizontal wind for dry and wet periods of 2014 And ( σ ) is the mean wind (standard deviation) based on multiyear ECMWF data for the corresponding dry and wet periods of 2014. The mean wind speed over the WG for the dry period of 2014 is approximately 8.34 (±7.4) m/s, while that for wet spell of 2014 is 13.45 (±4.8) m/s.…”

“…In the past, studies on orographic precipitation (e.g., Biasutti et al, 2012;Kumar & Bhat, 2017;Kumar et al, 2014;Maheskumar et al, 2014;Nesbitt & Anders, 2009;Romatschke & Houze, 2011;Sahany et al, 2010;Shige & Kummerow, 2016;Shrestha et al, 2015;Tawde & Singh, 2015;Xie et al, 2006), and its intraseasonal variability (e.g., Romatschke & Houze, 2011;Shige et al, 2017) over the WG, were mainly based on satellite and reanalysis data sets. There are few studies on the role of WG on simulating the summer precipitation characteristics using regional model (e.g., Flynn et al, 2017;Sijikumar et al, 2013;Wu et al, 1999;Zhang & Smith, 2018).…”

Using the Airborne Snow Observatory to Assess Remotely Sensed Snowfall Products in the California Sierra Nevada

“…Further analysis using the three in situ observations (section 2.2) suggests that on 8 February, the higher elevations and the northern side of the basin received larger accumulation while on 11 February TUM (at lower altitudes in the rain shadow) captured larger accumulation. The observed elevation dependency of precipitation suggests that a more skillful precipitation product should be able to capture this feature, which is not the strength of the current remote sensing products (e.g., Hirpa et al, ; Maggioni et al, ; Shige et al, ). During the second AR event, the temporal pattern of precipitation was more consistent among the products, but GSMaP shows a large isolated peak on 18 February that exceeds all other products and is not reflected in the in situ data.…”