Quantifying the added value of convection-permitting climate simulations in complex terrain: a systematic evaluation of WRF over the Himalayas

Karki, Ramchandra; Hasson, Shabeh ul; Gerlitz, Lars; Schickhoff, Udo; Scholten, Thomas; Böhner, Jürgen

doi:10.5194/esd-8-507-2017

Cited by 55 publications

(52 citation statements)

References 61 publications

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“…Highest temperatures are observed over the lowlands and high mountains in May and June, respectively, where the highest maximum temperature exceeds 45°C over the southern lowlands (Shrestha and Aryal, ). Lowest temperatures are observed during December or January, whereas high mountain regions above 5,000 m asl features sub‐zero mean daily temperature throughout the year (Karki et al ., ). For example, mean daily temperatures observed at the southern slopes of Mt.…”

Section: Study Areamentioning

confidence: 99%

Rising mean and extreme near‐surface air temperature across Nepal

Karki

Hasson

Gerlitz

et al. 2019

Intl Journal of Climatology

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Owing to unique topographic and ecological diversity, central Himalayan state of Nepal is exposed to adverse impacts of climate change and associated disasters. However, countrywide historical assessment of mean and extreme temperature changes, a prerequisite for devising adequate adaptation strategies, is still lacking. Here, we present a comprehensive picture of mean and extreme temperature trends across Nepal over the 1980–2016 period, based on high‐quality daily temperature observations from 46 stations. Our results suggest that besides winter cooling in southern lowlands, the country features a widespread warming, which is higher for maximum temperature (~0.04°C⋅year−1) than for minimum temperature (~0.02°C⋅year−1), over the mountainous region than in valleys and lowlands and during the pre‐monsoon season than for the rest of the year. Consistently, we found a higher increasing trend for warm days (13 days⋅decade−1) than for warm nights (4 days⋅decade−1), whereas the rates of decrease for cold days and cold nights are the same (6 days⋅decade−1). Further investigations reveal that pronounced warming in maximum temperature over mountain regions can be attributed to less cloud cover and snowfall in recent decades during non‐monsoon seasons as a result of positive geopotential height anomalies and strengthening of anticyclonic circulations in the mid‐to‐upper troposphere. Similarly, increased stability of lower atmosphere during winter and post‐monsoon seasons caused prolonged and frequent periods of fog over lowlands, resulting in significant winter cooling there.

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Section: Study Areamentioning

confidence: 99%

Rising mean and extreme near‐surface air temperature across Nepal

Karki

Hasson

Gerlitz

et al. 2019

Intl Journal of Climatology

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“…The high spatial variability of the wind acceleration components and the dominance of the pressure gradient both result from the impact of the tremendously complex terrain that characterizes the Dudh Koshi river basin as well as the wider HKKH region, which requires modeling with a resolution of around 1 km in order to realize accurate output (Collier & Immerzeel, ; Karki et al, ; Orr et al, ; Zängl et al, ). The importance of the local pressure gradient and turbulent vertical mixing additionally informs us that the representation of the land surface (and planetary boundary layer) to compute heat and moisture fluxes is crucial to produce accurate results in the near‐surface wind field.…”

Section: Discussionmentioning

confidence: 99%

“…Two month‐long runs were conducted using version 3.8.1 of the WRF model (Skamarock et al, ) over the Dudh Koshi river basin for July 2013 and January 2014 (hereafter referred to as the summer and winter runs, respectively). Previous high‐resolution modeling studies in the Nepalese Himalaya suggest that a horizontal resolution of around 1 km is necessary to accurately represent valley winds (Collier & Immerzeel, ; Karki et al, ; Orr et al, ). This is selected, therefore, as the resolution of the innermost domain, which is nested within three other domains at resolutions of 27, 9, and 3 km (Figure a).…”

Section: Observations Model and Methodsmentioning

confidence: 99%

“…The summer monsoon is the predominant large‐scale driver of precipitation in the eastern areas of the HKKH (Shea, Wagnon, et al, ; Wagnon et al, ; Yang et al, ), while in the western areas it is the winter westerly disturbances (Ueno et al, ). In addition to the synoptic scale systems, the local valley wind regimes also affect precipitation by transporting moisture and clouds up the slopes and valleys during the day, especially at high altitudes (Bollasina et al, ; Egger et al, ; Karki et al, ; Orr et al, ; Shea, Wagnon, et al, ; Tartari et al, ). They also affect snow redistribution (Wagnon et al, ).…”

Section: Introductionmentioning

confidence: 99%

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Dynamical Drivers of the Local Wind Regime in a Himalayan Valley

et al. 2018

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Understanding the local valley wind regimes in the Hindu-Kush Karakoram Himalaya is vital for future predictions of the glacio-hydro-meteorological system. Here the Weather Research and Forecasting model is employed at a resolution of 1 km to investigate the forces driving the local valley wind regime in a river basin in the Nepalese Himalaya, during July 2013 and January 2014. Comparing with observations shows that the model represents the diurnal cycle of the winds well, with strong daytime up-valley winds and weak nighttime winds in both months. A momentum budget analysis of the model output shows that in the summer run the physical drivers of the near-surface valley wind also have a clear diurnal cycle, and are dominated by the pressure gradient, advection, and turbulent vertical mixing, as well as a nonphysical numerical diffusion term. By contrast, the drivers in the winter run have a less consistent diurnal cycle. In both months, the pressure gradient, advection, numerical diffusion, and Coriolis terms dominate up to 5,000 m above the ground. The drivers are extremely variable over the valley, and also influenced by the presence of glaciers. When glaciers are removed from the model in the summer run, the wind continues further up the valley, indicating how the local valley winds might respond to future glacier shrinkage. The spatial variability of the drivers over both months is consistent with the complex topography in the basin, which must therefore be well represented in weather and regional climate models to generate accurate outputs. Plain Language SummaryThe rain and snow in Himalayan valleys, and the formation and melting of glaciers, are affected by the wind in the valleys. Exactly what drives this wind is not fully understood. Around the world, wind in valleys generally travels up the valley, and up the sides of mountains, during the day. This is due to the sun heating different areas by different amounts, creating areas of low and high air pressure. Here we use a computer simulation to determine whether the difference in pressure is the main cause of the acceleration of the wind in a valley in the Himalayas, or whether there are other forces which also affect the wind. We compare the simulation to measurements taken in the valley. We find that the pressure difference is the main process affecting the acceleration of the wind. However, the winds in the valley are also driven by the effects of turbulence and affected by the shape of the valley. The forces are consistent over the month, accelerating the wind in the morning and decelerating the wind in the afternoon every day.

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“…Due to the resolution of the input topography, the model topography slightly underestimates the actual topography (especially around the mountain peaks). WRF is run using 2‐way nesting with the physical and dynamical parameters based on previous studies for similar domains (Karki et al, ; Norris et al, ), as presented in Table . D1 is forced at its lateral boundaries by ERA‐Interim data.…”

Section: Methodsmentioning

confidence: 99%

A Relationship Between Ural‐Siberian Blocking and Himalayan Weather Anomalies

Tiwari

Bush

2019

Earth and Space Science

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Because of their distinctly high elevation, the Himalaya are directly influenced by upper‐tropospheric jet stream winds and circulation anomalies that on many occasions have brought unusual weather patterns to the region. In this study, we establish the dynamical relationship between upper‐tropospheric blocks formed over the Ural‐Siberian region and anomalous weather patterns over the Himalaya using a combination of reanalysis data and mesoscale atmospheric model simulations. We identify two distinct blocks (an omega (Ω) block and a dipole (Rex) block) using an appropriate blocking index and computed the anomalous atmospheric fields associated with these types of blocks. In both cases, the low‐pressure component of the block lies westward of the western Himalaya and the upper‐level convergence/divergence of ageostrophic winds along the upstream/downstream portion of the trough creates anomalous positive/negative sea level pressure at the surface. The local sea level pressure is enhanced over the Arabian Peninsula and the Arabian Sea, while downwind over the Himalaya there is a negative pressure anomaly. There is entrainment of high potential vorticity air that descends and flows equatorward. During the Ω blocking event, the trough remains quasi‐stationary over the Western Himalayan Notch and its circulation induces moisture transport from the Arabian Sea and the Bay of Bengal leading to strong precipitation events over the Western and Eastern Himalayan Notches. During the dipole blocking event, the trough gradually shifts southeastward and leads to widespread precipitation over the entire Himalayan arc.

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Quantifying the added value of convection-permitting climate simulations in complex terrain: a systematic evaluation of WRF over the Himalayas

Cited by 55 publications

References 61 publications

Rising mean and extreme near‐surface air temperature across Nepal

Rising mean and extreme near‐surface air temperature across Nepal

Dynamical Drivers of the Local Wind Regime in a Himalayan Valley

A Relationship Between Ural‐Siberian Blocking and Himalayan Weather Anomalies

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