Jesse Norris

WRF simulations of two extreme snowfall events associated with contrasting extratropical cyclones over the western and central Himalaya

Two extreme snowfall events associated with extratropical cyclones, one interacting with the western and one with the central Himalaya, are simulated with the Weather Research and Forecasting (WRF) model over 8 days. One event in January 1999 was driven by a longwave trough over west Asia, with the cyclone becoming terrain-locked in the western Himalayan notch. Another event in March 2006 was driven by a trough further south and east, facilitating the passage of two successive cyclones along the entire Himalayan ridge, drawing moisture from warm tropical waters. These flow patterns are typical for extreme winter precipitation in the western and central Himalaya, respectively, but were amplified in these two cases. In the WRF simulations, snowfall is confined to the western Himalaya in the January simulation, while a near-continuous band of accumulated snowfall along the Himalayan ridge forms in the March simulation. Precipitation rate throughout both simulations is largely determined by cross-barrier moisture flux, which is generally greatest wherever the cyclonic winds converge on the mountains at each time. However, the March 2006 simulation evolves in an environment with greater convective instability upwind of and moisture transport toward the mountains than in the January 1999 event. Hence, greater precipitation rates and more solid snowbands are generated in the March than in the January simulation. However, due to the terrain-locking of the cyclone in the January event, individual locations receive more persistent snowfall, so that the greatest 8 day accumulations are similar between the two events, although these accumulations are more widespread in the March event.

Winter westerly disturbance dynamics and precipitation in the western Himalaya and Karakoram: a wave-tracking approach

Extratropical cyclones, including winter westerly disturbances (WWD) over central Asia, are fundamental features of the atmosphere that maintain energy, momentum, and moisture at global scales while intimately linking large-scale circulation to regional-scale meteorology. Within high mountain Asia, WWD are the primary contributor to regional precipitation during winter. In this work, we present a novel WWD tracking methodology, which provides an inventory of location, timing, intensity, and duration of events, allowing for a comprehensive study of the factors that relate WWD to orographic precipitation, on an individual event basis and in the aggregate. We identify the relationship between the strength of disturbances, the state of the background environment during their propagation, and precipitation totals in the Karakoram/western Himalaya. We observe significant differences in convective and mechanical instability contributions to orographic precipitation as a function of the relationship between the intensity of WWD and the background temperature and moisture fields, which exhibit strong intraseasonal variability. Precipitation is primarily orographically forced during intense WWD with strong cross-barrier winds, while weaker WWD with similar precipitation totals are observed to benefit from enhanced instability due to high moisture content and temperature at low levels, occurring primarily in the late winter/premonsoon. The contribution of these factors is observed to fluctuate on a per-case basis, indicating important influences of intraseasonal oscillations and tropical-extratropical interactions on regional precipitation.

The influence of tropical forcing on extreme winter precipitation in the western Himalaya

Within the Karakoram and western Himalaya (KH), snowfall from winter westerly disturbances (WD) maintains the region’s snowpack and glaciers, which melt seasonally to sustain water resources for downstream populations. WD activity and subsequent precipitation are influenced by global atmospheric variability and tropical-extratropical interactions. On interannual time-scales, El Niño related changes in tropical diabatic heating induce a Rossby wave response over southwest Asia that is linked with enhanced dynamical forcing of WD and available moisture. Consequently, extreme orographic precipitation events are more frequent during El Niño than La Niña or neutral conditions. A similar spatial pattern of tropical diabatic heating is produced by the MJO at intraseasonal scales. In comparison to El Niño, the Rossby wave response to MJO activity is less spatially uniform over southwest Asia and varies on shorter time-scales. This study finds that the MJO’s relationship with WD and KH precipitation is more complex than that of ENSO. Phases of the MJO propagation cycle that favor the dynamical enhancement of WD simultaneously suppress available moisture over southwest Asia, and vice versa. As a result, extreme precipitation events in the KH occur with similar frequency in most phases of the MJO, however, there is a transition in the relative importance of dynamical forcing and moisture in WD to orographic precipitation in the KH as the MJO evolves. These findings give insight into the dynamics and predictability of extreme precipitation events in the KH through their relationship with global atmospheric variability, and are an important consideration in evaluating Asia’s water resources.

Effects of Topographic Smoothing on the Simulation of Winter Precipitation in High Mountain Asia

Numerous studies have projected future changes in High Mountain Asia water resources based on temperature and precipitation from global circulation models (GCM) under future climate scenarios. Although the potential benefit of such studies is immense, coarse grid-scale GCMs are unable to resolve High Mountain Asias complex topography and thus have a biased representation of regional weather and climate. This study investigates biases in the simulation of physical mechanisms that generate snowfall and contribute to snowpack in High Mountain Asia in coarse topography experiments using the Weather Research and Forecasting model. Regional snowpack is event driven, thus 33 extreme winter orographic precipitation events are simulated at fine atmospheric resolution with 6.67 km-resolution topography and smoothed 1.85°x1.25°-GCM topography. As with many modified topography experiments performed in other regions, the distribution of precipitation is highly dependent on first-order orographic effects, which dominate regional meteorology. However, we demonstrate that topographic smoothing enhances circulation in simulated extratropical cyclones, with significant impacts on orographic precipitation. Despite precipitation reductions of 28% over the highest ranges, due to reduced ascent on windward slopes, total precipitation over the study domain increased by an average of 9% in smoothed topography experiments on account of intensified extratropical cyclone dynamics and cross-barrier moisture flux. These findings identify an important source of bias in coarse-resolution simulated precipitation in High Mountain Asia, with important implications for the application of GCMs toward projecting future hydroclimate in the region.

Intraseasonal-to-Interannual Variability of the Indian Monsoon Identified with the Large-Scale Index for the Indian Monsoon System (LIMS)

AbstractThe Indian monsoon system (IMS) is among the most complex and important climatic features on land. This study proposes a simple and robust method to investigate large-scale variations and changes in the IMS that accounts for fluctuations in amplitude, onset, and duration of the summer monsoon, including active and break phases, and the postmonsoon season. This study uses 35 years (1979–2013) of daily data from the National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR) at 1° resolution and indicates great potential for application to other reanalyses and climate model datasets. The method is based on combined EOF (CEOF) analysis of variables associated with the IMS’s seasonal cycle (precipitation, circulation at 10 m, and temperature and specific humidity at 2 m). The first CEOF (CEOF-1) explains ~40% of the total variance and represents the continental-scale Asian monsoon. The second CEOF (CEOF-2) explains 11% of the variance and characterizes the Indian mon...

Deciphering the contrasting climatic trends between the central Himalaya and Karakoram with 36 years of WRF simulations

Glaciers over the central Himalaya have retreated at particularly rapid rates in recent decades, while glacier mass in the Karakoram appears stable. To address the meteorological factors associated with this contrast, 36 years of Climate Forecast System Reanalyses (CFSR) are dynamically downscaled from 1979 to 2015 with the Weather Research and Forecasting (WRF) model over High Mountain Asia at convection permitting grid spacing (6.7 km). In all seasons, CFSR shows an anti-cyclonic warming trend over the majority of High Mountain Asia, but distinctive differences are observed between the central Himalaya and Karakoram in winter and summer. In winter and summer, the central Himalaya has been under the influence of an anti-cyclonic trend, which in summer the downscaling shows has reduced cloud cover, leading to significant warming and reduced snowfall in recent years. Contrastingly, the Karakoram has been near the boundary between large-scale cyclonic and anti-cyclonic trends and has not experienced significant snowfall or temperature changes in winter or summer, despite significant trends in summer of increasing cloud cover and decreasing shortwave radiation. This downscaling does not identify any trends over glaciers in closer neighboring regions to the Karakoram (e.g., Hindu Kush and the western Himalaya) where glaciers have retreated as over the central Himalaya, indicating that there are other factors driving glacier mass balance that this downscaling is unable to capture. While this study does not fully explain the Karakoram anomaly, the identified trends detail important meteorological contributions to the observed differences between central Himalaya and Karakoram glacier evolution in recent decades.