Characteristics of historical precipitation in high mountain asia based on a 15-year high resolution dynamical downscaling

The mountains of High Mountain Asia serve as an important source of water for roughly one billion people living downstream. This research uses 15 years of dynamically downscaled precipitation produced by the Weather Research and Forecasting (WRF) model to delineate contrasts in precipitation characteristics and events between regions dominated by the Indian Summer Monsoon (ISM) versus westerly disturbances during the cool season (December to March). Cluster analysis reveals a more complex spatial pattern than indicated by some previous studies and illustrates the increasing importance of westerly disturbances at higher elevations. Although prior research suggests that a small number of westerly disturbances dominate precipitation in the western Himalaya and Karakoram, the WRF-downscaled precipitation is less dominated by infrequent large events. Integrated vapor transport (IVT) and precipitation are tightly coupled in both regions during the cool season, with precipitation maximizing for IVT from the south-southwest over the Karakoram and southeast-southwest over the western Himalaya. During the ISM, Karakoram precipitation is not strongly related to IVT direction, whereas over the western Himalaya, primary and secondary precipitation maxima occur for flow from the west-southwest and northwest, respectively. These differences in the drivers and timing of precipitation have implications for hydrology, glacier mass balance, snow accumulation, and their sensitivity to climate variability and change

The Community Foehn Classification Experiment

Strong winds crossing elevated terrain and descending to its lee occur over mountainous areas worldwide. Winds fulfilling these two criteria are called “foehn” in this paper although different names exist depending on region, sign of temperature change at onset, and depth of overflowing layer. They affect local weather and climate and impact society. Classification is difficult because other wind systems might be superimposed on them or share some characteristics. Additionally, no unanimously agreed-upon name, definition nor indications for such winds exist. The most trusted classifications have been performed by human experts. A classification experiment for different foehn locations in the Alps and different classifier groups addressed hitherto unanswered questions about the uncertainty of these classifications, their reproducibility and dependence on the level of expertise. One group consisted of mountain meteorology experts, the other two of Masters degree students who had taken mountain meteorology courses, and a further two of objective algorithms. Sixty periods of 48 hours were classified for foehn/no foehn at five Alpine foehn locations. The intra-human-classifier detection varies by about 10 percentage points (interquartile range). Experts and students are nearly indistinguishable. The algorithms are in the range of human classifications. One difficult case appeared twice in order to examine reproducibility of classified foehn duration, which turned out to be 50% or less. The classification dataset can now serve as a testbed for automatic classification algorithms, which - if successful - eliminate the drawbacks of manual classifications: lack of scalability and reproducibility

Life Cycle and Mesoscale Frontal Structure of an Intermountain Cyclone

High resolution analyses and surface observations are used to examine the structure and evolution of an lntermountain cyclone and its attendant surface fronts. The cyclone, known locally as the Tax Day Storm, produced the lowest sea level pressure on record and strongest cold frontal passage (based on the 2 h temperature fall) observed in the past 25 y at the Salt Lake City lnternational Airport (KSLC). Three surface features develop downstream of the Sierra Nevada prior to the incipient cyclogenesis. The first is a barrier parallel lee trough that forms in response to strengthening cross-barrier flow. The second is a decaying elongated trough draped over the northern Great Basin. The third is a confluence zone that extends downstream over the Great Basin normal to the Sierra Nevada. Strong contraction (i.e., deformation and convergence) within this Great Basin Confluence Zone (GBCZ) forms an airstream boundary that is initially nonfrontal, but becomes the locus for surface frontogenesis as it collects isentropes and cooler air from northern Nevada. As the elongated trough rotates into phase with the GBCZ and developing surface front, cyclogenesis occurs as an upper-level cyclonic potential vorticity anomaly traverses the Sierra Nevada and quasi-geostrophic forcing for ascent spreads over the Great Basin. Differential diabatic heating further sharpens the front over northern Utah where dramatic frontal distortions are produced by the local orography. This analysis further establishes the critical role that the GBCZ plays in lntermountain frontogenesis and identifies it as an important mesoscale feature accompanying Nevada cyclogenesis. Recognition of this role may improve short-range forecasting and help advance our understanding of these cyclones and their attendant fronts, which are the primary source of cool season precipitation over the Great Basin

Quantifying take-effect precipitation in the Great Salt Lake Basin

The Great Salt Lake Basin of the Intermountain West is one of the fastest growing regions of the United States. Consistent with the rapid population growth is the increased need for water resources, which are limited in the Intermountain West. Since most of the water resources in the Great Salt Lake Basin come from precipitation and related runoff, it is essential to understand and quantify the sources of precipitation in order to plan for future water usage. Lake-effect storms initiated over the Great Salt Lake are often overlooked in this regard, but are suspected to contribute significantly to overall precipitation amounts. Many studies have been conducted on the synoptic and mesoscale processes associated with Great Salt Lake-Effect, but none have tried to quantify the amount of precipitation produced. This study examines the relationship between the Great Salt Lake-Effect and the hydroclimate of the Great Salt Lake Basin by resolving the contribution of lake-effect precipitation to annual precipitation. Imagery from the WSR-88D radar at Promontory Point (KMTX) is used to identify all lake-effect events occurring from September 1997 to May 2009, noting the spatial extent, strength, and brevity of each storm. 24-hour precipitation observations from SNOTEL, NWS, and cooperative observing stations are disaggregated into hourly observations using a methodology that combines radar reflectivity with gauge measurements. Disaggregated hourly estimates are used to assess precipitation amounts from each lake-effect event. This approach is expected to yield a more objective calculation of the contribution of lake-effect precipitation to annual precipitation