Synoptic atmospheric circulation patterns associated with deep persistent slab avalanches in the western United States

Deep persistent slab avalanches are capable of destroying infrastructure and are usually unsurvivable for those who are caught. Formation of a snowpack conducive to deep persistent slab avalanches is typically driven by meteorological conditions occurring in the beginning weeks to months of the winter season, and yet the avalanche event may not occur for several weeks to months later. While predicting the exact timing of the release of deep persistent slab avalanches is difficult, onset of avalanche activity is commonly preceded by rapid warming, heavy precipitation, or high winds. This work investigates the synoptic drivers of deep persistent slab avalanches at three sites in the western USA with long records: Bridger Bowl, Montana; Jackson, Wyoming; and Mammoth Mountain, California. We use self-organizing maps to generate 20 synoptic types that summarize 5899 daily 500 mbar geopotential height maps for the winters (November–March) of 1979/80–2017/18. For each of the three locations, we identify major and minor deep persistent slab avalanche seasons and analyze the number of days represented by each synoptic type during the beginning (November–January) of the major and minor seasons. We also examine the number of days assigned to each synoptic type during the 72 h preceding deep persistent slab avalanche activity for both dry and wet slab events. Each of the three sites exhibits a unique distribution of the number of days assigned to each synoptic type during November–January of major and minor seasons and for the 72 h period preceding deep persistent slab avalanche activity. This work identifies the synoptic-scale atmospheric circulation patterns contributing to deep persistent slab instabilities and the patterns that commonly precede deep persistent slab avalanche activity. By identifying these patterns, we provide an improved understanding of deep persistent slab avalanches and an additional tool to anticipate the timing of these difficult-to-predict events.

THE USE OF AUTOMATED SYNOPTIC TYPING FOR CONDITIONAL VERIFICATION OF NUMERICAL WEATHER PREDICTION IN THE PERM REGION

The article discusses the possibility of verification of short-term 2-meter air temperature forecasts with the Global Forecast System and Global Environment Multiscale numerical weather prediction models depending on the observed synoptic type (a case study of the Perm region for the period 2018–2019). As part of the study, we have developed a system of automated determination of synoptic type based on a two-stage procedure, including decomposition of mean sea level pressure fields via principal component analysis and the subsequent clustering of decomposition coefficients using K-means. It has been established that GFS forecasts are more dependent on synoptic type in summer than in winter. The decline of forecast quality, expressed in systematic underestimation of forecast temperature by 0.6°–1.2°, is noted for synoptic types associated with warm air advection.: In contrast, GEM forecasts tend to lack accuracy in winter. A sharp decrease in forecast quality has been discovered in the central area of anticyclone at night, when the forecast accuracy drops to 44%. The obtained results could be useful in operational forecasting and model postprocessing.

Synoptic characteristics of an extreme weather event: The tornadic waterspout in Tivat (Montenegro), on June 9, 2018

Recently Montenegro has often been faced with extreme weather events. The aim of this paper is to provide a detailed synoptic analysis of a severe weather event, a waterspout, and to confirm an indication that in most cases such events could potentially be forecasted, which is of great practical significance, since human lives and property can be saved. The paper presents the research results of synoptic and mesoscale weather conditions which created a favourable meteorological environment for a waterspout development in Tivat (Montenegrin coast) on June 9, 2018, around 01 UTC (03 CET). Based on field survey analysis, the rating of tornado intensity by the Fujita scale (F-scale) has been done by assessing the damage. The synoptic type for this situation was CLOSED-SW and was determined by a detailed examination of atmospheric circulation. The results presented in the manuscript can help decision makers in Montenegro to take certain adaptation measures (above all, in tourism and construction) in order to mitig te the negative consequences of weather extremes.

Synoptic atmospheric circulation patterns associated with deep persistent slab avalanches in the western United States

Deep persistent slab avalanches are capable of destroying infrastructure and are usually unsurvivable to those who are caught. Formation of a snowpack conducive to deep persistent slab avalanches is typically driven by meteorological conditions occurring in the beginning weeks to months of the winter season, and yet the avalanche event may not occur for several weeks to months later. While predicting the exact timing of the release of deep persistent slab avalanches is difficult, onset of avalanche activity is commonly preceded by rapid warming, heavy precipitation, or high winds. This work investigates the synoptic drivers of deep persistent slab avalanches at three sites in the Western USA with long records: Bridger Bowl, Montana; Jackson, Wyoming; and Mammoth Mountain, California. We use self-organizing maps to generate twenty synoptic types that summarize 5,899 daily 500 mb geopotential height maps for the winters (November–March) of 1979/80–2017/18. For each of the three locations, we identify major and minor deep persistent slab avalanche seasons, and analyze the number of days represented by each synoptic type during the beginning (November–January) of the major and minor seasons. We also examine the number of days assigned to each synoptic type during the 72 hours preceding deep persistent slab avalanche activity for both dry and wet slab events. Each of the three sites exhibits a unique distribution of the number of days assigned to each synoptic type during November–January of major and minor seasons, and for the 72-hour period preceding deep persistent slab avalanche activity. This work identifies the synoptic scale atmospheric circulation patterns contributing to deep persistent slab instabilities, and the patterns that commonly precede deep persistent slab avalanche activity. By identifying these patterns, we provide an improved understanding of deep persistent slab avalanches, and an additional tool to anticipate the timing of these difficult-to-predict events.

Quantifying the impact of synoptic weather types and patterns on energy fluxes of a marginal snowpack

Synoptic weather patterns are investigated for their impact on energy fluxes driving melt of a marginal snowpack in the Snowy Mountains, southeast Australia. K-means clustering applied to ECMWF ERA-Interim data identified common synoptic types and patterns that were then associated with in situ snowpack energy flux measurements. The analysis showed that the largest contribution of energy to the snowpack occurred immediately prior to the passage of cold fronts through increased sensible heat flux as a result of warm air advection (WAA) ahead of the front. Shortwave radiation was found to be the dominant control on positive energy fluxes when individual synoptic weather types were examined. As a result, cloud cover related to each synoptic type was shown to be highly influential on the energy fluxes to the snowpack through its reduction of shortwave radiation and reflection/emission of longwave fluxes. As single-site energy balance measurements of the snowpack were used for this study, caution should be exercised before applying the results to the broader Australian Alps region. However, this research is an important step towards understanding changes in surface energy flux as a result of shifts to the global atmospheric circulation as anthropogenic climate change continues to impact marginal winter snowpacks.

Quantifying the impact of synoptic weather types and patterns on energy fluxes of a marginal snowpack

Synoptic weather patterns are investigated for their impact on energy fluxes driving melt of a marginal snowpack in the Snowy Mountains, southeast Australia. K-means clustering applied to ECMWF ERA-Interim data identified common synoptic types and patterns that were then associated with in-situ snowpack energy flux measurements. The analysis showed that the largest contribution of energy to the snowpack occurred immediately prior to the passage of cold fronts through increased sensible heat flux as a result of warm air advection (WAA) ahead of the front. Shortwave radiation was found to be the dominant control on positive energy fluxes when individual synoptic weather types were examined. As a result, cloud cover related to each synoptic type was shown to be highly influential on the energy fluxes to the snowpack through its reduction of shortwave radiation and reflection/emission of longwave fluxes. This research is an important step towards understanding changes in surface energy flux as a result of shifts to the global atmospheric circulation as anthropogenic climate change continues to impact marginal winter snowpacks.

Regionalization of Northeast US moisture conditions: analysis of synoptic-scale atmospheric drivers

Previous investigations have documented relationships between global-scale forcings and Northeast United States moisture conditions, yet the physical pathways from global-scale forcing to sub-regional moisture deficit or surplus are not well understood. This research uses eigenvector-based regionalization to confirm the existence of sub-regional moisture environments within the Northeast. Synoptic classification is used to derive daily weather types that impact these moisture environments, and evaluate the relationship between global and synoptic scales. The Palmer Drought Severity Index (PDSI) regionalization identifies 3 sub-regions across the Northeast with homogeneous moisture conditions including New England, the Eastern Great Lakes, and Mid-Atlantic Regions. All 3 regions’ PDSI conditions are predominantly associated with variations in precipitation, rather than thermal characteristics. The frequency of key precipitation-associated synoptic types can inform PDSI variability in the regions, where drier conditions are observed during growing seasons with a reduced frequency of precipitation-inducing synoptic types and an enhanced frequency of dry synoptic types. Variations in the frequencies of these synoptic types are partially explained by the phase of the various teleconnection patterns. In the case of the New England region, 14% of the variance in PDSI is explained by the frequency of synoptic type D2, and 12% of the variance in D2 is explained by variations in the Summer Atmospheric Drought Index. The New England region became significantly wetter (positive PDSI) from 1950 to 2016. This study suggests a partial cause of this trend is the increased and decreased frequencies of wet and dry synoptic types, respectively, both related to the phase of the Summer Atmospheric Drought Index.

Changing Intrasynoptic Type Characteristics and Interannual Frequencies of Circulation Patterns Conducive to Lake-Effect Snowfall

Using a temporal synoptic index, synoptic-scale atmospheric patterns suitable for lake-effect snow downwind of Lakes Erie and Ontario are identified and analyzed from 1950 to 2009. In response to prior research noting a trend reversal of snowfall in this region, changes in the inherent meteorological characteristics and winter-season frequencies of lake-effect synoptic weather types are evaluated as possible forcing mechanisms. Four atmospheric patterns are identified during the December–February winter season as lake-effect synoptic types. Changes in inherent meteorological characteristics and winter frequencies of these types are attributed to between 88% and 95% of the observed snowfall changes during the study period. Decreasing air temperatures and surface pressures, increasing boundary layer instability, and changes toward stronger zonal flow are noted for multiple lake-effect synoptic types from 1950 to 1979 as likely forcing mechanisms of observed snowfall increases, on the order of nearly 1.0 cm yr−1 downwind of Lake Ontario per individual synoptic type. Similarly, the significant increases in the winter frequency of multiple lake-effect synoptic types also are attributed to some of the increases in snowfall. From 1980 to 2009, however, the lake-effect synoptic types remained relatively unchanged or decreased in frequency, as did snowfall totals. The results of this study indicate that changes in the synoptic-scale environment are a viable mechanism forcing snowfall trends, in addition to the more commonly considered seasonal temperature and lake-ice considerations, and should be incorporated into future discussions of lake-effect snowfall projections.

An analysis of the cloud environment over the Ross Sea and Ross Ice Shelf using CloudSat/CALIPSO satellite observations: the importance of synoptic forcing

We use the 2B-GEOPROF-LIDAR R04 (2BGL4) and R05 (2BGL5) products and the 2B-CLDCLASS-LIDAR R04 (2BCL4) product, all generated by combining CloudSat radar and CALIPSO lidar satellite measurements with auxiliary data, to examine the vertical distribution of cloud occurrence around the Ross Ice Shelf (RIS) and Ross Sea region. We find that the 2BGL4 product, used in previous studies in this region, displays a discontinuity at 8.2 km which is not observable in the other products. This artefact appears to correspond to a change in the horizontal and vertical resolution of the CALIPSO dataset used above this level. We then use the 2BCL4 product to examine the vertical distribution of cloud occurrence, phase, and type over the RIS and Ross Sea. In particular we examine how synoptic conditions in the region, derived using a previously developed synoptic classification, impact the cloud environment and the contrasting response in the two regions. We observe large differences between the cloud occurrence as a function of altitude for synoptic regimes relative to those for seasonal variations. A stronger variation in the occurrence of clear skies and multi-layer cloud and in all cloud type occurrences over both the Ross Sea and RIS is associated more with synoptic type than seasonal composites. In addition, anomalies from the mean joint histogram of cloud top height against thickness display significant differences over the Ross Sea and RIS sectors as a function of synoptic regime, but are near identical over these two regions when a seasonal analysis is completed. However, the frequency of particular phases of cloud, notably mixed phase and water, is much more strongly modulated by seasonal than synoptic regime compositing, which suggests that temperature is still the most important control on cloud phase in the region.

An analysis of the cloud environment over the Ross Sea and Ross Ice Shelf using CloudSat/CALIPSO satellite observations: The importance of synoptic forcing

We use the 2B-GEOPROF-LIDAR R04 (2BGL4) and R05 (2BGL5) products and the 2B-CLDCLASSLIDAR R04 (2BCL4) product, all generated by combining CloudSat radar and CALIPSO lidar satellite measurements with auxiliary data, to examine the vertical distribution of cloud occurrence around the Ross Ice Shelf (RIS) and Ross Sea region. We find that the 2BGL4 product, used in previous studies in this region, displays a discontinuity at 8.2 km which is not observable in the other products. This artefact appears to correspond with a change in the horizontal and vertical resolution of the CALIPSO dataset used above this level. We then use the 2BCL4 product to examine the vertical distribution of cloud occurrence, phase, and type over the RIS and Ross Sea. In particular we examine how synoptic conditions in the region, derived using a previously developed synoptic classification, impact the cloud environment and the contrasting response in the two regions. We observe large differences between the cloud occurrence as a function of altitude for synoptic regimes relative to those for seasonal variations. A stronger variation in the occurrence of clear skies and multi-layer cloud and in all cloud type occurrences over both the Ross Sea and RIS is associated with synoptic type than seasonal composites. In addition, anomalies from the mean joint histogram of cloud top height against thickness display significant differences over the Ross Sea and RIS sectors as a function of synoptic regime, but are near identical over these two regions when a seasonal analysis is completed. However, the frequency of particular phases of cloud, notably mixed phase and water, is much more strongly modulated by seasonal than synoptic regime compositing which suggests that temperature is still the most important control on cloud phase in the region.