Ronald E. Stewart

A U.S. Clivar project to assess and compare the responses of global climate models to drought-related SST forcing patterns: Overview and results

Abstract The U.S. Climate Variability and Predictability (CLIVAR) working group on drought recently initiated a series of global climate model simulations forced with idealized SST anomaly patterns, designed to address a number of uncertainties regarding the impact of SST forcing and the role of land–atmosphere feedbacks on regional drought. The runs were carried out with five different atmospheric general circulation models (AGCMs) and one coupled atmosphere–ocean model in which the model was continuously nudged to the imposed SST forcing. This paper provides an overview of the experiments and some initial results focusing on the responses to the leading patterns of annual mean SST variability consisting of a Pacific El Nino–Southern Oscillation (ENSO)-like pattern, a pattern that resembles the Atlantic multidecadal oscillation (AMO), and a global trend pattern. One of the key findings is that all of the AGCMs produce broadly similar (though different in detail) precipitation responses to the Pacific for...

Characteristics through the Melting Layer of Stratiform Clouds

Abstract Thermodynamic and hydrometeor measurements from an aircraft flown through the melting layer of stratiform clouds over the California Valley are discussed and are compared with radar observations. An isothermal layer ∼200 m thick existed at 0°C, and radar bright bands up to 36 dB(Ze) were measured. The largest concentrations of ice particles occurred near −5°C and snowflakes melted by ∼2°C. Aggregation, and possibly ice multiplication, contributed to the characteristics of the radar bright band.

Science of Nowcasting Olympic Weather for Vancouver 2010 (SNOW-V10): a World Weather Research Programme Project

A World Weather Research Programme (WWRP) project entitled the Science of Nowcasting Olympic Weather for Vancouver 2010 (SNOW-V10) was developed to be associated with the Vancouver 2010 Olympic and Paralympic Winter Games conducted between 12 February and 21 March 2010. The SNOW-V10 international team augmented the instrumentation associated with the Winter Games and several new numerical weather forecasting and nowcasting models were added. Both the additional observational and model data were available to the forecasters in real time. This was an excellent opportunity to demonstrate existing capability in nowcasting and to develop better techniques for short term (0–6xa0h) nowcasts of winter weather in complex terrain. Better techniques to forecast visibility, low cloud, wind gusts, precipitation rate and type were evaluated. The weather during the games was exceptionally variable with many periods of low visibility, low ceilings and precipitation in the form of both snow and rain. The data collected should improve our understanding of many physical phenomena such as the diabatic effects due to melting snow, wind flow around and over terrain, diurnal flow reversal in valleys associated with daytime heating, and precipitation reductions and increases due to local terrain. Many studies related to these phenomena are described in the Special Issue on SNOW-V10 for which this paper was written. Numerical weather prediction and nowcast models have been evaluated against the unique observational data set now available. It is anticipated that the data set and the knowledge learned as a result of SNOW-V10 will become a resource for other World Meteorological Organization member states who are interested in improving forecasts of winter weather.

Precipitation types in winter storms

The characteristics of and the evolution between snow, rain, ice pellets, and freezing rain are discussed. Precipitation type and the nature of its size distribution and extent are related to the melting behaviour of snow. Model calculations of this melting show the progression of precipitation type from freezing rain to ice pellets and finally to snow, as melting systematically erodes an upper level inversion within about 5 h for a precipitation rate of 1mm h−1. The increase in temperature of the low level subfreezing region associated with ice pellet formation (up to ≈ 1°C) should furthermore be detectable. These phase transitions between the various precipitation types, as affected by atmospheric cooling by melting, are predicted to occur over mesoscale distances.

Global Meteorological Drought: A Synthesis of Current Understanding with a Focus on SST Drivers of Precipitation Deficits

AbstractDrought affects virtually every region of the world, and potential shifts in its character in a changing climate are a major concern. This article presents a synthesis of current understanding of meteorological drought, with a focus on the large-scale controls on precipitation afforded by sea surface temperature (SST) anomalies, land surface feedbacks, and radiative forcings. The synthesis is primarily based on regionally focused articles submitted to the Global Drought Information System (GDIS) collection together with new results from a suite of atmospheric general circulation model experiments intended to integrate those studies into a coherent view of drought worldwide. On interannual time scales, the preeminence of ENSO as a driver of meteorological drought throughout much of the Americas, eastern Asia, Australia, and the Maritime Continent is now well established, whereas in other regions (e.g., Europe, Africa, and India), the response to ENSO is more ephemeral or nonexistent. Northern Euras...

The 2013 flood event in the South Saskatchewan and Elk River basins: Causes, assessment and damages

In late June 2013, heavy rainfall and rapidly melting alpine snow triggered flooding throughout much of the southern half of Alberta. Heavy rainfall commenced on 19 June and continued for 3 days. When the event was over, more than 200 mm and as much as 350 mm of precipitation had fallen over the Front Ranges of the Canadian Rocky Mountains. Tributaries to the Bow River including the Ghost, Kananaskis, Elbow, Sheep and Highwood, and many of their tributaries, all reached flood levels. The storm had a large spatial extent causing flooding to the north and south in the Red Deer and Oldman Basins, and also to the west in the Elk River in British Columbia. Convergence of the nearly synchronous floodwaters downstream in the South Saskatchewan River system caused record high releases from Lake Diefenbaker through Gardiner Dam. Dam releases in Alberta and Saskatchewan attenuated the downstream flood peak such that only moderate flooding occurred in Saskatchewan and Manitoba. More than a dozen municipalities declared local states of emergency; numerous communities were placed under evacuation order in Alberta and Saskatchewan. The media reported that more than 100,000 people needed to evacuate their homes, and five people died. At CAD

Characterization and Summary of the 1999–2005 Canadian Prairie Drought

6 billion, the Province of Alberta estimated that it was the costliest natural disaster in Canadian history. At their peak, the water levels were the highest in the past 60 years, and nearing those of historic events of the late 1800s and early 1900s where records exist. There was major damage to infrastructure roads, bridges and culverts. Downtown Calgary was inundated, as were many communities such as High River and Bragg Creek. Debris flows and debris torrents affected communities such as Canmore and Exshaw, and isolated many mountain communities including closing the TransCanada Highway for several days. Environment Canada called it the “Flood of Floods.” Insured losses of CAD

Supercooled Liquid Water and Ice Crystal Distributions Within Sierra Nevada Winter Storms

2 billion included thousands of cars and homes demolished and damaged by backed-up sewers. Flood damage losses and recovery costs from the flood are projected to exceed CAD

On the Dependence of Winter Precipitation Types on Temperature, Precipitation Rate, and Associated Features

6 billion.

A Parameterization of the Microphysical Processes Forming Many Types of Winter Precipitation

Droughts are among the worlds most costly natural disasters and collectively affect more people than any other form of natural disaster. The Canadian Prairies are very susceptible to drought and have experienced this phenomenon many times. However, the recent 1999–2005 Prairie drought was one of the worst meteorological, agricultural and hydrologic droughts over the instrumental record. It also had major socio-economic consequences, adding up to losses in the billions of dollars. This recent drought was the focus of the Drought Research Initiative (DRI), the first integrated network focusing on drought in Canada. This article addresses some of the key objectives of DRI by providing a collective summary, understanding and synthesis of the 1999–2005 drought. Bringing together the many datasets used in this study was in itself a major accomplishment. This drought exhibited many important, and sometimes surprising, features. This includes, for example, (1) a non-steady large-scale atmospheric circulation (and sea surface temperature) pattern that mainly resulted in subsidence over the region but also cold and warm periods in its evolution; such features have typically not occurred in previous droughts; (2) large spatial gradients between wet and dry areas that, in some instances, were linked with major precipitation events; and (3) many impacts at and below the earths surface that occurred with varying temporal lags from the meteorological conditions and, in response, these impacts would have fed back onto the character of the drought (e.g., the surface-convection feedback). The droughts complexity poses enormous challenges for its simulation and prediction at all temporal scales. High-resolution models coupled with the surface are needed to address these and many other issues identified in this article. R ésumé u2003[Traduit par la rédaction] Les sécheresses sont parmi les catastrophes naturelles les plus coûteuses et, collectivement, affectent plus de gens que toute autre forme de catastrophe naturelle. Les Prairies canadiennes sont très vulnérables aux sécheresses et ont souvent subi ce phénomène. Toutefois, la récente sécheresse de 1999–2005 dans les Prairies a été lune des pires sécheresses météorologiques, agricoles et hydrologiques enregistrées depuis que lon effectue des relevés. Elle a aussi eu dimportantes conséquences socio-économiques, les dommages se chiffrant en milliards de dollars. Cest à cette sécheresse récente que sest intéressé le Réseau de recherche sur la sécheresse (DRI), le premier réseau intégré consacré à la sécheresse au Canada. Le présent article traite de certains des objectifs clés du DRI en fournissant un résumé général, une compréhension et une synthèse de la sécheresse de 1999–2005. Le seul fait de rassembler les nombreux ensembles de données utilisés dans cette étude était en soi un accomplissement remarquable. Cette sécheresse présentait plusieurs caractéristiques importantes et quelquefois surprenantes. Parmi celles-ci : (1) une configuration de circulation atmosphérique transitoire à grande échelle (et de température de la surface de la mer) qui a généralement causé de la subsidence dans la région mais aussi des périodes de froid et de chaleur au cours de son évolution; ces caractéristiques ne se sont généralement pas produites lors des sécheresses précédentes; (2) de forts gradients spatiaux entre les zones humides et sèches qui, dans certains cas, étaient liés à des événements de précipitations extrêmes; et (3) plusieurs conséquences à la surface et sous la surface de la terre qui se sont produites avec des retards variables par rapport aux conditions météorologiques et qui auraient à leur tour rétroagi sur le caractère de la sécheresse (par exemple, rétroaction de convection de surface). La complexité de la sécheresse pose des défis énormes pour sa simulation et sa prévision à toutes les échelles temporelles. Des modèles à haute résolution couplés avec la surface sont nécessaires pour traiter ces questions et plusieurs autres mentionnées dans cet article.