Lynn A. McMurdie

Major Numerical Forecast Failures over the Northeast Pacific

Abstract Strong North Pacific storms that impact the North American west coast are sometimes poorly predicted in the short term (up to 48 h) by operational models, with cyclone position errors of hundreds of kilometers and central pressure errors of tens of millibars. These major numerical forecast failures still occur despite continuing improvements in modeling and data assimilation. In this paper, the frequency and intensity of sea level pressure errors at buoy and coastal locations are documented by comparing the National Centers for Environmental Prediction (NCEP) Eta Model forecasts to observations and through case studies of two poorly forecast cyclones from the 2001/02 winter season. Using data from October 1999 through March 2003 at coastal and offshore sites along the west coast of North America, it was found that large forecast errors (48-h sea level pressure errors greater than 10 mb) by the Eta Model occur 10–15 times each winter, and extremely large errors (48-h errors greater than 15 mb) occ...

Atmospheric water distribution in a midlatitude cyclone observed by the Seasat Scanning Multichannel Microwave Radiometer

Abstract Patterns in the horizontal distribution of integrated water vapor, integrated liquid water and rainfall rate derived from the Seasat Scanning Multichannel Microwave Radiometer (SMMR) during a 10–12 September 1978 North Pacific cyclone are studied. These patterns are compared with surface analyses, ship reports, radiosonde data, and GOES-West infrared satellite imagery. The SMMR data give a unique view of the large mesoscale structure of a midlatitude cyclone. The water vapor distribution is found to have characteristic patterns related to the location of the surface fronts throughout the development of the cyclone. An example is given to illustrate that SMMR data could significantly improve frontal analysis over data-sparse oceanic regions. The distribution of integrated liquid water agrees qualitatively well with corresponding cloud patterns in satellite imagery and appears to provide a means to distinguish where liquid water clouds exist under a cirrus shield. Ship reports of rainfall intensity...

On the Relationship Between Scatterometer-Derived Convergences and Atmospheric Moisture

Abstract Fields of divergence calculated from the Seasat-A Satellite Scatterometer winds and fields of integrated water vapor and rainrate from the Scanning Multichannel Microwave Radiometer on Seasat are constructed for three different midlatitude cyclones. These storms include an explosively deepening cyclone that occurred in the North Atlantic (also known as the Queen Elizabeth II cyclone), a storm that occurred in the North Pacific, and a Southern Ocean storm. In all three cases, the regions of convergence and atmospheric water (vapor and rain) are consistent with each other and help to define features of each storm. The vertical distribution of moisture is inferred for one case using both the convergence pattern and the integrated water vapor field. In another, interpretation of the convergence field in a data gap region is aided by the water vapor field. In all three cases, surface low pressure centers, fronts, and even frontal waves are clearly evident as areas of convergence, and increased water v...

The Olympic Mountains Experiment (OLYMPEX)

OLYMPEX is a comprehensive field campaign to study how precipitation in Pacific storms is modified by passage over coastal mountains.

Satellite-derived atmospheric characteristics of spiral and comma-shaped southern hemisphere mesocyclones

Mesoscale cyclones in cold airstreams poleward of major frontal zones are generated frequently over the Southern Ocean. Quantitative information regarding the atmospheric water and wind fields of these systems can be retrieved from satellite-borne microwave instruments. These instruments include the Special Sensor Microwave Imager (SSM/I), the first European Remote Sensing satellite (ERS-1) scatterometer and the TIROS-N operational vertical sounder (TOVS). In this study, we summarize the structures found in the SSM/I-derived integrated water vapor (IWV), cloud liquid water, wind speed, and liquid and solid precipitation fields for 33 Southern Ocean mesolows. We present the ERS-1 scatterometer surface wind vector fields for nine of these mesolows, and we analyze the TOVS-derived geopotential thickness fields for one case. This study confirms earlier results and also examines several features not previously investigated in Southern Ocean mesolows. We find that (1) solid precipitation is more prevalent than liquid precipitation in most mesocyclones; (2) the higher-latitude, smaller-scale mesolows with spiral-shaped cloud systems have lower IWV content than comma-shaped mesolows; and (3) the surface cyclonic circulation of the mesolows is confirmed by the ERS-1 winds. Surface fluxes of heat and moisture were estimated from European Centre for Medium-Range Weather Forecasting (ECMWF) gridded fields for two highlighted cases and found to be moderate during the formation of the mesolow that occurred near the Antarctic ice edge and low for a comma cloud case. In addition, TOVS-derived thickness patterns show that the Antarctic case lacked strong baroclinic structure, whereas comma cloud cases tend to develop in baroclinic environments.

Comparison of Model Forecast Skill of Sea Level Pressure along the East and West Coasts of the United States

Abstract Despite recent advances in numerical weather prediction, major errors in short-range forecasts still occur. To gain insight into the origin and nature of model forecast errors, error frequencies and magnitudes need to be documented for different models and different regions. This study examines errors in sea level pressure for four operational forecast models at observation sites along the east and west coasts of the United States for three 5-month cold seasons. Considering several metrics of forecast accuracy, the European Centre for Medium-Range Weather Forecasts (ECMWF) model outperformed the other models, while the North American Mesoscale (NAM) model was least skillful. Sea level pressure errors on the West Coast are greater than those on the East Coast. The operational switch from the Eta to the Weather Research and Forecasting Nonhydrostatic Mesoscale Model (WRF-NMM) at the National Centers for Environmental Prediction (NCEP) did not improve forecasts of sea level pressure. The results als...

Satellite derived integrated water vapor and rain intensity patterns - Indicators of rapid cyclogenesis

Abstract Rapidly deepening cyclones in midlatitudes are characterized by large cloud shields and abundant condensation qualitatively evident in infrared and visible satellite images. With the availability of passive microwave measurements from polar-orbiting satellites, it is now possible to characterize rapidly deepening cyclones quantitatively in terms of integrated water vapor and precipitation intensity. In this study, fields of integrated water vapor, integrated water vapor anomaly (defined as the observed water vapor content minus the monthly mean water vapor content at the particular location), and rainfall intensity index derived from the Special Sensor Microwave Imager (SSM/I) on the F-8 satellite of the Defense Meteorological Satellite Program are examined for 12 North Atlantic rapidly deepening and 11 North Atlantic non-rapidly deepening storms that occurred during the 1988 and 1989 winter months. By correlating concurrent 6-h deepening rates with the satellite-derived parameters for a region w...

Satellite-Derived Integrated Water-Vapor Distribution in Oceanic Midlatitude Storms: Variation with Region and Season

Abstract With the atmospheric water-vapor content information available from the SEASAT and Nimbus-7 Scanning Multichannel Microwave Radiometers (SMMR), differences in water-vapor distribution between cyclonic storms in different regions of the global ocean can be examined in more detail than previously possible from radiosondes. SMMR-derived integrated water vapor is a robust and dependable variable of the same accuracy as integrated radiosonde soundings. In this study, maximum and minimum water-vapor content in the vicinity of cold fronts of 80 storms that occurred in the North Atlantic, North Pacific and Southern oceans are compared. North Atlantic storms were found to have significantly higher maximum and minimum water-vapor content near cold fronts on average than North Pacific or Southern ocean storms for both the warm and cold seasons. These differences are attributed to warmer sea surface temperatures and air temperatures in the North Atlantic, and higher baroclinity and consequently stronger upwa...

Predictability Characteristics of Landfalling Cyclones along the North American West Coast

AbstractThe predictability of North Pacific cyclones can vary widely, from highly accurate prediction of storm intensity and location to forecast position errors of hundreds of kilometers and central pressure errors of tens of hectopascals. In this study, a Weather Research and Forecasting Model (WRF) ensemble Kalman filter is used to investigate predictability of landfalling cyclones on the west coast of North America over two winter seasons (2008/09 and 2009/10). Predictability is defined as the ensemble spread of cyclone central pressure at the final forecast time (24 h) where large spread means low predictability. Both ensemble spread and ensemble initial-condition sensitivity are examined for a wide variety of cyclones that occurred during the two seasons. Storms that are deepening and track from the southwest exhibit the largest ensemble initial-condition sensitivity and highest ensemble spread compared to decaying storms and storms that track from other directions. Storms that end south of 40°N, ty...

Weather Regimes and Forecast Errors in the Pacific Northwest

Abstract Despite overall improvements in numerical weather prediction and data assimilation, large short-term forecast errors of sea level pressure and 2-m temperature still occur. This is especially true for the west coast of North America where short-term numerical weather forecasts of surface low pressure systems can have large position and central pressure errors. In this study, forecast errors of sea level pressure and temperature in the Pacific Northwest are related to the shape of the large-scale flow aloft. Applying a hierarchical limited-contour clustering algorithm to historical 500-hPa geopotential height data produces four distinct weather regimes. The Rockies ridge regime, which exhibits a ridge near the axis of the Rocky Mountains and nearly zonal flow across the Pacific, experiences the highest magnitude and frequency of large sea level pressure errors. On the other hand, the coastal ridge regime, which exhibits a ridge aligned with the North American west coast, experiences the highest mag...