Stephen J. Colucci

Verification of Eta–RSM Short-Range Ensemble Forecasts

Abstract Motivated by the success of ensemble forecasting at the medium range, the performance of a prototype short-range ensemble forecast system is examined. The ensemble dataset consists of 15 case days from September 1995 through January 1996. There are 15 members of the ensemble, 10 from an 80-km version of the eta model and five from the regional spectral model. Initial conditions include various in-house analyses available at the National Centers for Environmental Prediction as well as bred initial conditions interpolated from the medium-range forecast ensemble. Forecasts from the 29-km mesoeta model were archived as well for comparison. The performance of the ensemble is first evaluated by the criterion of “uniformity of verification rank.” Assuming a perfect forecast model, equally plausible initial conditions, and the verification is a plausible member of the ensemble, these imply the verification when pooled with the 15 ensemble forecasts and sorted is equally likely to occur in each of the 16 ...

Evaluation of Eta-RSM Ensemble Probabilistic Precipitation Forecasts

The accuracy of short-range probabilistic forecasts of quantitative precipitation (PQPF) from the experimental Eta‐Regional Spectral Model ensemble is compared with the accuracy of forecasts from the Nested Grid Model’s model output statistics (MOS) over a set of 13 case days from September 1995 through January 1996. Ensembles adjusted to compensate for deficiencies noted in prior forecasts were found to be more skillful than MOS for all precipitation categories except the basic probability of measurable precipitation. Gamma distributions fit to the corrected ensemble probability distributions provided an additional small improvement. Interestingly, despite the favorable comparison with MOS forecasts, this ensemble configuration showed no ability to ‘‘forecast the forecast skill’’ of precipitation—that is, the ensemble was not able to forecast the variable specificity of the ensemble probability distribution from day-to-day and location-to-location. Probability forecasts from gamma distributions developed as a function of the ensemble mean alone were as skillful at PQPF as forecasts from distributions whose specificity varied with the spread of the ensemble. Since forecasters desire information on forecast uncertainty from the ensemble, these results suggest that future ensemble configurations should be checked carefully for their presumed ability to forecast uncertainty.

Explosive cyclogenesis and large-scale circulation changes: implications for atmospheric blocking

Abstract Large-scale circulation changes attending explosive surface cyclogenesis are quantitatively examined in two cases selected from recent winter seasons. Both cases feature a rapidly deepening surface cyclone over the western Atlantic Ocean, but changes in the 500 mb geopotential height field near the cyclone differ in each case. One event, during January 1977, is characterized by the retrogression of an anticyclonic vortex (blocking high) in the 500 mb height field downstream of the surface cyclone. The second case, in February 1978, is distinguished by the formation of a 500 mb cyclonic vortex (cutoff low) upstream of the surface cyclone, but no downstream anticyclonic vortex is observed. The retrogression of the January 1977 block over the Atlantic Ocean coincides with the migration of a 500 mb synoptic-scale perturbation (associated with the surface cyclone) from a planetary- scale trough over North America toward a planetary-scale ridge over Europe. In the February 1978 case, the blocking cyclo...

Winter Cyclone Frequencies over the Eastern United States and Adjacent Western Atlantic

Abstract Analyses of winter cyclone frequency and deepening rates are presented for a 10-year period over the eastern United States and western Atlantic Ocean. Results are presented for 1° latitude-longitude quadrangles. The data source was microfilmed copies of NOAAs North American Surface Charts series routinely available over facsimile every 3 h. The analyses reveal a concentration of storms in a band from Cape Hatteras to New England, over the northern edge of the Gulf Stream current, and over the eastern Great Lakes. In addition, distinct minimums of winter cyclones are evident over the Appalachian Mountain range and, to a lesser degree, over the Florida peninsula. Analysis on a similar scale of 3 h pressure changes in these cyclones indicates that deepening was most favorable over the southern Appalachians, immediate Carolina coastal strip, the northern edge of the Gulf Stream. and the eastern Great Lakes. Significant positive or negative departures from normal winter precipitation along the East C...

An East Coast Winter Storm Climatology

A climatology of East Coast winter storms (ECWS) was developed using an automated procedure. This routine was used along with the NCEP‐NCAR reanalysis dataset (1948, 1951‐97) to identify storms over the October‐ April winter season. An array of statistical analyses was used to empirically analyze the interannual variability of these cyclones. To be classified as an ECWS, an area of low pressure was required to have a closed circulation, be located along the east coast of the United States (within the quadrilateral bounded at 45 8 Nb y 658 and 708W and at 308 Nb y 758 and 858W), show general movement from the south-southwest to the north-northeast, and contain winds greater than 10.3 m s21 (20 kt) for at least one time period (6 h). Storms meeting the above criteria were also required to have a closed circulation and be located within the quadrilateral during one additional 6-h period (not necessarily consecutive with the first). On average, 12 ECWS occurred per season with a maximum in January. Significant trends in storm frequency over the 46-yr period beginning in 1951 are not evident. However, a marginally significant (a 5 0.10) increase in average storm minimum pressure is noted. Spectral analysis of the ECWS time series shows significant cycles with periods of 2.3, 2.8, 3.4, 4.8, and 10.2 yr, which are in agreement with documented periodicities in joint Atlantic SST and sea level pressure data. Average monthly ECWS frequency anomalies are significantly higher ) ,

Comparative Diagnosis of Blocking Versus Nonblocking Planetary-Scale Circulation Changes during Synoptic-Scale Cyclogenesis

Abstract A 17-day period during November 1980 is investigated to obtain insight into differing large-scale 500 mb circulation changes during three consecutive synoptic-scale cyclone “events” Each event is defined by at least one rapidly intensifying surface cyclone over eastern North America or the western Atlantic Ocean. The first event is followed in time by the retrogression of a downstream split-flow type of block at 500 mb. The second event is characterized by the intensification of a large-scale 500 mb trough into a blocking cyclonic vortex. During the third event, no blocking systems are established or intensified; instead, the blocking cyclonic vortex is ejected downstream. Quasi-geostrophic model diagnosis reveals that during the first two (blocking) events the large-scale waves are reinforced by relatively large and spatially and temporally persistent transports of potential vorticity associated with 500 mb synoptic-scale waves linked with the surface cyclones. In the third (nonblocking)event, d...

Evaluation of Ensemble Predictions of Blocking in the NCEP Global Spectral Model

Abstract Ensemble forecasts from the National Centers for Environmental Prediction (NCEP) Global Spectral Model (GSM) have been used to develop a probabilistic scheme for the prediction of blocking over the Northern Hemisphere. An evaluation of these forecasts during three recent cool seasons revealed that they underpredicted the frequency of blocking at all ranges considered. Probabilistic forecasts for blocking over a sector on a particular day were constructed via binary logistic regression from all ensemble forecasts verifying on that day. Forecasts from two cool seasons served as the developmental dataset; the probabilistic forecasts generated from these data were tested on data from a third, independent cool season. The resulting calibrated (bias corrected) ensemble forecasts were compared to the climatology of blocking derived from the 40-yr NCEP–NCAR reanalysis data, and to the probability of blocking expected from the uncalibrated ensemble forecasts. The calibrated forecasts had higher skill, rel...

Climatology of 500 mb Cyclones and Anticyclones, 1950–85

Abstract The geographical and monthly frequencies of 500 mb cyclones and anticyclones in the National Meteorological Center analyses over the western half of the Northern Hemisphere are investigated for the period 1950–85. These cyclones and anticyclones, defined by the appearance of at least one closed (approximately) 6-dekameter contour around relatively low or high heights in the 500 mb height field, are generally observed less than ten percent of the time in any 10° by 10° latitude-longitude quadrangle, with cyclones being more numerous than anticyclones. The 500 mb cyclones are found primarily at middle and high latitudes, while anticyclones are observed most frequently over the subtropics. Cyclone frequency increases over the northern oceanic regions during summer, while anticyclone frequency increases throughout the subtropics during summer, especially over southwestern North America. Exceptions to these rules are observed; relatively high springtime 500 mb anticyclone frequency is found over the n...

Rapid 500-mb cyclogenesis and anticyclogenesis

Abstract Rapid intensification of cyclones and anticyclones in National Meteorological Center 500-mb height analyses is investigated during the 1983–84 through 1986–87 cool seasons (September through May). Rapid intensification is defined by analyzed height decreases (increases) of at least 10(5) dam in 24 h at the centers of 500-mb cyclones (anticyclones), defined here by closed contours at 6-dam intervals around low (high) 500-mb heights. The threshold 24-h central height changes for rapidly intensifying 500-mb cyclones (anticyclones) are two standard deviations lower (higher) than the mean central height changes for all 500-mb cyclones (anticyclones) during the 1983–84 cool season. Rapid 500-mb cyclogenesis is climatologically preferred over the northwestern Pacific Ocean and northeastern North America, while anticyclogenesis is favored in high latitudes with little longitudinal preference, but with some concentration in frequency near Alaska and Greenland. Both phenomena are observed preferentially no...

Planetary-scale climatology of explosive cyclogenesis and blocking

Abstract A climatology of the relationship between explosive cyclogenesis and atmospheric blocking has been constructed from an investigation of sea level pressure and 500-mb height analyses over the Northern Hemisphere during seven winter seasons. Planetary-scale 500-mb geostrophic u and v wind components and component anomalies (u′ and v′) were calculated over each explosive cyclone center and compared to the times and locations of block onsets. Blocking episodes were defined by the persistence, for at least 5 days, of negative zonal index (500-mb height at 40°N less than that at 60°N) spanning at least 18.75° of longitude. Thirty-three percent (43%) of the explosive cyclones that were not analyzed during blocking episodes over the Atlantic (Pacific) sector of the Northern Hemisphere and over which there were anomalously strong planetary-scale southerly winds (v′ > 0), anomalously weak westerly winds (u′ u/2 were followed, in not more than 5 days and within 60° of longitude, by the onset o...