TWICE-DAILY NORTHERN HEMISPHERE EXTRATROPICAL CYCLONE DATA SET (1966-1993) a. General Description This data set comprises a 28-year record (1 May 1966-31 December 1993) of twice-daily extratropical cyclone statistics for the Northern Hemisphere, obtained from application of the updated Serreze [1995] algorithm to National Meteorological Center (NMC) sea level pressure (SLP) fields for the 47x51 Octagonal Grid. The data set includes the position and central pressure of each cyclone, whether the observation represents a cyclogenesis or cyclolysis event (from the first and last observation of each system), and the local Laplacian (10-5 mb m-2) and sea level pressure tendency (SLPT) (mb (12h-1)) at each cyclone center. The local Laplacian is proportional to the geostrophic relative vorticity, and unlike cyclone central pressure, provides an index of cyclone intensity largely independent of changes in the background pressure field [Murray and Simmonds, 1991]. In turn, SLPT provides a useful index of synoptic development [Sanders and Gyakum, 1980; Roebber, 1984, 1989; Serreze, 1995], provided that the decrease in cyclone central pressure is not embedded within a region of generally falling or rising pressure and the storm maintains an approximately constant size through the 12 hour analysis period [Roebber, 1989]. b. Cyclone Detection Cyclone detection is based on a series of nested searches, testing whether a grid-point SLP value is enclosed by grid-point values at least 2 mb higher than the central point being tested. As the size of the NMC grids range from 274x274 km at 20oN to 408x408 km at the North Pole, the minimum diameter over which the 2 mb threshold can be satisfied at Arctic latitudes is roughly 800 km (by initial inspection of the eight grid values surrounding the central point). As up to three grid points away from the central point are tested, the maximum detection diameter is about 2400 km. The diameter of the entire closed system, of course, can be considerably larger. c. Cyclone Tracking Starting on the first 12-hourly chart in the record (chart 1), each cyclone is ascribed a number. A 3x3 NMC grid array is then centered over each system on the next 12-hourly chart (chart 2). If a cyclone on chart 1 falls within a given 3x3 array, the chart 2 cyclone at the center of the array is taken to be a continuation of the chart 1 system. This immediately tracks stationary or slow-moving cyclones. The minimum distances from all remaining untracked systems on chart 2 are then determined with respect to the remaining numbered systems on chart 1. Typically, two or more untracked chart 2 systems have their minimum distance with respect to the same chart 1 system. The number of the chart 1 system is carried over to the closest chart 2 system, provided that the minimum distance between them is <1200 km and several other checks involving the 12 hourlly SLPT, laplacian of SLP at the cyclone centers between the candidate pairing and the direction of system motion are satisfied. Otherwise, the chart 2 system is taken as new (cyclogenesis), and successively more distant chart 2 systems are tested in the same manner, up to the 1200 km limit. At the end of the search process, all remaining chart 1 systems which could not be paired with a chart 2 system are considered to have filled (cyclolysis). The process is then repeated for each subsequent pair of charts. If any chart is missing, all systems on the next available chart are taken as new and numbered accordingly. SYsten numbers are also reinitialized on January 1 of each year. d. Quality Control From a one-month comparison with manual analyses, Serreze [1995] found the algorithm (see for comparison the algorithms of Murray and Simmonds [1991] and Sinclair [1995]) to track systems correctly better than 98% of the time. This essentially reflects the tendency for the distances that individual cyclones move in 12 hours to be much smaller than the typical separation between cyclones on a given chart. Changes from the original Serreze [1995] algorithm improve the performance in synoptically "complex" situations, such as when two separate systems enclosed by a common isobar merge together or when a "parent low" is dissipating in conjunction with separate, but related, development of a new system. Some minor "edge effect" problems at low latitudes may be encountered when determining cyclogenesis and cyclolysis, at it may be indeterminate whether a system formed (decayed) within or moved into (out of) the map area. Limiting any analyses to the region north of 25oN should eliminate this problem. Apart from possible temporal inhomogeneities in the SLP data, smoothing over the relatively coarse NMC grid results in loss in detail compared with the "true" field. Furthermore, cyclone central pressures will tend to be overestimated (too high) with this effect increasing poleward. Because of the tendency for greater overestimation of central pressures as systems deepen, SLPT will also tend to be underestimated, with this effect greater for intense systems with strong pressure gradients near their center. It can be argued that the algorithm will tend to depict cyclogenesis (cyclolysis) as occurring after (before) its "true" occurrence, hence possibly impacting upon where these events are depicted because of the 2 mb cyclone detection threshold and the relatively coarse NMC grid. Finally, the algorithm is limited to the detection of synoptic-scale systems - mesoscale "polar lows" will generally be missed. Although tropical cyclones will be detected and tracked, no attempt was made to differentiate them from extratropical disturbances. f. Suggestions to Users Because of possible edge effect problems, analyses of cyclogenesis and cyclolysis should be limited to north of 25oN. It is also suggested that the SLPT values be adjusted to a chosen reference latitude using the relationship: SLPTA = SLPT*sinLATREF/sinLAT where SLPTA is the adjusted pressure tendency, LATREF is the reference latitude and LAT is the latitude of the original ("raw") SLPT observation [see Roebber, 1984]. This accounts for the fact that storms at different latitudes with identical pressure gradients do not produce the same maximum geostrophic wind. The choice of the reference latitude is somewhat arbitrary; 60oN has been used in several studies [e.g., Sanders and Gyakum, 1980; Serreze, 1995]. Since even in a synoptically "active" area there can be a mix of filling and deepening events, analyses of average pressure tendency rates are generally not useful. More information is obtained through analysis of maximum deepening rates for individual systems or the frequency of occurrence of deepening greater than some selected threshold [e.f., Serreze, 1995]. The NMC Octagonal Grid is referenced to a polar stereographic projection, true at 60oN (where the grid size is 381x381 km). As the area represented by an NMC grid increases with latitude, maps of counts of system centers, cyclogenesis and cyclolysis should also be adjusted to 60oN latitude. The NMC grid size at any latitude (SIZELAT) needed for these adjustments can be determined as: SIZELAT = 381.0/((1+sin60)/(1+sinLAT)) g. References Murray, R.J. and I. Simmonds, 1991: A numerical scheme for tracking cyclone centers from digital data, Part I: development and operation of the scheme. Australian Met. Mag.39(3),155-166. Roebber, P.J., 1984: Statistical analysis and updated climatology of explosive cyclones. Mon. Wea. Rev., 112, 1577-1589. Roebber, P.J., 1989: On the statistical analysis of cyclone deepening rates. Mon. Wea. Rev., 117, 2293-2298. Sanders, F. and J.R. Gyakum, 1980: Synoptic-dynamic climatology of the "bomb". Mon. Wea. Rev., 108, 1589-1606. Serreze, M.C., 1995: Climatological aspects of cyclone development and decay in the Arctic. Atmosphere-Ocean, 33(1), 1-23. Sinclair, M.R., 1995: An objective cyclone climatology for the Southern Hemisphere. Mon. Wea. Rev., 122(10), 2239-2256. c h. Variables and Format 1) julian day 2) year 3) month 4) day of month 5) hour (00 or 12 UTC) 6) total number of systems for that day 7) number of grid points defining the central pressure 8) flag to indicate if previous day(s) were skipped due to missing chart(s) (yes=1, no=0) 9) cyclone central pressure (mb) 10) local laplacian of pressure (10-5 mb m-2) 11) distance traveled from last observations (-999 when iskip=1 or is cyclogenesis event) 12) 12 hour pressure tendency from last observation (-999 when iskip=1 or is cycogenesis event) 13) latitude of system center 14) longitude of system center (0 to 360 deg.) 15) NMC grid row 16) NMC grid column 17) flag for cyclogenesis event (yes=1,no=0) 18) flag for cyclolysis event (yes=1, no=0) 19) system number (reinitialized on January 1)