NOGAPS 4.0 Note: On 24 June 1998 NOGAPS was upgraded to version 4.0. The primary change was an increase in the number of vertical levels from 18 to 24. Model tendencies are not expected to change dramatically from 3.4. This section will be modified as more experience is gained with 4.0. a. Surface lows: (ACPE = average central pressure error (bias). It is defined as the forecast central pressure minus the verifying analysis central pressure for each low being tracked. Negative ACPE indicates a bias toward being overforecast (deep); positive ACPE indicates a bias toward being underforecast or weak.) 1) Deepening land low ACPE is slightly overforecast, and filling (mature) lows tend to be -3 to -4 mb deep and slow to fill in the cool seasons. * 2) Developing oceanic lows tend to be slightly underforecast and slow to deepen with an ACPE near zero through 72 hours. Mature, filling oceanic lows tend to be -2 to -3 mb overforecast and slow to fill. 3) NOGAPS tendencies for land and ocean basin surface lows are: a)Europe, China and Siberia deepening land ACPE is near zero by 48 hours. Mature land low ACPE is -3 to -4 mb deep and slow to fill. o b)North America deepening land low ACPE is near zero by 48 h. Filling land lows are -3 to -4 mb deep and slow to fill. c)Atlantic developing low ACPE is slightly weak and slow to deepen by 48 hours. Atlantic mature lows are mostly deep and slow to fill. Filling low ACPE is -3 to -4 mb by 48/72 hours. d)Pacific developing low ACPE is slightly weak and slow to deepen by 72 hours. Pacific mature lows are -3 to -4 mb deep and slow to fill by 72 hours. e)In view of the general NOGAPS model tendency to underforecast oceanic developing SLP lows and overforecast oceanic mature, filling surface lows; associated surface wind speed forecasts also exhibit similar biases in the areas of higher wind speeds. Surface wind forecasts associated with deepening (filling) lows are underforecast (overforecast). 4)NOGAPS shows no consistent directional bias or tendency in the placement of forecast surface lows. In very general terms and synoptic flow patterns: a)Meridional flow: deepening oceanic lows are usually behind the analysis track (slow to move), especially at the extended forecast period. Filling oceanic lows are typically biased to the left of the analysis track (toward the upper level cold air). o b)Strong zonal flow or broad flow: deepening oceanic lows are ahead of the analysis track (fast to move), especially at the extended forecast period. Mature oceanic lows are usually to the left of the analysis track, and only occasionally to the right of the analysis track at the extended forecast period in strong U/L flow. c)Former West Pacific tropical cyclones are typically slow to move during and after transition to extra-tropical. 5) Secondary cyclogenesis continues to be underforecast. Lee cyclogenesis off Southeast Kamchatka and Greenland is underforecast. 6) NOGAPS continues to merge complex lows into one, usually deeper low pressure system, especially at the extended forecast period. 7) Surface lows forming south of the polar jet and bottom-up type of lows are slow to deepen. 8) Surface lows north of the polar jet at high latitude (>50N latitude) tend to be too deep. These are usually mature lows which have bottomed-out and tend to be slow to fill. 9) The forecast deepening rate for NOGAPS rapid/explosive cyclogenesis (deepening rate 15 mb or greater in 24 hours) is mostly underforecast, although for a small percent of cases it may be overforecast. 10) Surface lows associated with the formation of upper level cut-off lows in the cool season are usually overforecast (too deep), especially at the extended forecast period. Mature cut-off lows are slow to fill after bottoming-out. Instances of major overforecasting of cut-off lows may reflect the occasions where valid ship reports are sparse. 11) In the Central Mediterranean, rapidly deepening surface lows tend to be -4 to -6 mb overforecast (too deep) as upper level troughs deepen over the region. The overdeepening bias is primarily associated with digging troughs over the region as opposed to west-east moving troughs. The tendency to overdeepen lows is mostly observed in the cool seasons. 12) Sea-level pressure analyses and forecasts over the very high terrain of Greenland, Himalayas, Antarctica are suspect and should be used with caution. 13) In the warm seasons, late Spring to early Fall, a spuriously deep surface low is observed in the analysis and forecasts over the very high terrain of the Himalayas (vicinity 30N-090E). This "lock-in" feature is caused by model reduction of station pressure to sea-level, and the warm season surface air temperatures. b. Tropical cyclones: 1. NOGAPS exhibits several model tendencies associated with a tropical cyclone's stages of evolution: a) Tropical cyclone genesis: *NOGAPS tends to generate spurious tropical cyclones in the extended forecast periods (tau 72 and beyond). This is most pronounced in the Indian Ocean. o b) Tropical cyclone phase: * Due to the resolution of the NOGAPS global model, sizes of tropical cyclones in the NOGAPS analyses and forecasts are almost always too large in areal extent and this may cause false interaction with nearby tropical cyclones. + * In the deepening stage, forecast tropical cyclone directional bias is behind the analysis track and slow to move. After reaching maximum intensity, mature tropicals continue to be slow to move. c) Transition and extra-tropical phase: * For tropical cyclones undergoing transition to extra-tropical, the forecast surface low is usually overforecast (deep), slow to fill, and slow to move. + * In the re-deepening extra-tropical phase, former tropical cyclones are underforecast (weak) and slow to move. Directional bias is usually behind and to the left of the analysis track (slow to move and toward the U/L cold air), especially is zonal flow. * After filling begins, extra-tropical cyclones (former tropicals) are overforecast (deep) and slow to fill with directional bias slightly ahead of the analysis track (fast to move). c. Surface highs: 1) Western ocean high cells tend to be slightly strong by 48 hours. * 2) Mid-ocean high cells tend to be 1 to 2 mb strong by 48 to 72 hours. 3) The forecast central pressure of the offshore Eastern Pacific high cell is also somewhat strong. d. Upper-level: 1) Forecast upper level troughs and associated surface lows moving in strong zonal flow tend to be fast to move, especially at extended forecast periods. * 2) Upper level short-wave troughs in strong zonal and broad meridional flow are slightly weak. The associated developing surface low tends to be slow to deepen and 3 to 4 mb weak. 3) Upper level lows north of the polar jet tend to be deep. 4) Upper level highs south of the polar jet are slightly strong. 5) The formation of upper level cut-off lows associated with a digging trough continues to be well forecast in the transition seasons. The associated surface low is slightly overforecast (deep) throughout. 6) Splitting upper level troughs moving in zonal flow typically show the poleward portion to be well defined, but too deep. The equatorward portion of the trough is less defined and typically underforecast. 7) NOGAPS wind speed forecast variability is greatest in the 300 to 250 mb jetstream region of the upper troposphere. Intense jet level winds may be underforecast due to the limited vertical resolution of the model. 8) During the cool seasons, upper level troughs digging southeast out of the Gulf of Alaska to the U. S. West Coast are generally overforecast