及金 第五章： 大气环流中的纬向环流系统 5.1 Storm Tracks 授课教师：张洋 2022.12.4
第五章： 大气环流中的纬向环流系统 5.1 Storm Tracks 授课教师：张洋 2022. 12. 4
及图乐 Non-zonal circulations 1992010100 授课教师：张洋2
授课教师：张洋 2 Non-zonal circulations
及图乐 Outline Observed features from two basic approaches seasonal variation inter-annual.decadal variations Storm track dynamics Baroclinic eddy life cycle Transient eddy energy budget Summary and discussion 授课教师：张洋3
授课教师：张洋 3 Outline n Observed features n from two basic approaches n seasonal variation n inter-annual, decadal variations n Storm track dynamics n Baroclinic eddy life cycle n Transient eddy energy budget n Summary and discussion
Observed features Two basic approaches to diagnosing storm tracks: The traditional one:track the position of individual weather systems,produce statistics for their distributions,e.g.track densities,storm life span... The bandpass filtering approach (in synoptic time scales): estimate the statistics at a set grid points in analyzed fields,which can provide a 3-d picture of storm tracks. 授课教师：张洋4
授课教师：张洋 4 Observed features n Two basic approaches to diagnosing storm tracks: n The traditional one: track the position of individual weather systems, produce statistics for their distributions, e.g. track densities, storm life span... n The bandpass filtering approach (in synoptic time scales): estimate the statistics at a set grid points in analyzed fields, which can provide a 3-d picture of storm tracks
Observed features across the Atlantic towards/ northern Europe from the East China sea across the Pacific EQAT0R SOUTH +" TROPIO OF CA AUSTRALIA FIG.1.A figure from an 1888 geography text showing storm frequency distribution as viewed in the mid-nineteenth century.The stipling denotes high storm frequency,while the arrows indicate individual storms.Reproduced from Hinman(1888). 授课教师：张洋5
授课教师：张洋 5 Observed features 2164 JOURNAL OF CLIMATE VOLUME 15 FIG. 1. A figure from an 1888 geography text showing storm frequency distribution as viewed in the mid-nineteenth century. The stipling denotes high storm frequency, while the arrows indicate individual storms. Reproduced from Hinman (1888). er activity over central Asia; and a third weak maxima in cyclone activity located over the Mediterranean, extending into central Asia. Consistent with the eastward propagation of disturbances, cyclogenesis preferentially occurs on the westward fringe of the areas of maximum cyclone occurrence. The advent of gridded atmospheric analyses at regular time intervals in the late 1970s heralded a new and dynamically more complete picture of storm track structure. Blackmon (1976) and Blackmon et al. (1977), following a methodology that can be traced to Klein (1951), showed that the atmosphere is described by a dispersion relation of sorts, as time filtering a series of gridded analyses maps to isolate disturbances with periods of 2–7 days (see also Hartmann 1974; Randel and Stanford 1985) isolates the O(1000 km) spatial-scale mobile transients familiar from the above synoptic classification of storm tracks. Further, this ‘‘bandpass’’ filtering has the distinct advantage vis-a´-vis synoptic classification that it can be carried out at all levels in the atmosphere, allowing the development of a true threedimensional picture of storm tracks. The original diagnoses of Blackmon and collaborators, along with numerous others since, provide an alternative definition of storm tracks as geographically localized maxima in bandpass transient variance. Examples of storm track structure that emerge from such an analysis are shown in Figs. 2a–c, where the storm tracks are marked in the various bandpass standard deviation fields by enhanced variability off the east coasts of Asia and North America, more or less coinciding with the regions of maximum cyclone occurrence described above. With their strong connection to sensible weather, storm tracks play a prominent part in midlatitude climate dynamics. Regardless of how one chooses to define storm tracks, a systematic shift in either their geographical location or the level of storm activity will lead to substantial precipitation anomalies with consequent impacts on regional climates. A particularly pointed example of precipitation anomalies resulting from a change in storm track structure occurs during strong El Nin˜o events, when the Pacific storm track extends much farther downstream than it does during ‘‘normal’’ winters. This downstream extension brings more active landfalling cyclones to California, resulting in flooding, landslides, and beach erosion. However, it is not only the ‘‘obvious’’ changes in precipitation patterns associated with shifts in storm track structure that explains why storm tracks are a topic of such vital importance to climate dynamics. Rather, over the past decade there has been a growing realization that storm tracks are symbiotically linked (following the terminology of Cai and Mak 1990) to the planetaryscale flow. To be concrete, consider a common problem in climate dynamics; namely, diagnosing an anomaly in the planetary-scale flow associated with some imposed external forcing, that is, anomalous tropical heating associated with El Nin˜o SST anomalies. In general, a corresponding shift in the storm track structure will accompany the anomaly in the planetary-scale flow (Branstator 1995). However, diagnoses have shown that the storm track shift, through anomalous fluxes of heat and momentum, often forces a larger component of the observed planetary-scale flow anomaly than the imposed from the East China sea across the Pacific across the Atlantic towards northern Europe
Observed features b)NCEP/NCAR storm tracks 90N 60N 30N EO 60E 120E 180 120W 60 Shaded:standard eviation of 24-h filtered 500-hl geopotential height ontour interval 20 m)computed from the Januaries of 982-1994(NCEP/NCAR reanaly is) (From Chang's homepage) Two storm track zones in N.H. Fig.7.9.The tracks of low pressure centres over the North Atlantic for the period December 1985 to February 1986.The shading indicates the region where the high frequency元l/ exceeded 90m in the ECMWF analyses for the same period. 授课教师：张洋 6
授课教师：张洋 6 Observed features 224 Three-dimensional aspects of the global circulation Fig. 7.9. The tracks of low pressure centres over the North Atlantic for the period December 1985 to February 1986. The shading indicates the region where the high frequency Za exceeded 90 m in the ECMWF analyses for the same period. 'storm track' as observed in the northern hemisphere, namely, the elongated maximum in geopotential height variance, the large vertical temperature flux at low levels, and the dipolar structure of the poleward momentum flux towards the downstream end of the storm track. Attempts to correlate the tracks of synoptic systems with the variance maximum are less successful than in the northern hemisphere. There is a tendency for the cyclonic systems to spiral polewards from cyclogenesis regions on the equatorward flank of the 'storm track' to decay regions in the 'circumpolar trough', the region of low pressure around the Antarctic coast. Partly at least, this is the result of attempting to identify the centres of synoptic weather systems by means of extrema in the surface pressure field. Because the surface wind field is strong around the southern hemisphere baroclinic zone, there is a natural tendency for centres of low pressure to be displaced poleward of the vortex centre, and for centres of high pressure to be displaced equatorward. But, partly, it seems that this spiral trajectory of weather systems is real. Perhaps it would be better to describe the regions of large high frequency variance as 'storm zones' rather than 'storm tracks'. However, the latter nomenclature is in general use despite being rather misleading. Each of the three major storm zones has a distinctive seasonal behaviour. Shaded: standard deviation of 24-h filtered 500-hPa geopotential height (contour interval 20 m) computed from the Januaries of 1982-1994 (NCEP/NCAR reanalysis) Two storm track zones in N.H. (From Chang’s homepage)
Observed features b)NCEP/NCAR storm tracks 90N 60N The storm zones occur in 30N association with the jet EO streams; 60E 120E 180 120W 60W 0 DJF mean at 500 hpa from 1980 to 2000 NP 60N The storm zones are most 295 intense near the longitude of 30N the jet exits. EQ 50 100 150 200 250 300 350 Zonal wind from NCEP/NCAR data 授课教师：张洋7
授课教师：张洋 7 Observed features − 5 10 15 15 20 25 20 −5 30 0 5 10 20 25 10 35 10 10 DJF mean at 500 hpa from 1980 to 2000 0 50 100 150 200 250 300 350 EQ 30N 60N NP Zonal wind from NCEP/NCAR data The storm zones occur in association with the jet streams; The storm zones are most intense near the longitude of the jet exits
Observed features b)NCEP/NCAR storm tracks 90N Temperature distribution from NCEP/NCAR data 60N DJF mean at 850 hpa from 1980 to 2000 NP 30N 60N EO 60E 120E 180 120W 60W 0 30N DJF mean at 500 hpa from 1980 to 2000 EQ NP 50 100 150 200 250 300 350 60N DJF mean at 500 hpa from 1980 to 2000 NP 295 60N 30N 30N 260 EQ 50 100 150 200 250 300 350 EQ 50 100 150 200 250 300 350 授课教师：张洋 8
授课教师：张洋 8 Observed features − 5 10 15 15 20 25 20 −5 30 0 5 10 20 25 10 35 10 10 DJF mean at 500 hpa from 1980 to 2000 0 50 100 150 200 250 300 350 EQ 30N 60N NP − 5 10 15 15 20 25 20 −5 30 0 5 10 20 25 10 35 10 10 DJF mean at 500 hpa from 1980 to 2000 235 240 250 260 0 50 100 150 200 250 300 350 EQ 30N 60N NP 250 255 265 275 285 290 250 DJF mean at 850 hpa from 1980 to 2000 0 50 100 150 200 250 300 350 EQ 30N 60N NP Temperature distribution from NCEP/NCAR data
及图 Observed features 3.00 (b)Track Density 0.0 (c)Genesis Density 0.20 0.00 Using ECMWF,MSLP. from Hoskins and Hodges,2002 授课教师：张洋 9
授课教师：张洋 9 15 MARCH 2002 HOSKINS AND HODGES Observed features 1049 FIG. 5. Attributes for the tracking of negative, cyclonic MSLP features: (a) feature density, (b) track density, (c) genesis density, (d) lysis density, (e) mean intensity (hPa), (f ) mean growth rate (day 1), (g) mean velocity (m s1), and (h) mean lifetime (days). Feature density suppression threshold is 0.5, track density suppression threshold is 0.2. 15 MARCH 2002 HOSKINS AND HODGES 1049 FIG. 5. Attributes for the tracking of negative, cyclonic MSLP features: (a) feature density, (b) track density, (c) genesis density, (d) lysis density, (e) mean intensity (hPa), (f ) mean growth rate (day 1), (g) mean velocity (m s1), and (h) mean lifetime (days). Feature density suppression threshold is 0.5, track density suppression threshold is 0.2. Using ECMWF, MSLP, from Hoskins and Hodges, 2002
及金乐 Observed features 7=仙≈03发 Eddy kinetic energy 0.9 (a) (b) 1350 0.8 300 250 0.5 200 150 0 02 100 0. 50 Fig.2.Left:the Eady growth rate,oL,at 500 hPa in units of I/days.Right:The average eddy kinetic energy at 250 hPa in units of (m/s)2.Both are for the Northern Hemisphere winter (DJF),computed from the NCEP/NCAR re-analysis.The maxima in EKE are downstream of the maxima in growth rate,and the Pacific storm track does not fully decay before the beginning of the Atlantic storm track.The prime meridian (Greenwich)is at 6 O'clock. from Vallis and Gerber,2008 授课教师：张洋10
授课教师：张洋 10 Observed features from Vallis and Gerber, 2008 = kci ⇡ 0.3 ⇤fo N Eddy kinetic energy