GUIDE WAVE ANALYSIS AND FORECASTING - WMO

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(for the possible influence of land-sea breezes) and level of station maintenance. 2.2.4 Satellite data During the past decade or so, there have been a number of oceanographic satellites on which active and passive microwave sensors, such as scatterometers and radiometers (1978 on SEASAT) and radar altimeters (1985–89 on GEOSAT), have demonstrated the capability of measuring ocean surface winds. Since 1987, the Special Sensor Microwave/Imager (SSM/I) (US Defense Meteorological Satellite Program spacecraft) has been providing surface wind speed data over the global oceans in real time. These microwave measurements, from polar-orbiting satellites in 102-minute orbits around the Earth, provide several orders of magnitude greater surface wind observations compared to conventional ship and buoy data. Most recently the European Space Agency also has started to provide vector wind data from scatterometers and wind speed and wave data from radar altimeters on board the ERS-1 satellite and its successor ERS-2 launched in April 1995. Research efforts now are concentrating on how best to use these remotely sensed ocean surface wind data in improving the initial ocean surface wind analyses and subsequent forecasts. Table 2.3 is a summary of the current operational wind data available from satellites*. An example of an ocean surface wind data plot using SSM/I wind speed data is shown in Figure 2.3. This plot represents data centred at 00 UTC with a ± 3-hour window. The wind directions are assigned using the lowest level winds (about 45 m) from the US National Meteorological Center global operational analysis system. Winds are missing either where there is precipitation or where wind speeds are in excess of 50 kn (~ 25 m/s) (the satellite sensor’s upper limit). It should be noted that the quality of the wind measurements obtained from satellite-borne sensors depends on the accuracy of the algorithms used to derive wind-related parameters (speed and, if applicable, direction) from the sensor measurements (brightness temperatures from passive microwave sensors, and radar backscatter cross-section and antenna parameters from active microwave sensors) and various corrections that need to be applied for atmospheric water vapour and liquid water contamination. Furthermore, the sensor response may drift in time and careful quality-control procedures should be used to monitor the retrievals. _________ * The altimeter footprint depends on the pulse duration, satellite height, the sea state and the method for retrieving the backscatter value. The footprint is about 10 km. Measurements are reported as one second averages (so the area sampled is effectively about 17 km x 10 km) centred at about 7 km intervals along the sub-satellite track. OCEAN SURFACE WINDS 21 TABLE 2.3 Satellite derived winds Instrument Mode Swath Footprint Measurement Altimeter Active Nadir ≈10 km Surface wind microwave speed Scatterometer Active 500 km 50 km Surface wind microwave speed and direction Special Sensor Passive 1 500 km 25 km Surface wind Microwave/ microwave speed Imager 2.2.5 Common height adjustment In order to perform wind and pressure analyses, the data must be adjusted to a standard height. However, there is no fixed height at which the observations are made. Anemometer heights on fixed buoys range between 3 m and 14 m, on ships they range from 15 m to over 40 m, and on platforms or at coastal stations the heights may range up to 200 m or more above the sea. The theory of Monin-Obukhov (1954) is well known for calculating the shape of the wind profile through the lowest part of the atmosphere — known as the constant flux layer. The calculation requires the wind speed, at a known height within the layer, and the stability, which is derived from the air-sea temperature difference. Methods for adjusting winds to a standard height are reviewed and wind adjustment ratios are presented in table form in a WMO document by Shearman and Zelenko (1989). In practice, when an ocean surface wind field is being analysed from observations, the wind field is assumed to be prescribed at 10 m. For further discussion of wind profiles in the lower atmospheric boundary layer see Section 2.4. 2.3 Large-scale meteorological factors affecting ocean surface winds In common practice a forecaster may have to work from surface weather maps, because prognostic or diagnostic models for producing ocean surface wind fields may not be available. The forecaster can subjectively produce an ocean surface wind chart based on a knowledge of a few principles of large-scale atmospheric motions and some knowledge of boundary-layer theory. The simplest approach to obtain ocean surface winds would be to: • Calculate the geostrophic wind speed; • Correct it for curvature to derive the gradient wind speed; • Simulate the effect of friction by reducing this wind to approximately 75 per cent and rotating the wind direction by approximately 15° (anti-clockwise in the northern hemisphere and clockwise in the southern hemisphere). In relation to the geostrophic wind that is towards the region of lower pressure.
22 As a quick approximation of ocean surface winds this approach may be satisfactory. However, there are several important factors which should be considered when particular meteorological situations are identified. Some important meteorological relationships that govern the speed and direction of ocean surface winds are: (1) Surface-pressure gradient — geostrophic wind; (2) Curvature of isobars — gradient wind; (3) Vertical wind shear of the geostrophic wind — thermal wind; GUIDE TO WAVE ANALYSIS AND FORECASTING Figure 2.3 — Example of a combined Aviation Model (global spectral model) — Special Sensor Microwave/Imager (SSM/I) ocean surface wind analysis, valid at 00 UTC on 6 April 1992 for the area of the Grand Banks, North Atlantic (from NWS) (4) Rapidly changing pressure gradient in time — isallobaric wind; (5) Rapidly changing pressure gradient downstream — difluence and confluence; (6) Friction — Ekman and Prandtl layers; (7) Stability of air over the sea — air-sea temperature difference. These relationships (discussed in the following sections) may be considered independently and then combined to give an estimate of the wind field. It should
Page 1 and 2: WORLD METEOROLOGICAL ORGANIZATION G
Page 3 and 4: © 1998, World Meteorological Organ
Page 5 and 6: IV Chapter 7 - WAVES IN SHALLOW WAT
Page 7 and 8: ACKNOWLEDGEMENTS The revision of th
Page 9 and 10: VIII has been included particularly
Page 11 and 12: X considerable attention. Annex III
Page 13 and 14: 2 η Crest Zero level a H = 2a a Tr
Page 15 and 16: 4 Figure 1.5 — Paths of the water
Page 17 and 18: 6 When waves propagate into shallow
Page 19 and 20: 8 1.3.2 Wave groups and group veloc
Page 21 and 22: 10 wavelengths in a given sea state
Page 23 and 24: 12 for instance, a frequency of 0.1
Page 25 and 26: 14 in which E(f) is the variance de
Page 27 and 28: 16 2.1.1 Wind and pressure analyses
Page 29 and 30: 18 GUIDE TO WAVE ANALYSIS AND FOREC
Page 31: 20 Figure 2.2(a) (right) — Usual
Page 35 and 36: 24 GUIDE TO WAVE ANALYSIS AND FOREC
Page 37 and 38: 26 Gr G Gr ∇p C Cnf ∇p C Cnf
Page 39 and 40: 28 POINT C — The effect of warm a
Page 41 and 42: 30 a general sense, and can be appl
Page 43 and 44: 32 Free atmosphere Ekman layer Cons
Page 45 and 46: 3.1 Introduction This chapter gives
Page 47 and 48: ange of directions. Also, waves at
Page 49 and 50: small enough that swell can survive
Page 51 and 52: Figure 3.7 — Structure of spectra
Page 53 and 54: 4.1 Introduction CHAPTER 4 WAVE FOR
Page 55 and 56: necessary to forecast waves for a p
Page 57 and 58: TABLE 4.4 Additional wave informati
Page 59 and 60: TABLE 4.6 Ranges of swell periods a
Page 61 and 62: this situation, the angular spreadi
Page 63 and 64: the energy flux is c gH 2 . This is
Page 65 and 66: α0, degrees Solution: 80° 70° 60
Page 67 and 68: 5.1 Introduction National Meteorolo
Page 69 and 70: only once, since it is usual to sto
Page 71 and 72: Frequency 0.050 0.067 0.083 0.100 0
Page 73 and 74: The models may differ in several re
Page 75 and 76: The CH class may include many semi-
Page 77 and 78: 6.1 Introductory remarks Since the
Page 79 and 80: in some applications, the zero up/d
Page 81 and 82: OPERATIONAL WAVE MODELS 71 Figure 6
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give large differences in the compa
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Wind (m/s) Waves (m) Buoy/GSOWM Wav
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TABLE 6.2 Numerical wave models ope
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TABLE 6.2 (cont.) Country Name of m
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7.1 Introduction The evolution of w
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water boundary into shallow water.
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eflected waves, although the latter
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Energy (cm 2 /Hz) Energy (cm 2 /Hz)
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8.1 Introduction Wave data are ofte
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matter and timing the intervals bet
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a Directional Waverider (Barstow et
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80°N 60°N 40°N 20°N 0° -20°S
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a combination of sea-surface temper
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8.7.1 Digital analysis of wave reco
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9.1 Introduction Chapter 8 describe
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Presentation of data and wave clima
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the monsoon season of June to Septe
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Probability of non-exceedance case
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individual wave height derived usin
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emote, data-sparse areas. However,
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TABLE 9.2 (cont.) Country Source of
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For a storm hindcast, select the mo
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TABLE 9.3 (cont.) Country Model use
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ANNEX I ABBREVIATIONS AND KEY TO SY
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Abbreviation/ Definition Symbol MOS
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CODE FORM: D . . . . D or IIiii* SE
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sensors, spectral peak wave period
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cs1cs1 The ratio of the spectral de
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M jM j n mn m n smn sm n bn bn b P
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1744 Im Indicator for method of cal
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The following distributions are bri
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4. Weibull distribution Probability
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8. Fisher-Tippett Type I (FT-I) (or
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ANNEX IV THE PNJ (PIERSON-NEUMANN-J
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ANNEX IV 141 Duration graph. Distor
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REFERENCES Abbott, M. B., H. H. Pet
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REFERENCES 145 Carter, D. J. T., P.
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REFERENCES 147 Gumbel, E. J., 1958:
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REFERENCES 149 Maat, N., C. Kraan a
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Slutz, R. J., S. J. Lubker, J. D. H
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SELECTED BIBLIOGRAPHY CERC, 1984: C
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156 Energy . . . . . . . . . . . .
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158 steepness . . . . . . . . . . .
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GUIDE WAVE ANALYSIS AND FORECASTING - WMO

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