Different region of the globe. Lecture 3, 2009, Tropics & thunderstorms. Tropics. Tropics. Streamline 16/04/06. Rossby number

Transcription

1 Different region of the globe Lecture 3, 2009, Tropics & thunderstorms. Angular momentum of earth is larger at the equator than the pole => move south W ly momentum increases, moves north E ly momentum increases. Knowing that angular momentum per unit mass, L is: From L for a ring of air around a latitude: 100m/s E ly moving 30S to 10S Need friction! From Bureau of Meteorology. Tropics From [11] Tropics There is a much smaller Coriolis effect near the tropics. Tropical weather characterised by: Small Coriolis parameter, small variations in P, T. Barotropic rather than baroclinic. E ly trades. => Thunderstorms & deep convection balances meridional T gradient - monsoons. Cyclones. Put dp ~ 2 hpa, L ~ 1000 km, U ~ 10 m/s, T = L/U, rho = 1kg/m 3 1 x ~ 10-4 m/s 2 ~ 10 x m/s 2 ~ 10-4 m/s 2 Winds drive the pressure gradient in tropics, instead of pressure gradient driving winds. => Use streamline charts Rossby number Streamline 16/04/06 Defines relative importance of coriolis force Cyclone Monica For large scale motions - like gravity waves, Rossby number is small. For the mid-latitudes where f large, R 0 is also small. Wind field drives pressure field in tropics Monsoon Trough ITCZ? Equatorial Trough Monsoon Flow Streamlines converge into low pressure areas and diverge out of high pressure areas. 1

2 Meridional heat transport? From [13] Tropical up branch of Hadley circulation driven by convection. So let s talk about convection and stability, ie: vertical motion in the atmosphere. Occurs when atmosphere is unstable, ie: less dense underlaying more dense (we see overturning in the ocean too). Hadley cell shifts with the monsoons as area of maximum heating shifts annually - monsoons. Ramage criteria: 1.Prevailing wind direction shifts by at least 120 o between January & July 2.Average frequency of prevailing wind direction exceeds 40% in both months. 3.The mean resultant winds in a least one of the months exceeds 3ms -1 4.Less than one cyclone anticyclone alteration occurs on average every two years in any one month in a 5 o latlon rectangle. Heat or parcel energy Now we can relate P and T. Start with The change in heat, dq, of a parcel is the sum of internal and external energy changes. In terms of known quantities, one way to write is: Volume held constant Then, for an adiabatic process P 0 + dp T 0 + dt Or Constant of Work done proportionality in expanding How are C p, C v related? Use eqn state & chain rule Pressure held constant Integrate P 0, T 0 Set dq=0 (adiabatic) and equate This the Potential temperature T 0 often taken as 1000 hpa Gives a way of measuring temperature or density that is independent of the local pressure, ie: can answer the question, Will convection occur? Increases upwards and towards the equator. Dry Adiabatic Lapse Rate How does T change with height? P 0 + dp T 0 + dt Lines of constant potential temperature Now use binomial expansion: P 0, T 0 Pole 340K 320K 300K Eq. Rearrange 2

3 Often, put, DALR = Cp (dry air) ~ 1004 J/K Saturated Adiabatic Lapse Rate Amount of heat released from parcel when water condenses = L c dq. Including in DALR eqn: From [14] EXERCISE: If the temperature in Sydney is 25 degrees C, what temperature would you expect to feel in Blackheath, 1100m higher? Assume both locations are sunny and the atmosphere is dry. ANSWER: 14 deg C Dividing by C p T Since Lc, Cp T > 0 < Lines of constant saturated potential temperature Stability Start with buoyancy B has units of acceleration. We can see it equivalently as the force per unit mass acting on the parcel due to the re-arrangement of mass vertically. Now Integrating the buoyancy, B, over a depth of atmosphere, z: Also, B is the vertical acceleration per unit mass, so: Equating: P T we can see from temperature sondes => one of the most useful tools in meteorology. Updraft strength Values of CAPE can be up to: T Stability summary UNSTABLE CONDITIONALLY UNSTABLE STABLE rain What sort of trace? storm From [3] stable unstable stable A parcel of air displaced upwards will follow a trajectory between the DALR and SALR. The environment is not constrained by the lapse rate. 3

4 Storms Frequent tropical occurrence - main weather of the tropics. Updraft weakened by coincident downdraft - instability removed. Longer timescales - Waves Kelvin Waves Madden-Julian Oscillation Rossby Waves Key driver of active & break periods of monsoon Tracked via OLR, 850 hpa and 200 hpa zonal wind anomalies. Also known as day oscillation. Travels W to E at 5-10 m/s. Origin/physical mechanisms not well understood. From [5] Updraft separated from downdraft - instability maintained. From [6] Kelvin Waves Remember the shallow water equations (p replaced by h) Then and where H is the unperturbed depth of the fluid and we assume a barotropic fluid, ie: pressure changes due only to depth changes. Before: f=0, v=0 and d/dy all quanties = 0. Now d/dy is not zero and allow f=f0 Assume solutions like: Kelvin Waves have been known to trigger El Nino s (see ) Maximum div.east of sfce low. Moves east fast ~ 10m/s Boundary is the equator. Non-equatorial Kelvin waves occur near coastlines. Oceanic Kelvin waves can trigger El Nino Kelvin wave propagates eastwards across the Pacific, then follows the coast of South America. Trigger: weakening of the E ly trades. Key driver of active & break Madden-Julian Oscillation periods of monsoon Tracked via OLR, 850 hpa and 200 hpa zonal wind anomalies. Also known as day oscillation. Travels W to E at 5-10 m/s. Origin/physical mechanisms not well understood. From [15] 4

5 MJO Monitoring Kelvin W & MJO 1971 Roland Madden and Paul Julian noticed periodic pattern in upper wind anomalies over the Pacific. Known as MJO. Driver of wet/dry spells in wet season. Hence, indexes Tropical oceans drive climate variability in the tropics and further afield via teleconnections. [ Pdiff - Pdiffav ] SOI = SD(Pdiff) IOD From [15] From [16] Cyclones Intense tropical storms where winds average over 64 knots. In the absence of winds, a central pressure of 995 hpa is used. Warm cored. Different mechanisms at play near the centre and on the fringes. Warm cored. Cyclostrophic balance From [5] 5

6 In the Australian region, C ~ 2.5, x~0.7 (Crane 1985) Hence, for cyclone of central pressure 995hPa, embedded In an ambient pressure of 1010hPa, v c ~39m/s = 76 kts = 270 km/h. which is hurricane force. If central pressure is only 925 hpa (Thelma), 1050 km/h! Empirical, Method gives 205 km/h. Real winds were ~ 350 km/h. Dvorak technique Intensity References [1] Stewart, R. H. and Johnson, D., 2005: Introduction to Physical Oceanography, Texas A and M University, Accessed November , [2] Charts, Bureau of Meteorology, Accessed May , [3] Holton, J. R., 1994: Dynamic Meteorology: an Introduction. Daya Publishing House, ISBN , Tri nagar, Delhi, 390 pp [4] Stewart, R. H. and Johnson, D., 2005: Introduction to Physical Oceanography, Texas A and M University, Accessed November , [5] Holland, G.J., 1997: The Maximum Potential Intensity of Tropical Cyclones. Journal of Atmospheric Sciences, 1 November 1997, [6] Rensch, P.V., Pezza, A. B., & Wenju Cai: A Tracking Climatology for Heat Waves in Southern Australia, http: Accessed. Joint publication of Bureau of Meteorology and CSIRO. [7] Visconti, Guidio, Fundamentals of physics and chemistry of the Atmosphere. Springer, ISBN , , 593 pages [8] Special Climate Statement 17: The exceptional January-February heatwave in south eastern Australian, 4th February 2009, National Climate Centre, Bureau of Meteorology. [9] Wheeler, Matthew, Background to the MJO a.ka. the intraseasonal oscillation, Accessed May10, ww.bom.gov.au/bmrc/clfor 10] Geerts, B. and Wheeler, M.: The Madden Julian Oscillation, Bureau of Meteorology, May 1998, Accessed July 08, [11] Baroclinic Instability: Accessed May 28, 2009, [12] Northern Territory Library, Accessed July 08, 2008, [13] Pilot Friend: Prevailing Winds, Accessed June 03, 2009, [14] Atkins, Nolan, Dry versus Moist-Adiabatic Process. London State College Survey of Meteorology, Accessed November , [15] National Weather Service Forecast Office, Flagstaff, AZ, What is El Nino?. Acessed June 03, 2009, [16] Ummenhofer, C. et. al., at article by Bob Beale, Accessed June 02,