Cloud Dynamics, Structure and..

Transcription

1 Cloud Dynamics, Structure and.. Prof. Steven Rutledge Department of Atmospheric Science Colorado State University Presented to the ASP Remote Sensing Symposium 3 June 2009

2 Outline Introduction Instability, shear and storm organization Air mass and multicell thunderstorms Supercellular convection Mesoscale Convective Systems

3 The mystery of clouds Precipitating clouds are of course needed for life on our planet Fundamental components of the hydrologic cycle Clouds play a key role in climate change Precipitating clouds present challenges for both remote and in-situ sensing We will focus mainly on clouds that are convective in nature, forming in the troposphere where temperature generally decreases with height Clouds have been the subject of studies for centuries

4 The big controls on clouds Atmospheric instability CAPE is integrated thermal buoyancy assuming parcel reaches LFC, level of free convection CAPE represent maximum kinetic energy achievable by an ascending parcel that does not exchange momentum, heat and moisture with its environment Vertical shear (drives degree of organization) Aerosols, receiving increased attention as of late African aerosols for example

5 Convection as a function of shear and CAPE: Individual convective storms and mesoscale convective systems DIVIDING LINE BETWEEN SEVERE AND NON-SEVERE (Jorgensen and Weckwerth 2003)

6 Convection as a function of shear and CAPE Bulk Richardson No. Lowest 0-4-km shear Weisman and Klemp Multicell R>30 Supercell 10<R<40 R. Johnson (CSU), class notes, Mesoscale Dynamics

7 Mature Stage Conceptual Model AIRMASS THUNDERSTORM The classical conceptual model of an airmass thunderstorm from The Thunderstorm Project. Byers and Braham, Thunderstorm Project was the first field project dedicated to weather research. More on this in a bit.. Doswell, C. A. III, 2007: Historical overview of severe convective storms research. Electronic J. Severe Storms Meteor., 2(1), In the low shear environment that airmass storms form in, precipitation produced downdraft chokes off the updraft, leading to rapid storm dissipation. Lifecycle around 30 min.

8 Cloud dynamics and microphysical processes can be described by a simple 1-D entraining cumulus model. Concept of Buoyancy, allows parcels to accelerate in the vertical Vertical momentum equation can be formulated by considering a slight perturbation from the base hydrostatic state dw / dt g / (1 / ) p / z First term on RHS is the buoyancy term Second term on RHS is the pressure perturbation term Neglecting the pressure perturbation term, parcel dw / dt ( / )g e Since the cloud area is small compared to the area of the cloud s environment, e dw / dt ( e / e)g

9 A more general form of the Buoyancy term can be arrived at my combining the Ideal Gas Law and the virtual temperature expression in perturbation form; g[t / T e p / p e (q vc q ve ) q L ] The various contributions to buoyancy are apparent. Temperature, pressure, vapor and condensate. Here the primed terms are short hand notation for the difference in the quantity between the cloud and the environment. Level of neutral buoyancy, neglecting pressure and vapor effects, occurs when q L (T c T e ) / T e

10 A host of cloud dynamical problems can be studied via the use of a simple 1-D entraining plume model. Lets look at a quick formulation that includes microphysical processes. Average properties of a rising cumulus element must include the effects of turbulent exchange with the cloud s environment. This is of course the entrainment process. Entrained air is brought in from sides and mixed instantaneously across the cloud Process occurs continuously as the cloud parcel rises c is some arbitrary variable such a rain water content, heat, etc. m is mass. m dm i dm 0, c d c e d e dm i dm 0 c,m e

11 From conservation principles.. [m (dm) i (dm) 0 ]( c d c ) m c e (dm) i c (dm) 0 Smdt It can be shown after some algebra that d c / dt 1/ m(dm / dt) i ( c e ) S (1) where 1 / m(dm / dt) i is the entrainment rate. The entrainment rate is parameterized based on conceptual models for shape of thermal. Jet, thermal and starting plume models are invoked. Based on Eqn. (1), a set of equations describing parcel properties in rising cumulus elements can easily be arrived at. Vertical momentum equation: dw c / dt 1 / m(dm / dt) i (w e w c ) S o

12 Water continuity equations can be introduced to describe vapor and each condensate field: dq c / dt 1 / m(dm / dt) i (q e q c ) S Microphysical processes (S) can be formulated via bulk parameterization or explicit methods. Finally, the thermodynamic energy equation can be derived from principles of Moist Static Energy: dt c / dz g / c p L / c p (dq vc / dz) 1/ m(dm / dt) i [(T e T c ) L / c p (q ve q vc )] These equations can easily be solved to describe the vertical motion, temperature, water vapor and rain/precipitation ice fields. This simple model can be used to investigate a host of problems in the area of cumulus dynamics.

13 From Houze (1993) Cloud Dynamics The common airmass thunderstorm provides a nice illustration of the couplings between storm dynamics, microphysics and electrification. Mixed phase processes aloft promote intracloud lightning in developing/mature phase. Descent of ice and charge then lead to cloud-to-ground lightning in the mature/dissipating phases.

14 Storm charge structure Normal polarity Ice based precipitation provides for electrification Dipole/tripole Vertically separated, oppositely charged regions Charge regions tied to specific temperature regions

15 Basic premise is that large and small ice particles along with supercooled droplets, collide and rebound in a cloud, with charge of opposite sign being retained on the graupel and small ice particles, respectively. Graupel charges negatively under certain conditions and positively under other conditions. Depositional surface state and water coated surface state associated with positive charge on that particle. Adapted from Williams, Scientific American Takahashi (1978) J. Atmos Sci esu = 33 fc Femto = 10e- 15

16 The Thunderstorm Project Directed by Professors H. Byers and R. Braham at the University of Chicago. It was clear at the end of World War II that both civilian and military aircraft could avoid flying in and around thunderstorms. To promote aviation safety, information was needed concerning the internal structure and behavior of thunderstorms. The Thunderstorm Project took advantage of equipment and people that were at the end of World War II. Twenty two railroad freight cars and ten P-61C Black Widow aircraft were made available. Radars, sounding equipment and surface instrumentation were deployed. Instrumented sailplane observations of thunderstorms were also carried out. Results published in The Thunderstorm (1949). Revealed detailed information on airmass and multicell thunderstorms.

17 Conceptual model for the multicell storm from The Thunderstorm Project.. From Houze (1993) Cloud Dynamics A collection of air mass storms undergoing individual lifecycles Young cells contain a single updraft with developing precipitation Mature cells have both an updraft and a downdraft and produce heavy rain Dissipating cells contain a downdraft and produce light rain

18 An example of a multicell as viewed by the CSU-CHILL radar

19 Corresponding image of differential reflectivity

20 The more organized multicell thunderstorm Cells undergo the three stage lifecycle as they move through the storm Browning et al. (1976), Quart. J. Roy. Met. Soc.

21 Adapted from Houze (1993) Cloud Dynamics Supercell storm. Often accompanied by severe weather, wind, hail, tornadoes. Conceptual model identifies a single updraft. Ambient shear allows for storm-scale rotation, the mesocyclone. Characterizes the pre-tornadic phase. Mesocyclone is mainly at storm mid-levels at this stage.

22 Supercell conceptual model CHARACTERISTICS Weak echo vault Leading shelf cloud denoting updraft -40 C Hail formation process, rapid growth by accretion as graupel pass through areas of large supercooled liquid water content Hail fallout Browning and Foote (1976) Fallout of heavy rain

23 29 June 2000 supercell storm observed during STEPS 2000 Dual-Doppler derived flow structure One large updraft, downdraft structure. Characteristic of supercell storm. Tessendorf et al. (2005), J. Atmos. Sci.

24 Cold outflow Tornadic phase Warm, moist inflow Key dynamics of the supercell Forward and rear flank downdrafts critical for generation of low level horizontal vorticity and then vertical vorticity through tilting. The tilting of the low level vorticity associated with the RFD/FFD converging with the warm inflow is a major source of vorticity for lower portion of the mesocyclone. Tornado is thought to form when lower portion of the mesocyclone intensifies via this mechanism. Lemon and Doswell (1979)

25 Mesocyclone and TVS signatures Example of supercell as viewed by the CSU-CHILL radar, 21 May 2004

26 What physical interpretations can you make here..

27 But here in Colorado Nonsupercell tornadoes prevail. Some times referred to as gustnadoes. Stretching of low-level horizontal vorticity by storm updraft Wakimoto and Wilson (1989)

28 Classic supercell, CG lightning predominately positive 29 June 2000 overview Positive CGs Max reflectivity ~ 70 dbz, Max updraft ~ 50 m/s

29 29 June charge structure (+CG supercell) Inverted tripole in precipitation; dipole in updraft Lower negative charge present in region of +CGs Wiens et al Radar data time: 2325 UTC NLDN data time: UTC LMA data time: 23:24:42-23:24:57 UTC

30 The Mesoscale Convective System Houze, Rutledge, Biggerstaff and Smull (1989), BAMS Driven by organized, linear convection forming on a feature like a surface front or outflow boundary. Convective line contains individual, intense convective elements that slope slightly rearward with height owing to vorticity considerations between ambient low level shear and cold pool. Rottuno, Klemp and Wiesman (1988).

31 From Newton and Newton, J. of Meteorology, 1959.

32 L Johnson and Hamilton (1988) Trailing stratiform region contains a number of interesting pressure features. Mesohigh Wake low

33 8 May 2009 In a more weakly-sheared environment, PV dynamics produces mid-level vortex in trailing stratiform region. Vortex produces asymmetric pattern to stratiform precipitation.

34 20 June 2007 MCS

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36 Convection initiated along colliding gust fronts. Gust front clearly depicted in Zdr field from CSU-CHILL radar.

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