Storm charge structure

Transcription

1 Storm charge structure Dipole/tripole structure Vertically separated, oppositely charged regions/layers Typical charge pattern has negative charge sandwiched between upper and lower positive charge Exceptions to this charge structure exist, for example the inverted tripole where positive charge is sandwiched between upper and lower negative charge. Inverted dipole structures have also been documented.

2 Observations of Charge Structures Location of negative charge layer is largely invariant with temperature! How are these charge structures inferred? Balloonborne electric field meter observations Coulomb s Law applications

3 Normal charge structure Temperature ( C) Anomalous charge structure Why do storms develop midlevel or low level dominant positive charge; why are these storms confined to specific geographical locations? 3

4 Zajac and Rutledge (2000) Positive storms hang out in western and upper Great Plains

5 Williams et al Atmospheric Research What is this telling you?

6 Williams et al Wet bulb potential temperature Theta w ridge clearly evident Cloud base height above local terrain Proportional to TT d Summary of studies on positive ground flashes 22 Intersection between high CAPE and moderate CBH

7 We will return to PPCG (or inverted storms) in a bit but now let us talk about charging mechanisms, in general terms. How do these charge structures develop, say for a normal polarity storm?

8 Any charging theory must address the following The average duration of precipitation and electrical activity from a single convective storm is about 30 min Charge separation in thunderstorms results in cloud electric fields of 10 5 to 10 6 V/m. A single lightning stroke may transfer upwards of 100 coulombs of charge to the Earth s surface. Typical current associated with a cloud to ground discharge is 10 ka. Positive CG s can carry higher currents. Up to several hundred ka s from trailing stratiform regions. Electrical charge is typically located between about 5ºC and 40ºC. This charge resides on primarily ice particles of various types. (H, OH are charge carriers). Electrification is closely tied to the development of mixed phase precipitation in clouds. So the mixed phase region in thunderclouds is of particular interest for explaining charge separation.

9 Conceptual Model of charging mechanisms Two basic charging mechanisms. One involves precipitation processes, the socalled precipitation mechanism. The second mechanism involves convective air motions transporting free ions in the troposphere. This is called the convective charging mechanism. Precipitation Based Air Motion Based (Convective Charging) For the precipitation based mechanism, the electrical energy is derived from the gravitational power of precipitation particles. The electrical power is a fraction of the gravitational power. E. Williams, Scientific American

10 Noninductive charging (Precipitationbased charging) Consistent with lots of observational data that suggest strong E fields and lightning occur in clouds that develop a robust mixedphase precipitation process involving ice particles in the presence of supercooled liquid water (in form of cloud droplets)

11 Precipitation or noninductive mechanism Basic premise is that large and small ice particles collide and rebound in a cloud (in the presence of supercooled water), 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. Williams, Scientific American

12 Noninductive charging Exact mechanisms for this type of charging are not completely understood Lab experiments have shown that magnitude and sign of charge are dependent on: Temperature (Takahashi, 1978) Liquid water content (Takahashi, 1978; Saunders et al., 1991; Saunders and Brooks, 1992) Size of the colliding ice crystal (Jayaratne et al., 1983 and Keith and Saunders, 1990); size of the graupel particle Takahashi (1978) was one of the first to report on laboratory data to show the sign and magnitude of charge on a rod of ice whirling though a field of small ice crystals in the presence of supercooled water droplets at various temperatures and liquid water contents. But as we will see from the student talks, the literature on NIC began much earlier than the Takahashi study.

13 Takahashi, 1978, JAS 10 4 esu = 33 fc Femto = Three distinct charge regions found! Negative charging to graupel for temperatures colder than 10 C over modest to substantial liquid water contents. Positive charging to graupel at very low and extremely high liquid water contents. There are basic differences between Takahashi and the UMIST NIC experiments related to sign of charge on the rimer at moderate LWC s. Resulting in fundamental differences in modeled or inferred charge structures.

14 Convective Charging Theoryinvolving convective air motions Normal fairweather E field establishes charge concentration in lower troposphere (via corona processes at Earth s surface), which when carried by updrafts to the top of storms (or even dry convection), attracts negative ions, which are then carried down by downdrafts on cloud edges Charge is separated by the up and downdrafts Chiu and Klett (1976) argued that this method is unlikely to produce sufficient cloud charging to generate lightning and this mechanism has been generally dismissed

15 Inductive Charging General overview: Electric field induces charge on surface of hydrometeors Hydrometeor will acquire charges of opposite polarity on top and bottom surfaces of hydrometeor. This charge is induced by the ambient electric field caused by noninductive charging. Fair weather field is too weak to do anything. E When particles collide: large precipitation particle acquires negative charge and fall relative to the updraft, smaller cloud particles get positive charge and are carried upwards by updraft

16 A neat example of inductive charging NSF GV aircraft getting ready to penetrate a thin anvil cloud attached to a thunderstorm during the DC3 field campaign in 2012 Documentation of FDA s, frozen drop aggregates consisting of chains of small, frozen droplets in the core of the anvil. The droplets likely froze via homogeneous freezing and then subsequently aggregated due to inductive charging in the thunderstorm updraft. The normal graupel based charging mechanism produced strong electric fields in the storm updraft region that allowed inductive charging to build the FDA s. Stith et al. (2014), Stith et al. (2016)

17 How do hydrometeors get charged via the noninductive mechanism?

18 Electrical Properties of Water Quasiliquid layer Dash (1989), Golecki and Jaccard (1978) and Elbaum et al. (1992) showed the presence of a QLL, an ultrathin water phase transition between vapor and ice to temperatures as cold as 30ºC The QLL may be about 10 molecular layers in thickness

19 Electrical properties of water Electric doublelayer (Faraday layer) A dipole charge layer exists within the QLL (some H 2 O is always disocciated into H and OH ions) Studies have shown that negative () charge is in the outer most portion of the QLL with positive charge below. This is called an electrical double layer. Charging from double layer When two particles with different quasiliquid layer thicknesses contact one another, mass (negative charge) will flow from the thicker QLL to the thinner QLL, to establish chemical equilibrium between the QLL s. Negative charge moves in the direction of the mass transfer. When the particles rebound, the particle with the thicker QLL will have net positive charge. The particle with the thin QLL will have net negative charge by acquiring mass from the particle with the thicker QLL. As a rule, when two particles collide, the particle growing more rapidly by deposition will acquire (retain) net positive charge. More rapid deposition implies a thicker QLL. Air Water/ice

20 Now examine environmental variables in these regions Suspect we are again seeing high FR with high CBH which is common in Colorado Colorado region; highest flash rates DC region lowest Flash rates via clustering algorithm developed by E. Bruning and others

21 N NCAPE: CAPE divided by the height difference between the LFC and equilibrium level. J/kg/m. NCAPE is related to parcel kinetic energy. OK and CO are the winners in terms of NCAPE. Yet CO flash rates are larger.

22 Colorado median LCL height ~ 3 times higher Cloud base height MSL = 1.4 km AGL for CO

23 The final parameter: WCDvertical distance between cloud base and the freezing level Colorado storms have higher cloud bases and smaller Warm Cloud Depths compared to other regions. Reduced warm cloud depth leads to higher SLW contents in mixed phase region due to reduced coalescence. Higher LCL/cloud base heights also reduce entrainment by producing broader updrafts. Both processes lead to a higher adiabatic liquid water content in mixed phase region. High liquid water contents linked to positive charging of rimer via non inductive charging. Sanders and Peck would predict positive charge on rimer. Takahashi negative! Lots to think about here and we will do so next week during the course of the presentations.

24 Plotting peak LMA source density as function of T AL/DC warm positive charge layers associated with decaying, low flash rate storms. EOSO In Colorado, significant amount of active storms have inverted or anomalous charge structures. Recall, large NCAPE s, high CBH s and shallow WCD s.