Is the Global Electric Circuit Modulating Winter Cyclone Vorticity in the Northern High Latitudes?
Abstract
Since the atmosphere is a weakly conducting medium, the potential difference between the ionosphere and the earth's surface drives a downward fair-weather current density Jz. Values of Jz at a given location depend on the global ionospheric potential Vi and the local columnar resistance, which is modulated by local ionization rates and aerosol content. According to Ohm's Law and Gauss' Law, space charge will be present at locations where Jz passes through conductivity gradients within clouds (e.g., at cloud boundaries and locations of gradients in droplet concentration). Monte Carlo simulations have shown that the presence of charges of like sign on cloud droplets and aerosols (as in regions of space charge) increases the rate of scavenging of large aerosols by cloud droplets, while simultaneously decreasing the rate of scavenging of the smaller aerosols. The predicted reduction in scavenging of smaller cloud condensation nuclei and increase in scavenging of large ones would result in smaller average droplet sizes, thus increasing the likelihood that these smaller droplets will be lifted upward to a cloud's freezing level before they can coalesce and precipitate as rain (Rosenfeld et al. 2008). The predicted increase in scavenging of larger aerosols, including ice-forming nuclei, by supercooled water droplets increases contact ice nucleation above the freezing level. Thus this "charge modulation of aerosol scavenging" should increase ice production within cyclonic clouds. Additional ice formation releases latent heat, thus warming the air within the cyclone's interior. As this warmer air rises, air from lower elevations flows inward, and conservation of angular momentum requires an increase in the cyclone's rotation rate, with a consequent increase in cyclonic vorticity. Thus, one might expect Jz to be positively correlated with cyclonic vorticity, particularly at northern high latitudes during the winter months. Jz values were measured at Mauna Loa Observatory (MLO) in Hawaii from 1960-61 and 1977-84. Because the columnar resistance above MLO varies little over a solar cycle and Vi is spatially uniform outside the magnetic polar caps, MLO Jz values should be proportional to Vi. Furthermore, MLO Jz values may also serve as proxies for high latitude values of Jz , provided that care is taken to exclude data measured during moderate-to-large Forbush decreases and heliospheric current sheet crossings (during periods of high stratospheric aerosol loading). MLO Jz data from six 5-month extended (northern hemisphere) winters were of sufficient quantity and quality to obtain proxies for daily average values of these high latitude Jz values, and these proxies were correlated with anomalies in the 500 hPa vorticity area index (VAI) for the 60-80° N latitude band. The resulting regression coefficients and R2 values were plotted as a function of lag. Strong signals (for lags close to zero days) were present after separating the data according to level of solar activity, and a moderately strong signal was present after combining the data from all six winters. This supports the theory outlined above.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMAE31A0255H
- Keywords:
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- 3304 ATMOSPHERIC PROCESSES / Atmospheric electricity;
- 3311 ATMOSPHERIC PROCESSES / Clouds and aerosols;
- 3329 ATMOSPHERIC PROCESSES / Mesoscale meteorology