Observations from the one year electric field Study-North Slope of Alaska (OYES-NSA) field campaign, and their implications for observing the distribution of global electrified cloud activity

Highlights

• Several unique Ez events were compared to simultaneous Micro-Pulse Lidar, Ka-band Radar and wind variables.

• A novel method for determining fair-weather electric fields was created utilizing the Lidar backscatter retrieval.

• Simultaneous fair-weather electric fields were analyzed at two sites on the yearly, monthly and sub-hourly time scales.

Abstract

For over a century, the electric Potential Gradient (PG) of the atmosphere has been measured and studied. The local vertical electric field (E_z) is strongly influenced by the presence of lightning, electrified clouds, rainfall, aerosols, and many others. The One Year Electric Field Study-North Slope of Alaska (OYES-NSA) field campaign was established in the summer of 2017 to measure the vertical electric field at the ARM site in Barrow, Alaska alongside a wide array of supplementary instrumentation, including a Micro-Pulse Lidar and upward facing Ka-band radar. Two years of observations (072017-062019) have shown the possibility to quantify the local effects from aerosols and clouds observed by the Lidar and Radar on the measured E_Z. Throughout the manuscript, the physics convention of negative downward fair-weather electric fields is used. Three cases (convective clouds, high concentration of near surface aerosols, and blowing snow) are used to demonstrate the localized effects on the measured E_Z. Utilizing the relationships between E_Z and backscatter/reflectivity, we have developed a methodology to distinguish samples with local influences. A fair-weather (FW) condition is determined to be associated with a low Lidar backscattering (less than 15 km⁻¹sr⁻¹), in the presence of no significant cloud activity (radar reflectivity less than −10dBZ). The samples satisfying these criteria are found with a 5-min averaged standard deviation of less than 15 V/m, and E_Z between −250 V/m to −50 V/m. Using only properties of the E_Z measurements allows for the simultaneous comparisons of FW at multiple sites, without the need for supplementary information of local weather conditions. Simultaneous E_Z measurements from 8 FW cases are shown between Barrow, AK and Corpus Christi, TX on the timescale of minutes to hours. Similar variation patterns in the FW E_Z are shown at both sites, providing evidence of the global nature of the atmospheric electric system. Furthermore, the seasonal-diurnal variability of FW at multiple sites shows similar distributions of the PG.

Introduction

Several atmospheric electricity variables such as lightning, thunder days, and the electric field are useful in monitoring the changing climate [Williams, 1992; Reeve & Toumi, 1999; Rycroft et al., 2000; Williams, 2005; Lavigne et al., 2019; among others]. In 2016, lightning was added for the first time to the Global Climate Observing System's (GCOS) list of Essential Climate Variables (ECVs). [Global Climate Observing System, 2016; Aich et al., 2018]. However, the major shortcoming in utilizing the lightning parameter to monitor the changing nature of global storms, is the relatively short time period of global coverage of lightning flash data [Christian et al., 1999; Aich et al., 2018]. Only several decades of optical imagers onboard satellites, as well as ground-based Very Low Frequency (VLF) networks are available on even the quasi-global scale. While these data are very useful for understanding the variability of lightning and thunderstorms at the diurnal, seasonal, and even inter-annual scales, they are not sufficient in understanding the longer-term trends (if any) that are occurring in thunderstorm activity during the past century [Christian et al., 1999; Rodger et al., 2006; Blakeslee et al., 2014a]. An interesting alternative is to instead monitor a global system that is largely driven by global thunderstorm and electrified cloud activity, that has a much longer data record. The Global Electric Circuit (GEC) of the atmosphere is a vast Earth system of electrical currents that are present between the Earth's surface and Ionosphere [Rycroft et al., 2008]. Even during fair-weather conditions, in the absence of significant clouds, aerosols, etc., a small current density of approximately 2 pA/m² is always present running from the Ionosphere in the upper atmosphere, down to the Earth's surface [Rycroft et al., 2000]. Dating back to the early 20th century, it was hypothesized that the temporal variability of this fair-weather electric field, was produced by the simultaneous temporal variability of the summation global thunderstorm activity [Wilson, 1909; Wilson, 1921]. Whipple, [1929], provided the first quantitative evidence of the link between thunderstorms and the fair-weather field by utilizing ship-borne vertical electric field data aboard the Carnegie cruise. The results indicated the maxima of both the GEC and thunder day area to occur at approximately 19 UTC, while the minima occurred at roughly 3 UTC. This diurnal cycle of the fair-weather electric field measured during this ground-breaking field work is now known as the classical Carnegie Curve [Harrison, 2013]. It remains the present-day view that the totality of thunderstorms around the globe at any given time, act as the main “battery” that continuously drives the GEC [Williams, 2009].

Throughout the next 90 years, many more details about the GEC have been revealed. With the addition of far greater amounts of electric field data measured around the globe, further details have been uncovered pertaining to the diurnal [Burns et al., 2005; Liu et al., 2010; Mach et al., 2011; Nicoll et al., 2019; among many others], seasonal [Adlerman and Williams, 1996; Liu et al., 2010; Burns et al. 2012; Blakeslee et al., 2014b; among many others], interannual [Burns et al., 2005; Williams and Mareev, 2014; Lavigne et al., 2017] and even decadal [Markson, 2007] variability of the GEC. In the past several decades, the addition of quasi-global radar measurements from space, as well as optical lightning imagers, have allowed for thunderstorms, and electrified precipitation systems to be analyzed in greater detail [Christian et al., 1999; Goodman et al., 2013]. The combination of the improvements in coverage of electric field measurements, as well as vast advancements in the global measurement of thunderstorm and cloud activity, has allowed for corroboration and extension of Wilson's and Whipple's findings.

The El Nino Southern Oscillation (ENSO) variability has also been observed in fair-weather electric field data [Hamid et al., 2001; Satori et al., 2009; Lavigne et al., 2017]. The regional increases and decreases in thunderstorm and electrified cloud occurrence on the ENSO time scales has been noted to also be simultaneously observed in the fair-weather electric field measured in Vostok Station, Antarctica [Lavigne et al., 2017]. For example, during the Southern Hemispheric summer months, an increase in both precipitation from thunderstorm and electrified clouds, as well as flash count was observed by the Tropical Rainfall Measurement Mission (TRMM) satellite during the hours of 16–24 UTC in La Nina periods. South America, which is known to be convectively active during this time, also observes an increase in thunderstorm and electrified clouds during these La Nina periods [Williams and Stanfill, 2002; Liu et al., 2010; Lavigne et al., 2017]. This increase in both electrified precipitation features, as well as the GEC during this time period, indicates that indeed the regional enhancement/suppression of thunderstorms as a result of ENSO can simultaneously be observed in the variation of GEC electric fields as well. This type of finding provides further evidence that the GEC is directly tied to the variability of global/regional thunderstorm and electrified cloud activity on a scale of natural climate variability (approximately 2–7 years). This allows for the next logical question to be asked; whether or not the GEC can monitor the longer-term climate variability over the past 100-years?

For all the progress that has been made on understanding and modelling the GEC of the atmosphere, there are still many unknowns pertaining to the smaller-scale contributing input parameters. Kalb et al., [2016] had some success at parameterizing storm conduction currents in the TRMM domain, and applying them to a global Earth model. However, the output models had a significantly smaller diurnal amplitude, and peaked approximately 4–6 h before the Carnegie curve. This could imply that there are several other factors not included in the budget that play an important role in driving the GEC system. As a general rule, thunderstorms and electrified showerclouds (defined as precipitation systems that produce significant charge separation but do not generate lightning) are the main driver of the GEC [Rycroft et al., 2007; Mach et al. 2009; Liu et al., 2010; Peterson et al. 2018]. However, it has been well established that many other physical processes contribute to the system. These include cosmic galactic rays, geomagnetic processes, energetic solar particles as well as many others [Tinsley, 2000; Siingh et al., 2007; Baumgaertner et al., 2013, among others]. In addition to these, many other localized processes are known to influence the local vertical electric field, such as aerosols, non-raining clouds, blowing dust and snow, fog, radon gas release, auroras, etc.

Several past studies have examined the influence of several of the above-mentioned localized influences on the measured vertical electric field. The typical magnitude of the physics convention fair-weather electric fields measured on the surface at sea level varies from approximately −100 to −200 V/m. It should be noted that throughout this manuscript, the physics convention of fair-weather electric fields will be used. Fair-weather electric fields will be represented as negative, and the potential gradient will be represented as positive values. Lucas et al., [2017] concluded using that during fog conditions, the Earth's electric field deviates from the background fair-weather electric field by roughly +150–200 V/m. The same study concluded that during an overcast day, the electric field varied by approximately −40 to −50 V/m from typical fair-weather values. This indicates that non-electrified clouds and fog contribute relatively weakly to the localized electric field. However, several studies have shown that blowing snow can cause a much larger influence on the electric field. Schmidt et al. [1999] concluded that surface electric field measurements during even a moderate blizzard can deviate the electric field on the order of +30,000 V/m. Model outputs conducted by Gordon and Taylor [2009] seem to corroborate this result, indicating that electric field magnitudes can exceed 25,000 V/m during surface blowing snow events. Chmielewski [2013] studied the influence of blowing dust on the surface vertical electric field in West Texas. The study found that a typical blowing dust event causes a +4000 to +5000 V/m effect on the electric field. However, during intense events, the effect can be as a large as +15,000 V/m based on case studies.

At high latitudes, snow cover may slow the release of radon from the ground which changes the conductivity of the near-surface atmosphere. Baumgaertner et al., [2013], found that direct natural radiation emitted from surface, as well as ground decay of radon gas, lead to approximately 10 ion pairs cm⁻³s⁻¹ over land between the latitudes of 60^oN-60^oS. In higher latitude regions where ground snow coverage is more prevalent, the rate was found to be reduced to half, creating a variation in the surface conductivity of up to 200% [Baumgaertner et al., 2013].

Furthermore, in high-latitude regions, aurorae are present. These solar wind disturbances can have intense effects on localized electric field measurements in polar regions. A case study, conducted by Olson [1971], concluded that during an incident of visual aurora near the measurement site, the surface electric field was disturbed on the order of 1000 V/m for several hours. During this time period, the sky was clear with no visible clouds indicating that the significant jump in the surface electric field was due to the solar event. The study further indicated that there are two main types of aurorae events: 1) events that produce negative E_z for approximately 30 min and then return to fair-weather magnitudes, and 2) events that more significantly shift the E_z towards negative values, and last on the order of several hours [Olson, 1971]. More recent studies on aurora influence such as Lucas et al., [2015], concluded that in arctic regions of the globe, the amplitude of magnetospheric perturbation can be as large as 50% of the GEC potentials, and can either constructively or destructively interfere. Reddell et al., [2004], conducted a magnetospheric correction due to the cross-cap potential of the vertical electric field. This diurnal correction was found to have the largest sinusoidal variability of +15 V/m at roughly 7 UTC and −25 V/m at approximately 21 UTC during periods of high magnetic activity. This correction factor was found to be in good agreement with several other past studies at high latitudes [Tinsley et al., 1998; Corney et al., 2003].

To address the mystery of the localized inputs to the electric field, as well as to build upon the understanding and possible practical uses of the global aspect of the GEC, a field campaign has been created in the unique location of the North American Arctic. The One-Year Electric Field Study-North Slope of Alaska (OYES-NSA) field campaign was established in June of 2017 at the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Northern Slope of Alaska (NSA) site. With the goal of understanding the contribution of the unique localized parameters in the region to the electric field, as well as to utilize the fair-weather electric field to monitor electrified cloud activity around the globe, this student-led field campaign was established in the northernmost town in the USA, Barrow, Alaska.

The North Slope of Alaska (NSA), provides a unique study site to monitor both local influences on the vertical electric field, as well as the fair-weather global component. The region observes unique Arctic cloud formations, which have shown at times to become significantly electrified. The North Slope of Alaska is an ideal location for measuring these electric field values, due to the stably stratified boundary layer that exists in the extremely cold temperatures [Burns et al., 2005]. In addition to the unique nature of the site (i.e. blowing snow, Arctic clouds), the site is also very well instrumented. The location has a co-located Ka-band radar, a Micro-Pulse Lidar (MPL), as well as much other supplementary meteorological information.

The unique vertical separation of two electric field meters (one at 2 m and the other at 5 m), allows for the investigation of local space charge concentration in the region. Marshall et al. [1999], studied the sunrise effect in the fair-weather electric field at Kennedy Space Center in Florida. Results from the study found that enhancement measured near sunrise was due to the upward mixing of the dense electrode layer very near the surface. The presence of two electric field mills separated by several meters can help determine if the conductivity changes above the electric field mills, or if local space charge is introduced such as in the cases that Marshall studied.

The site, which is one of the only electric field records in the Western Hemispheric Arctic, aims to shed light on the contribution of the unique local influences on the electric field budget, as well as to monitor the global component or GEC from the Arctic, in order to better understand the variability of global thunderstorms and electrified clouds at various timescales.