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The role of volcanic aerosols and relativistic electrons in modulating winter storm vorticity
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Advances in Space Research 50 (2012) 819–827 www.elsevier.com/locate/asr
The role of volcanic aerosols and relativistic electrons in modulating winter storm vorticity Brian A. Tinsley a,⇑, Limin Zhou b, Weiping Liu a b
a University of Texas at Dallas, WT15, Richardson, TX 75080, USA Key Laboratory of Geographic Information Science, Ministry of Education, East China Normal University, Shanghai, China
Available online 29 December 2011
Abstract Small changes in the vorticity of winter storms, responding to solar wind variations, are found in winters from 1957 to 2011, and are greater for winters with higher levels of stratospheric volcanic aerosols. Using 1993–2011 data, the response of the vorticity area index (VAI) is shown to be of larger amplitude when the days of minima in the relativistic electron flux (REF) precipitating from the radiation belts are used, instead of heliospheric current sheet (HCS) crossings, as key days in superposed epoch analyses. The HCS crossings mostly occur within a few days of the REF minima. The VAI is an objective measure of the area of high cyclonic vorticity, and for the present work is derived from ERA-40 and ERA-Interim reanalyses of global meteorological data. The VAI dependencies on the stratospheric aerosol content (SAC) and the REF are consistent with a model in which the ionosphere-earth current density (Jz) affects cloud microphysics. One of the ways in which Jz is modulated is by changes in stratospheric column resistance (S), which is increased by stratospheric aerosols. Because S is in series with the tropospheric column resistance (T), Jz modulation by REF requires that S be not negligible with respect to T. So the Jz modulation and the VAI response appear when the SAC is very high, or the REF reductions (which also increase S) are very deep, and when the product of the SAC and the reciprocal of the REF exceeds a threshold value dependent on T. Ó 2011 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Atmospheric electricity; Relativistic electrons; Stratospheric aerosols; Storm vorticity
1. Introduction 1.1. VAI responses to HCS crossings and relativistic electron flux It was discovered by Wilcox et al. (1973) and confirmed by Hines and Halevy (1977) that for the years 1964–1970 there was a small reduction in the vorticity of northern hemisphere winter storms, measured by the vorticity area index, VAI, at times that solar wind magnetic sector boundaries (Wilcox and Ness, 1965) crossed over the earth. The sector boundary crossings are now known as heliospheric current sheet, or HCS, crossings. The VAI is defined by the area of the atmosphere (in units of 105 km2) covered by values of absolute vorticity above a ⇑ Corresponding author.
E-mail address: [email protected] (B.A. Tinsley).
threshold of 20 105 s1, plus the area for vorticity above 24 105 s1. It was defined by Roberts and Olson (1973), for extended winters, November through March, for latitudes poleward of 20°N, and for 300 h Pa levels in the atmosphere, but can be evaluated for any latitude range; for any pressure level; with any threshold vorticity, and for any months. Systematic changes in the strength of cyclogenesis, measured by the VAI in a given hemisphere, affect the amplitude of Rossby waves, and these changes in atmospheric circulation, downstream of a cyclogenesis center, can affect regional climate. However, the significance of the present study for meteorology is that it provides evidence for a new physical mechanism, linking atmospheric electricity through cloud microphysics to atmospheric dynamics (see Section 1.4). It was shown by Tinsley et al. (1994) and Kirkland et al. (1996) that a link between the solar wind and the VAI could be provided by the relativistic electron flux (REF),
0273-1177/$36.00 Ó 2011 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2011.12.019
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of energy >2 Mev, that because of their quasi-trapped condition in the radiation belts were continually precipitating into the atmosphere, at rates which depend strongly on solar wind parameters. Because there is an association between minima in the REF and HCS crossings, minima in the VAI are associated with minima in the REF. The association of the VAI and HCS crossings and minima in the REF was evident at times of high concentrations of stratospheric aerosols, however, Prikryl et al. (2009a) have questioned whether the association depends on stratospheric aerosols, and have suggested a different mechanism that does not involve the REF link. Here we analyze 1957– 2011 observations with an improved VAI time series to examine the relationship to the stratospheric aerosol content (SAC). HCS crossings can deviate by up to several days from REF minima, and so it was suspected that if the REF was a physical link between the solar wind and the atmosphere, the VAI minima would be more easily detectable and sharply focused when the REF minima, rather than HCS crossings, were used as key days in superposed epoch analyses. This has been shown for 1997–2002 winters in the northern hemisphere for the 500 h Pa VAI by Mironova et al., 2011, using data from the GOES satellites at geosynchronous orbit. This was a period of relatively low average REF levels (see Reeves et al., 2011, Fig. 2). Here we extend that work from 1993 to 2011 using similar data, primarily from LANL satellite measurements (Reeves et al., 2011, and auxiliary material), but supplemented by GOES measurements. 1.2. Solar wind – Jz link It was pointed out by Tinsley et al. (1994) that REF penetration to upper stratospheric levels, with the Bremsstrahlung radiation that they produce which penetrates to lower stratospheric levels, increases ion production and conductivity, so that decreases in the REF would increase the stratospheric column electrical resistance. The consequent changes in the ionosphere-earth current density (Jz) that flows as the downward return current in the global electric circuit was considered to be the physical link to the tropospheric dynamical changes. The global electric circuit is constituted by an upward flow of about 1000 A to the highly conducting ionosphere from global thunderstorms and other highly electrified clouds, with the current spreading out and flowing downward over the whole globe with a current density (Jz) of a few picoamperes per square meter (Chalmers, 1949; Tinsley and Zhou, 2006; Rycroft et al., 2008). The value of Jz can be expressed by the Ohm’s Law relation (e.g., Tinsley and Zhou, 2006) as J z ¼ V i =ðT þ SÞ
ð1Þ
where Vi is the ionospheric potential with respect to Earth’s surface (ocean and land), T is the tropospheric column resistance, and S is the stratospheric column resistance. Markson (1976) and Mu¨hleisen (1977) estimated that S
was only 10% of T in the normal atmosphere, evidently without consideration of stratospheric aerosols, and the model of Hays and Roble, 1979 for an aerosol-free atmosphere also showed S small compared to T. In that situation decreases in S due to ionization by relativistic electrons, although important in the stratosphere, have negligible effect on Jz, by Eq. (1). However, stratospheric balloon conductivity measurements by Byrne et al. (1988) and Hu and Holzworth (1996) found values several times larger than calculated by for an aerosol-free atmosphere. Also, following a large volcanic eruption in Chile, Hoffman et al. (1985) found a large increase in aerosol concentration in the Antarctic stratosphere. The hypothesis of Tinsley et al. (1994) as developed by Kirkland et al. (1996), Tinsley (2005), and Tinsley and Zhou (2006) is that for several years following volcanic eruptions that inject large quantities of SO2 gas into the stratosphere, that form H2SO4 on a timescale of months, there is a production of ultrafine aerosol particles from the gaseous H2SO4 (Goodman et al., 1994). When carried into the higher temperatures of the upper stratosphere by the Brewer–Dobson circulation, the aerosol particles formed in the lower stratosphere evaporate and/or are dissociated by radiation and become gaseous but re-condense to form ultrafine aerosol particles as the air descends and diabatically cools in the downward branches of the circulation. The hypothesis is that the large concentrations of ultrafine aerosol particles at mid-high latitudes lower the stratospheric conductivity and increase S in those regions where precipitation of REF takes place. The values of S are estimated (Tinsley and Zhou, 2006) to be high enough compared with T so that by Eq. (1) Jz fluctuates as the REF fluctuates, especially at low values of the REF. Minima in the solar wind speed (SWS) as well as deep minima in the REF are associated with the HCS crossings, as shown by Tinsley et al. (1994, Fig. 5). The strong correlation between the REF and the SWS has been explored by Li et al. (2001a,b). Observations of the reductions of tropospheric potential gradient at HCS crossings, consistent with reductions in Jz, have been reported by Fischer and Mu¨hleisen (1980). Tinsley et al. (1994) showed that such reductions for a few years following the 1982 El Chicon volcanic eruption were greater than before or after. 1.3. Other atmospheric dynamics responses to Jz The hypothesized link between the reductions in Jz and changes in the vorticity of winter storms also explains reductions in the VAI associated with Forbush decreases of the galactic cosmic ray (GCR) flux. Tinsley and Deen (1991) had shown that small VAI reductions agreed in onset time (within a day) and duration with the Forbush decreases, and the amplitude of the response was proportional to the amplitude of the Forbush decrease. The variations of Jz due to Forbush decreases and solar cycle changes in the GCR are due to changes in tropospheric ion production and changes in T (Eq. (1)), and are several
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times greater at high latitudes, which is where the VAI changes are greatest, than at low latitudes. Also small changes in the northern hemisphere VAI, associated with day-to-day changes in Jz measured at Mauna Loa, Hawaii, that are due to varying upward current output of global thunderstorms (affecting Vi in Eq. (1)), have been found by Hebert et al. (submitted for publication). Responses of surface pressure in the Antarctic and Arctic to Jz changes have been found, with the Jz changes inferred from satellite measurements of local Vi changes due to solar wind inputs (Burns et al., 2007) or from ground based measurements of Ez changes due to changes in global thunderstorm current output (Burns et al., 2008). Ez is the near-surface potential gradient proportional to Jz and was measured at Vostok, Antarctica. Increases in atmospheric vorticity in response to solar energetic particle events, which increase Jz, have been reported by Veretenenko and Thejll (2004, 2005). Responses of cloud cover to HCS crossings have been found by Kniveton and Tinsley (2004) and to Forbush decreases by Veretenenko and Pudovkin (1997), and responses of global cloud cover to global Jz changes inferred from the Ez measurements at Vostok have been found by Kniveton et al. (2008). 1.4. The Jz – cloud microphysics link The mechanism for responses of winter storm vorticity and surface pressure and cloud cover to Jz is hypothesized to involve changes in nucleation processes in clouds. Tinsley and Deen, 1991, suggested electric charge-induced increases in the efficiency of ice nucleation processes, and Tinsley et al. (2000) focused on increases in the rate at which supercooled droplets scavenge charged ice-forming nuclei (electro-scavenging causing contact ice nucleation). More recently, electric charge effects on size-dependent in-cloud scavenging affecting the concentration and size distribution of cloud condensation nuclei (CCN), which in later episodes of cloud formation affects cloud droplet size distributions, has been discussed and modeled by Tinsley (2004, 2010). Electro-anti-scavenging increases in the concentration of small CCN, and the cumulative effect over a day or so in an air mass, in further cloud formation such as in the updrafts of cyclones, increases the number concentration and reduces the size of small droplets. Such CCN concentration changes have been shown to delay the onset of coalescence processes that would precipitate liquid water in convective storms before it is carried above the freezing level by updrafts (Rosenfeld et al., 2008). In addition, electro-scavenging reduces the concentration of large and giant CCN, which subsequently reduces the concentration of large droplets, which also reduces the rate of coagulation processes. It is the extra water, not precipitated, available for freezing that releases extra latent heat that invigorates updrafts. The invigorated updraft carries up even more water, and eventually increases total precipitation. Contact ice nucleation rates above the freezing level might also increase with Jz, and further increase the
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updraft speed. In the context of winter storms with significant latent heat release, associated with eddies in the winter circulation, the enhanced updrafts lead to increase in vorticity, tapping into the unstable baroclinic pressure and zonal wind shear gradients, completing a set of links between the solar wind, the REF, Jz and the VAI. 1.5. Alternate mechanism for VAI responses at HCS crossings The analysis of Prikryl et al. (2009a) showed that the reductions in the VAI at HCS crossings can be found in data for extended winters (May–September) in the southern hemisphere, as well as in the Northern hemisphere (November–March) and also for most years without high SAC. However, Prikryl et al. (2009b) sought to explain the VAI responding to the solar wind in terms of a completely different mechanism – one involving the generation of gravity waves by auroral energy deposition in the lower thermosphere near times of HCS crossings, with the gravity waves propagating down through the mesosphere and stratosphere to trigger instabilities in updrafts in winter storms. Whereas previous work (and the present work) has focused on short-term decreases in the VAI, Prikryl et al. (2009a,b) focused on the short-term increases, which follow the decreases. However, atmospheric dynamics has been shown to respond to solar wind changes causing Jz changes when no auroral energy input is occurring. As mentioned in Section 1.3, there are the polar surface pressure responses to changes in the interplanetary magnetic field By component that modulates local polar ionospheric potential Vi and thus local Jz (Burns et al., 2007), and the several atmospheric responses noted earlier that are associated with changes in global Vi due to changes in global thunderstorm current output: these include changes in polar surface pressure (Burns et al., 2008); changes in global cloud cover (Kniveton et al., 2008) and of changes in the VAI (Hebert et al., submitted for publication). These effects cannot be caused by the auroral gravity wave mechanism, but are consistent with the Jz/cloud microphysics mechanism. In this work we will use 300 h Pa and 500 h Pa VAI time series derived from the ERA-40 gridded meteorological data set (Uppala et al., 2005), its extension (ERA-Interim) with improved resolution (Dee et al., 2011). We will examine the response to HCS crossings in non-volcanic as well as volcanic winters for both VAI time series, and evaluate the response to minima in the REF, and estimate the changes in amplitude of the response with both changing levels of stratospheric volcanic aerosol and changing REF. The VAI time series are calculated from the ERA-40 data from 1957 to 2002, and scaled to match the ERA-Interim data from 1989 to 2010 by fitting in the period of overlap. The times of HCS crossings are from the list by Svalgaard (2011), and the REF time series are from satellite measurements at geosynchronous orbit. The LANL flux (in units of electrons (1.8–3.5 MeV) cm2 s1 sr1 kev1) was used
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as the primary time series to determine minima in the REF, but to fill data gaps use has been made of GOES (2011) measurements scaled to the same units. 2. Results Fig. 1 is a set of superposed epoch analyses of the responses to HCS crossings of the 500 h Pa VAI (top and third rows) and the 300 h Pa VAI (second and bottom rows) for 8 groupings of northern hemisphere extended winters (November through March) between 1957 and 1995. The latitude range is 20–80°N, and the groups are designated by the year of the beginning November and ending March, e.g., 57–63 is for the November through March months for the 6 winters between November 1957 and March 1963. The winters during periods of high SAC, i.e., 1963–1966 (Agung), 1982–1986 (El Chicon), and 1993–1995 (Pinatubo) show the minima in the VAI within a day or two of the HCS crossings, as observed in previous work, and more clearly at 300 h Pa than at 500 h Pa. Only at 500 h Pa for 1993–1995 is the minimum for these high ‘volcanic’ years not well defined. Several of the superposed epoch analyses for all the remaining ‘medium- or low-volcanic’ years also show some minima near HCS crossings, but with amplitude comparable to or smaller than the noise level. In 1957–1963, when there was a very low SAC (Meyerott et al., 1983, their Fig. 4), no response to the HCS crossings is apparent. Note that the mean level of the 300 h Pa VAI is about twice that of the 500 h Pa VAI, and that there are different offsets in many of the panels to accommodate the variation from year to year of the mean levels. Fig. 2 shows data for 1993–2011 when satellite data on relativistic electron flux is available. The top row of panels is the 300 h Pa VAI response to key days being HCS crossings (top row); the second row is the VAI response to the key days being minima in the solar wind speed, with minimum speed less than 370 km s1; the third row is the VAI response to the key days being minima in the REF of energy > 2 MeV measured at geosynchronous orbit, with the minimum flux being less than either 0.1 cm2 s1 sr1 kev1 or 0.2 cm2 s1 sr1 kev1; and the bottom row is the LANL REF response, with key days being the REF minima. The 1993–1995 HCS result is the same as in Fig. 1. Responses to HCS crossings are not apparent for the low volcanic years 1995–2007, and responses of the VAI to SWS minima are generally not evident. However, a clear response to REF minima can be seen in the third row in the high ‘volcanic’ years 1993– 1995, with smaller responses in the ‘low-volcanic’ periods 1995–1999, 1999–2003, 2003–2007. We discuss the 2007– 2011 response below. The second row has been included because minima in the REF tend to occur near minima in the SWS. However, the REF minima are intrinsically better than the SWS minima as markers of the solar influence, presuming the REF is the effective physical link, because the REF minima can be sorted by the absolute value of the minimum flux, which depends on the prior his-
tory of the REF energization processes, whereas this is not possible with the SWS minima. In the bottom row the first panel shows that in 1993– 1995 the average REF was high, and also that the average level of its minima was high, compared to the following years. In order to retain enough key days to bring out the signal relative to the noise, it was necessary to select minima for 1993–1995 where the flux level was less than 0.2 cm2 s1 sr1 kev1 i.e., set the upper limit a factor of two higher than the 0.1 cm2 s1sr1 kev1 used for the remaining four panels. For 2007–2011 there is no VAI minimum at days 1 and 2 as in the previous periods, but for days 3–5 there is a persistent minimum which is present with HCS crossings, SWS minima and REF minima as the key days. (It is also persistent when individual winters are examined). We have no explanation for this result, but note that 2007–2011 was an exceptionally low solar activity period with low average REF. Fig. 3 is in the same format as for Fig. 2, except it is for 500 h Pa pressure level. The VAI response to the 1993– 1995 HCS crossings, in the first panel in the top row, is the same as in Fig 1, being weaker than that at 300 h Pa. In the remaining panels of the first row, a VAI response to the HCS crossings is not evident. In none of the panels of the second row is a response to minima in the SWS evident. However, following the moderate VAI response to REF in the first panel of the third row for the 1993–1995 high volcanic period, there is a weak response in the panels for 1995–1999, 1999–2003, and 2003–2007. The response for 2007–2011 is negligible on day 1, but the dip for days 3–5 found for 300 h Pa is also apparent here, for HCS crossings, SWS minima and the REF minima. Overall, the results of Fig. 3 for the 500 h Pa VAI are similar to the results in Fig. 2 for the 300 h Pa VAI. To improve the signal/noise for evaluating the effect of the SAC, Fig. 4 shows in the first panels of the upper (for 500 h Pa) row and lower (for 300 h Pa) row all 262 ‘medium–low volcanic’ events up to 1993 where the key days are HCS crossings. The second column of panels shows all the 111 high ‘volcanic’ events combined, and the third column of panels is for the low volcanic years of 1995–2007, where the key days are minima in the REF. While a precise comparison is not meaningful, the results are consistent with the hypothesis that the level of response depends on the SAC. The response for the ‘medium’ and ‘low’ volcanic’ years is about half that for the ‘volcanic’ years, and the 1957–1963 result that shows no evidence for response with a very clean stratosphere is also consistent with the hypothesis. 3. Discussion 3.1. Effects of stratospheric aerosols The largest responses of the 500 h Pa and 300 h Pa VAI at HCS crossings was found for 1963–1966, which comes
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Fig. 1. Superposed epoch analyses of VAI responses to HCS crossings (day 0), in winters (November–March) for 1957–1995. The first and second rows are for the 500 h Pa and 300 h Pa VAI, respectively, for winters 1957–1963 (‘non-volcanic’); 1963–1966 (‘high volcanic’); 1966–1970 (‘medium-volcanic’); and 1970–1976 (‘low-volcanic’). The third and fourth rows are for 500 h Pa and 300 h Pa, respectively, for winters 1976–1982 (‘low-volcanic’); 1982–1986 (‘high-volcanic’); 1986–1993 (‘low-volcanic’): and 1993–1995 (‘high-volcanic’). The VAI values were calculated for the latitude range 20–80°N.
immediately after the evident lack of response in 1957– 1963, with the very low levels of SAC prior to the eruption of Mt. Agung. The decrease of amplitude of the effect during the next one or two decades is associated with the slow clearing out of the stratosphere, delayed by some small further eruptions (Taal and Redoubt, 1965; Fernadina, 1968; Helka, 1970). This decadal-long decay parallels the yearby-year decay of the temperature decrease at HCS crossings (3°C in 1965–1967 winters) in the Arctic at 500 h Pa found by Misumi (1983) and reviewed by Tinsley (2000). At 300 h Pa the next largest response to the 1963–1966 HCS crossings occurred in 1982–1986, following the eruption of El Chicon, with a comparable response in 1993– 1995, following the eruption of Pinatubo, with responses weak or absent in 1986-1993. These responses are similar to those shown in Kirkland et al., 1996, Fig. 2) for a 500 h Pa VAI time series derived from the National Mete-
orological Center (NMC) gridded data. The present work uses 300 h Pa and 500 h Pa time series derived from the ERA-40 and ERA-Interim gridded data sets as noted. While the NMC and ERA data sets are broadly similar, they differ in details, especially for small values of the VAI derived from them. This is evidently due to different assimilative models into which the intermittent raw meteorological data is incorporated. In one sense our results agree with those of Prikryl et al. (2009a) who showed, using a VAI time series derived from the ERA-40 data, that significant VAI responses at HCS crossings could be found for ‘non-volcanic’ winters. However, our results do not support their result that the amplitude of the response in ‘non-volcanic’ winters is comparable to that in ‘volcanic’ winters, as required by their alternate mechanism (Prikryl et al., 2009b) for tropospheric vorticity responses to solar wind changes. We note
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Fig. 2. Superposed epoch analyses for 300 h Pa VAI for winters 1993–2011, where key days in the top row are HCS crossings; in the second row are minima in the solar wind speed; and in the third row are minima in the relativistic electron flux. In the fourth row the key days are the same REF minima as in the third row, but the time series analyzed is the LANL REF (without GOES supplement). The labels for volcanic activity for each column are given at the top.
that only about 60% of the dates of HCS crossings they used (Prikryl et al., 2009a, Table 1) are the same as those of Svalgaard (2011) that were used for HCS crossings in the present work. According to Prikryl (personal communication, 2011) the reason for the changes was that the dates were adjusted to better align with the solar wind variations as judged by spacecraft measurements of interplanetary magnetic fields; solar wind speed variations, coronal holes, and optical measurements of the solar corona, with possible or observed dates of coronal mass ejections removed. Comparing the lists, it can be seen that there were a greater percentage of differences with Svalgaard’s list in ‘non-volcanic’ winters than in ‘volcanic’ winters, and this may partly explain the differences between the magnitude of the VAI responses of Prikryl et al. (2009a) in ‘non-volcanic’ winters as compared to the present results. Prikryl et al. (2009a) found that the VAI maxima immediately following the minima tended to be higher than the maxima immediately preceding them. This is apparent in Fig. 4 for 500 h Pa for the first and second panels in the top row, but not for the others. This could be related to the REF maxima following the minima being somewhat greater than the preceding maxima, as seen in the bottom rows of Figs. 2 and 3, but meteorological noise or physical processes in the dynamic recovery are other possibilities.
3.2. Use of REF minima preferable to HCS crossings Comparing the VAI responses to 300 h Pa HCS crossings in the top row of Fig. 2 with the VAI responses to REF minima in the third row of Fig. 2, we see that for 1995–1999, 1999–2003, and 2003–2007 there are VAI minima at day 0 or day 1 when REF minima are used as key days, but these are absent when HCS crossings are used as key days. For 2007–2011 in neither case is there a response. Again, in Fig. 3 for the 500 h Pa VAI, the use of REF minima as key days in the third row results in VAI minima on day 0 or day 1, with even a marginal VAI minimum for 2007–2011, but smaller or absent responses when HCS crossings are used as key days. These results support the hypothesis that the REF is a physical link between the space environment and the stratosphere, which, depending on the level of stratospheric aerosols, modulates Jz and ultimately affects cloud microphysics. 3.3. Threshold for the response The relationship expressed by Eq. (1) require that for the REF to modulate Jz the change in stratospheric column resistance S must be an appreciable fraction of the tropospheric column resistance T. This can be achieved with a
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Fig. 3. As for Fig. 2, but for the 500 h Pa VAI in the first three rows. The fourth row is the same as in Fig. 2.
Fig. 4. Combined superposed epoch analyses. The top row of panels is for 500 h Pa, and the bottom for 300 h Pa, with first column the combined analyses for medium-low volcanic winters 1966–1993 using HCS crossings as key days. The second column is for all high volcanic winters using HCS crossings as key days, and the third column is for the low volcanic years 1995–2007 using REF minima as key days.
high minimum level of the REF provided there is a very high SAC, as in 1993–1995 following the Pinatubo eruption (see Fig. 3, first panel on the bottom row). Alternatively, it could be achieved with a generally lower level near minimum for the REF with a lower SAC (as in the second and third panels of the bottom row of Fig. 3). A rule of thumb would be that S, at the REF minimum,
depends on the product of the SAC and the reciprocal of the REF, and that the amplitude of the day-to-day VAI response depends on the amplitude of the REF change, provided that S is non-negligible with respect to T. Thus, only when the ratio of the SAC to the minimum REF exceeds a threshold value, dependent on T, would there be the opportunity for modulation of Jz in this way,
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leading to the vorticity responses of the Wilcox effect. A more sophisticated analysis than this rule of thumb would take into account the altitude, latitude, and size distributions of the ultrafine aerosol particles, and the latitudinal and energy distributions of the REF. 3.4. Use of REF data for the 1960s through 1980s Given that the REF minima are better markers of the solar wind influence than HCS crossings, it should be possible to obtain data on REF levels at geosynchronous orbit, or at other altitudes, from the first observations in the 1960s through the 1980s, and process the data to ensure uniformity of flux levels, and extend the analysis to the earlier period. For future research, the optimum marker would be measurements of Jz, or simultaneous measurements of conductivity and electric field, at high tropospheric altitudes several times a day, at latitudes where the precipitation of relativistic electrons is occurring. An alternative would be measurements from satellites of upward fluxes of Bremsstrahlung, and/or REF spectra, together with solar occultation SAC measurements. 3.5. Use of forecasted VAI to improve signal/noise The VAI variations are seen in a background of meteorological ‘noise’ which is actually the ‘weather’ or internal variability. It was shown by Larsen and Kelley (1977) that forecasts of the 500 h Pa VAI made a day or two prior to an HCS crossing using the North American Fine Mesh meteorological grid data had a 20% lower correlation with the actual VAI than at other times, and this was for moderate to low SAC levels. This means that the HCS-related component of vorticity was superimposed on the forecasted vorticity, thus degrading the forecast success. Consequently, forecasting the meteorological vorticity and subtracting it from the actual VAI would seem likely to improve the signal/noise for the analysis of VAI responses to external forcing. This applies to responses to REF changes; Forbush decreases of the cosmic ray flux; solar energetic particles; Vi changes in the polar caps due to the solar wind, or to changes in thunderstorm output. In general, any day-to-day influence on the atmosphere that is not included in the physical processes in the assimilative models would be partly, but not completely assimilated into the model, and a further improvement in signal/noise might be obtained by analyzing perturbations of raw data from a forecasted output, rather than using the actual output of the assimilative model. 4. Conclusions This work has strongly supported the earlier published result that the amplitude of the response of the VAI to HCS crossings, where the dates of the crossings were determined independently of the analysis, depends strongly on the stratospheric aerosol content, contrary to the finding
of Prikryl et al. (2009a). It has shown that the VAI response is greater when minima in the relativistic electron flux (>2 MeV, measured at geosynchronous orbit) are used as key days in the superposed epoch analysis, instead of HCS crossings, consistent with an hypothesis that the REF is a physical link between the space environment and the stratosphere, where it modulates the stratospheric column resistance, and modulates the ionosphere-earth current density Jz to an extent depending on the stratospheric ultrafine aerosol content. The signal/noise of the analyses could be substantially improved by evaluating differences between a forecast of the VAI and the actual VAI. The phenomenon of the VAI response to REF changes is only one of a number of similar atmospheric responses to Jz changes, that are not included in current atmospheric models, and appears to be due to effects of electrical charges on the microphysics of clouds and cyclonic storms. Acknowledgements This work was supported by NSF Grants AGS 0855351 and AGS 0836171 to the University of Texas at Dallas, and by Chinese Fundamental Research Funds for the Central Universities, and Funds of the Shanghai Committee for Science and Technology, China (10dz0581600, 10dz1200703). We are appreciative of the ECMWF in making available the ERA-40 and ERA-Interim data. References Burns, G.B., Tinsley, B.A., Frank-Kamenetsky, A.V., Bering, E.A. Interplanetary magnetic field and atmospheric electric circuit influences on ground-level pressure at Vostok. J. Geophys. Res. 112, 1–10, D04103, 2007. Burns, G.B., Tinsley, B.A., French, W.J.R., Troshichev, O.A., FrankKamenetsky, A.V. Atmospheric circuit influences on ground-level pressure in the Antarctic and Arctic. J. Geophys. Res. 113, D15112, 2008. Byrne, G.J., Benbrook, J.R., Bering, E.A., Oro´, D., Seubert, C.O., Sheldon, W.R. Observations of the stratospheric conductivity and its variations at three latitudes. J. Geophys. Res. 93, 3879–3891, 1988. Chalmers, J.A. Atmospheric Electricity, first ed Oxford, Clarendon, 1949. Dee, D.P., Uppala, S.M., Simmons, A.J., et al. The ERA-Interim reanalysis, configuration and performance of the data assimilation system. Q. J. R. Meteorol. Soc. 137, 553–597, 2011. GOES. Energetic electron flux from the NOAA Space Weather Prediction site., 2011. Goodman, J., Snetsinger, K.G., Pueschel, R.F., Ferry, G.V. Evolution of Pinatubo aerosol near 19 km altitude over western North America. Geophys. Res. Lett. 21 (12), 1129–1132, 1994. Fischer, H.-J., Mu¨hleisen, R. The ionospheric potential and the solar magnetic sector boundary crossings. Rep. Astron. Inst., D-7980, Univ. Tu¨bingen, Ravensburg, Germany, 1980. Hays, P.B., Roble, R.G. A quasi-static model of global atmospheric electricity: 1. The lower atmosphere. J. Geophys. Res. 84, 3291–3305, 1979. Hebert, L. III, Tinsley, B.A., Zhou, L. Global electric circuit modulation of winter cyclone vorticity in the northern high latitudes. Adv. Space Res. 56, 55–56, submitted for publication. Hines, C.O., Halevy, I. On the reality and nature of a certain sun-weather correlation. J. Atmos. Sci. 34, 382–404, 1977.
B.A. Tinsley et al. / Advances in Space Research 50 (2012) 819–827 Hoffman, D.J., Rosen, J.M., Gringel, W. Delayed production of sulfuric acid condensation nuclei in the polar stratosphere from El chicon volcanic vapors. J. Geophys. Res. 90 (D1), 2341–2354, 1985. Hu, H., Holzworth, R.H. Observations and parameterization of the stratospheric electrical conductivity. J. Geophys. Res. 101 (D23), 29535–29552, 1996. Kirkland, M.W., Tinsley, B.A., Hoeksema, J.T. Are stratospheric aerosols the missing link between tropospheric vorticity and Earth transits of the heliospheric current sheet? J. Geophys. Res. 101 (D23), 29689– 29699, 1996. Kniveton, D.R., Tinsley, B.A. Daily changes in global cloud cover and Earth transits of the heliospheric current sheet. J. Geophys. Res. 109, D11201, 2004. Kniveton, D.R., Tinsley, B.A., Burns, G.B., Bering, E.A., Troshichev, O.A. Variations in global cloud cover and the fair-weather vertical electric field. J. Atmos. Solar-Terr. Phys. 70, 1633–1642, 2008. Larsen, M.F., Kelley, M.C. A study of an observed and forecasted meteorological index and its relationship to the interplanetary magnetic field. Geophys. Res. Lett. 4, 337–340, 1977. Li, X., Temerin, M., Baker, D.N., Reeves, G.D., Larsen, D. Quantitative prediction of radiation belt electrons at geostationary orbit based on solar wind measurements. Geophys. Res. Lett. 28, 1887–1890, 2001a. Li, X., Baker, D.N., Kanekal, S.G., Looper, M., Temerin, M. Long term measurements of radiation belts by SAMPEX and their variations. Geophys. Res. Lett. 28, 3827–3830, 2001b. Markson, R. Ionospheric potential variations obtained from aircraft measurements of potential gradient. J. Geophys. Res. 81, 1980–1990, 1976. Misumi, Y. The tropospheric response to the passage of solar sector boundaries. J. Meteorol. Soc. Japan 61, 686–694, 1983. Meyerott, R.E., Reagan, J.B., Evans, J.E. On the correlation between ionospheric potential and the intensity of cosmic rays, in: McCormac, B.M. (Ed.), Weather and Climate Responses to Solar Variations. Colorado Assoc. Univ. press, Boulder, Colorado, pp. 449–460, 1983. Mu¨hleisen, R. The global electric circuit and its parameters, in: Dolezalek, H., Reiter, R. (Eds.), Electrical Processes in Atmospheres. Steinkopft, Darmstadt, pp. 467–476, 1977. Mironova, I., Tinsley, B.A., Zhou, L. The links between atmospheric vorticity, radiation belt electrons and the solar wind. Adv. Space. Res., in press. Prikryl, P., Rusˇin, V., Rybansky, M. The influence of solar wind on extratropical cyclones – Part I: Wilcox effect revisited. Ann. Geophys. 27, 1–30, 2009a. Prikryl, P., Muldrew, D.B., Sofko, G.J. The influence of solar wind on extratropical cyclones –Part II: A link mediated by auroral gravity waves. Ann. Geophys. 27, 31–57, 2009b. Reeves, G.D., Morley, S.K., Friedel, R.H.W., et al. On the relationship between relativistic electron flux and solar wind velocity: Paulikas and Blake revisited. J. Geophys. Res. 116, A02213, and auxiliary material, 2011. Roberts, W.O., Olson, R.H. Geomagnetic storms and wintertime 300-mb trough development in the North Pacific–North America area. J. Atmos. Sci. 30, 135–140, 1973.
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Rosenfeld, D., Lohman, U., Raga, G.B., O’Dowd, C.D., Kumala, M., Fuzzi, S., Reissell, A., Andrease, M.O. Flood or drought: how do aerosols affect precipitation. Science 321, 1309–1313, 2008. Rycroft, M.J., Harrison, R.G., Nicoll, K.A., Mareev, E.A. An overview of Earth’s global electric circuit and atmospheric conductivity. Space Sci. Rev. 137, 83–105, 2008. Svalgaard, L. Sector Boundary List 1926–Present, Leif Svalgaard’s Research Page., 2011. Tinsley, B.A. Influence of the solar wind on the global electric circuit, and inferred effects on cloud microphysics, temperature, and dynamics in the troposphere. Space Sci. Rev. 94, 231–258, 2000. Tinsley, B.A. Scavenging of condensation nuclei in clouds: dependence of sign of electroscavenging effect on droplet and CCN sizes, in: Extended Abstracts, 14th Conference on Cloud Physics and Precipitation, ICCP, Bologna 18–23 July 2004, pp. 248–252, 2004. Tinsley, B.A. On the variability of the stratospheric column resistance in the global electric circuit. Atmos. Res. 76, 78–94, 2005. Tinsley, B.A. Electric charge modulation of aerosol scavenging in clouds: rate coefficients with Monte-Carlo simulation of diffusion. J. Geophys. Res. 115, D23211, doi:10.1029/2010JD014580, 2010. Tinsley, B.A., Deen, G.W. Apparent tropospheric response to MeV-GeV particle flux variations, a connection via electrofreezing of supercooled water in high level clouds? J. Geophys. Res. 96, 2283–2296, 1991. Tinsley, B.A., Zhou, L. Initial results of a global circuit model with stratospheric and tropospheric aerosols. J. Geophys. Res. 111, D16205, 2006. Tinsley, B.A., Hoeksema, J.T., Baker, D.N. Stratospheric volcanic aerosols and changes in air-earth current density at solar wind magnetic sector boundaries as conditions for the Wilcox tropospheric vorticity effect. J. Geophys. Res. 99, 16,805–16,813, 1994. Tinsley, B.A., Rohrbaugh, R.P., Hei, M., Beard, K.V. Effects of image charges on the scavenging of aerosol particles by cloud droplets and on droplet charging and possible ice nucleation processes. J. Atmos. Sci. 57, 2118–2134, 2000. Uppala, S.M., Ka˚llberg, P.W., Simmons, A.J., et al. The ERA-40 reanalysis. Q. J. Roy. Meteorol. Soc. 131, 2961–3012, 2005. Veretenenko, S.V., Pudovkin, M.I. Effects of galactic cosmic ray variations on the solar radiation input in the lower atmosphere. J. Atmos. Terr. Phys. 59, 1739–1746, 1997. Veretenenko, S., Thejll, P. Effects of energetic solar protons events on the cyclone development in the North Atlantic. J. Atmos. Solar Terr. Phys. 66, 393–405, 2004. Veretenenko, S., Thejll, P. Cyclone regeneration in the North Atlantic intensified by energetic solar proton events. Adv. Space Res. 35, 470– 475, 2005. Wilcox, J.M., Ness, N.F. Quasi-stationary corotating structure in the interplanetary medium. J. Geophys. Res. 70, 5793–5805, 1965. Wilcox, J.M., Scherrer, P.H., Svalgaard, L., Roberts, W.O., Olson, R.H. Solar magnetic structure: relation to circulation of the Earth’s atmosphere. Science 180, 185–186, 1973.
Advances in Space Research 50 (2012) 819–827 www.elsevier.com/locate/asr
The role of volcanic aerosols and relativistic electrons in modulating winter storm vorticity Brian A. Tinsley a,⇑, Limin Zhou b, Weiping Liu a b
a University of Texas at Dallas, WT15, Richardson, TX 75080, USA Key Laboratory of Geographic Information Science, Ministry of Education, East China Normal University, Shanghai, China
Available online 29 December 2011
Abstract Small changes in the vorticity of winter storms, responding to solar wind variations, are found in winters from 1957 to 2011, and are greater for winters with higher levels of stratospheric volcanic aerosols. Using 1993–2011 data, the response of the vorticity area index (VAI) is shown to be of larger amplitude when the days of minima in the relativistic electron flux (REF) precipitating from the radiation belts are used, instead of heliospheric current sheet (HCS) crossings, as key days in superposed epoch analyses. The HCS crossings mostly occur within a few days of the REF minima. The VAI is an objective measure of the area of high cyclonic vorticity, and for the present work is derived from ERA-40 and ERA-Interim reanalyses of global meteorological data. The VAI dependencies on the stratospheric aerosol content (SAC) and the REF are consistent with a model in which the ionosphere-earth current density (Jz) affects cloud microphysics. One of the ways in which Jz is modulated is by changes in stratospheric column resistance (S), which is increased by stratospheric aerosols. Because S is in series with the tropospheric column resistance (T), Jz modulation by REF requires that S be not negligible with respect to T. So the Jz modulation and the VAI response appear when the SAC is very high, or the REF reductions (which also increase S) are very deep, and when the product of the SAC and the reciprocal of the REF exceeds a threshold value dependent on T. Ó 2011 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Atmospheric electricity; Relativistic electrons; Stratospheric aerosols; Storm vorticity
1. Introduction 1.1. VAI responses to HCS crossings and relativistic electron flux It was discovered by Wilcox et al. (1973) and confirmed by Hines and Halevy (1977) that for the years 1964–1970 there was a small reduction in the vorticity of northern hemisphere winter storms, measured by the vorticity area index, VAI, at times that solar wind magnetic sector boundaries (Wilcox and Ness, 1965) crossed over the earth. The sector boundary crossings are now known as heliospheric current sheet, or HCS, crossings. The VAI is defined by the area of the atmosphere (in units of 105 km2) covered by values of absolute vorticity above a ⇑ Corresponding author.
E-mail address: [email protected] (B.A. Tinsley).
threshold of 20 105 s1, plus the area for vorticity above 24 105 s1. It was defined by Roberts and Olson (1973), for extended winters, November through March, for latitudes poleward of 20°N, and for 300 h Pa levels in the atmosphere, but can be evaluated for any latitude range; for any pressure level; with any threshold vorticity, and for any months. Systematic changes in the strength of cyclogenesis, measured by the VAI in a given hemisphere, affect the amplitude of Rossby waves, and these changes in atmospheric circulation, downstream of a cyclogenesis center, can affect regional climate. However, the significance of the present study for meteorology is that it provides evidence for a new physical mechanism, linking atmospheric electricity through cloud microphysics to atmospheric dynamics (see Section 1.4). It was shown by Tinsley et al. (1994) and Kirkland et al. (1996) that a link between the solar wind and the VAI could be provided by the relativistic electron flux (REF),
0273-1177/$36.00 Ó 2011 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2011.12.019
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of energy >2 Mev, that because of their quasi-trapped condition in the radiation belts were continually precipitating into the atmosphere, at rates which depend strongly on solar wind parameters. Because there is an association between minima in the REF and HCS crossings, minima in the VAI are associated with minima in the REF. The association of the VAI and HCS crossings and minima in the REF was evident at times of high concentrations of stratospheric aerosols, however, Prikryl et al. (2009a) have questioned whether the association depends on stratospheric aerosols, and have suggested a different mechanism that does not involve the REF link. Here we analyze 1957– 2011 observations with an improved VAI time series to examine the relationship to the stratospheric aerosol content (SAC). HCS crossings can deviate by up to several days from REF minima, and so it was suspected that if the REF was a physical link between the solar wind and the atmosphere, the VAI minima would be more easily detectable and sharply focused when the REF minima, rather than HCS crossings, were used as key days in superposed epoch analyses. This has been shown for 1997–2002 winters in the northern hemisphere for the 500 h Pa VAI by Mironova et al., 2011, using data from the GOES satellites at geosynchronous orbit. This was a period of relatively low average REF levels (see Reeves et al., 2011, Fig. 2). Here we extend that work from 1993 to 2011 using similar data, primarily from LANL satellite measurements (Reeves et al., 2011, and auxiliary material), but supplemented by GOES measurements. 1.2. Solar wind – Jz link It was pointed out by Tinsley et al. (1994) that REF penetration to upper stratospheric levels, with the Bremsstrahlung radiation that they produce which penetrates to lower stratospheric levels, increases ion production and conductivity, so that decreases in the REF would increase the stratospheric column electrical resistance. The consequent changes in the ionosphere-earth current density (Jz) that flows as the downward return current in the global electric circuit was considered to be the physical link to the tropospheric dynamical changes. The global electric circuit is constituted by an upward flow of about 1000 A to the highly conducting ionosphere from global thunderstorms and other highly electrified clouds, with the current spreading out and flowing downward over the whole globe with a current density (Jz) of a few picoamperes per square meter (Chalmers, 1949; Tinsley and Zhou, 2006; Rycroft et al., 2008). The value of Jz can be expressed by the Ohm’s Law relation (e.g., Tinsley and Zhou, 2006) as J z ¼ V i =ðT þ SÞ
ð1Þ
where Vi is the ionospheric potential with respect to Earth’s surface (ocean and land), T is the tropospheric column resistance, and S is the stratospheric column resistance. Markson (1976) and Mu¨hleisen (1977) estimated that S
was only 10% of T in the normal atmosphere, evidently without consideration of stratospheric aerosols, and the model of Hays and Roble, 1979 for an aerosol-free atmosphere also showed S small compared to T. In that situation decreases in S due to ionization by relativistic electrons, although important in the stratosphere, have negligible effect on Jz, by Eq. (1). However, stratospheric balloon conductivity measurements by Byrne et al. (1988) and Hu and Holzworth (1996) found values several times larger than calculated by for an aerosol-free atmosphere. Also, following a large volcanic eruption in Chile, Hoffman et al. (1985) found a large increase in aerosol concentration in the Antarctic stratosphere. The hypothesis of Tinsley et al. (1994) as developed by Kirkland et al. (1996), Tinsley (2005), and Tinsley and Zhou (2006) is that for several years following volcanic eruptions that inject large quantities of SO2 gas into the stratosphere, that form H2SO4 on a timescale of months, there is a production of ultrafine aerosol particles from the gaseous H2SO4 (Goodman et al., 1994). When carried into the higher temperatures of the upper stratosphere by the Brewer–Dobson circulation, the aerosol particles formed in the lower stratosphere evaporate and/or are dissociated by radiation and become gaseous but re-condense to form ultrafine aerosol particles as the air descends and diabatically cools in the downward branches of the circulation. The hypothesis is that the large concentrations of ultrafine aerosol particles at mid-high latitudes lower the stratospheric conductivity and increase S in those regions where precipitation of REF takes place. The values of S are estimated (Tinsley and Zhou, 2006) to be high enough compared with T so that by Eq. (1) Jz fluctuates as the REF fluctuates, especially at low values of the REF. Minima in the solar wind speed (SWS) as well as deep minima in the REF are associated with the HCS crossings, as shown by Tinsley et al. (1994, Fig. 5). The strong correlation between the REF and the SWS has been explored by Li et al. (2001a,b). Observations of the reductions of tropospheric potential gradient at HCS crossings, consistent with reductions in Jz, have been reported by Fischer and Mu¨hleisen (1980). Tinsley et al. (1994) showed that such reductions for a few years following the 1982 El Chicon volcanic eruption were greater than before or after. 1.3. Other atmospheric dynamics responses to Jz The hypothesized link between the reductions in Jz and changes in the vorticity of winter storms also explains reductions in the VAI associated with Forbush decreases of the galactic cosmic ray (GCR) flux. Tinsley and Deen (1991) had shown that small VAI reductions agreed in onset time (within a day) and duration with the Forbush decreases, and the amplitude of the response was proportional to the amplitude of the Forbush decrease. The variations of Jz due to Forbush decreases and solar cycle changes in the GCR are due to changes in tropospheric ion production and changes in T (Eq. (1)), and are several
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times greater at high latitudes, which is where the VAI changes are greatest, than at low latitudes. Also small changes in the northern hemisphere VAI, associated with day-to-day changes in Jz measured at Mauna Loa, Hawaii, that are due to varying upward current output of global thunderstorms (affecting Vi in Eq. (1)), have been found by Hebert et al. (submitted for publication). Responses of surface pressure in the Antarctic and Arctic to Jz changes have been found, with the Jz changes inferred from satellite measurements of local Vi changes due to solar wind inputs (Burns et al., 2007) or from ground based measurements of Ez changes due to changes in global thunderstorm current output (Burns et al., 2008). Ez is the near-surface potential gradient proportional to Jz and was measured at Vostok, Antarctica. Increases in atmospheric vorticity in response to solar energetic particle events, which increase Jz, have been reported by Veretenenko and Thejll (2004, 2005). Responses of cloud cover to HCS crossings have been found by Kniveton and Tinsley (2004) and to Forbush decreases by Veretenenko and Pudovkin (1997), and responses of global cloud cover to global Jz changes inferred from the Ez measurements at Vostok have been found by Kniveton et al. (2008). 1.4. The Jz – cloud microphysics link The mechanism for responses of winter storm vorticity and surface pressure and cloud cover to Jz is hypothesized to involve changes in nucleation processes in clouds. Tinsley and Deen, 1991, suggested electric charge-induced increases in the efficiency of ice nucleation processes, and Tinsley et al. (2000) focused on increases in the rate at which supercooled droplets scavenge charged ice-forming nuclei (electro-scavenging causing contact ice nucleation). More recently, electric charge effects on size-dependent in-cloud scavenging affecting the concentration and size distribution of cloud condensation nuclei (CCN), which in later episodes of cloud formation affects cloud droplet size distributions, has been discussed and modeled by Tinsley (2004, 2010). Electro-anti-scavenging increases in the concentration of small CCN, and the cumulative effect over a day or so in an air mass, in further cloud formation such as in the updrafts of cyclones, increases the number concentration and reduces the size of small droplets. Such CCN concentration changes have been shown to delay the onset of coalescence processes that would precipitate liquid water in convective storms before it is carried above the freezing level by updrafts (Rosenfeld et al., 2008). In addition, electro-scavenging reduces the concentration of large and giant CCN, which subsequently reduces the concentration of large droplets, which also reduces the rate of coagulation processes. It is the extra water, not precipitated, available for freezing that releases extra latent heat that invigorates updrafts. The invigorated updraft carries up even more water, and eventually increases total precipitation. Contact ice nucleation rates above the freezing level might also increase with Jz, and further increase the
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updraft speed. In the context of winter storms with significant latent heat release, associated with eddies in the winter circulation, the enhanced updrafts lead to increase in vorticity, tapping into the unstable baroclinic pressure and zonal wind shear gradients, completing a set of links between the solar wind, the REF, Jz and the VAI. 1.5. Alternate mechanism for VAI responses at HCS crossings The analysis of Prikryl et al. (2009a) showed that the reductions in the VAI at HCS crossings can be found in data for extended winters (May–September) in the southern hemisphere, as well as in the Northern hemisphere (November–March) and also for most years without high SAC. However, Prikryl et al. (2009b) sought to explain the VAI responding to the solar wind in terms of a completely different mechanism – one involving the generation of gravity waves by auroral energy deposition in the lower thermosphere near times of HCS crossings, with the gravity waves propagating down through the mesosphere and stratosphere to trigger instabilities in updrafts in winter storms. Whereas previous work (and the present work) has focused on short-term decreases in the VAI, Prikryl et al. (2009a,b) focused on the short-term increases, which follow the decreases. However, atmospheric dynamics has been shown to respond to solar wind changes causing Jz changes when no auroral energy input is occurring. As mentioned in Section 1.3, there are the polar surface pressure responses to changes in the interplanetary magnetic field By component that modulates local polar ionospheric potential Vi and thus local Jz (Burns et al., 2007), and the several atmospheric responses noted earlier that are associated with changes in global Vi due to changes in global thunderstorm current output: these include changes in polar surface pressure (Burns et al., 2008); changes in global cloud cover (Kniveton et al., 2008) and of changes in the VAI (Hebert et al., submitted for publication). These effects cannot be caused by the auroral gravity wave mechanism, but are consistent with the Jz/cloud microphysics mechanism. In this work we will use 300 h Pa and 500 h Pa VAI time series derived from the ERA-40 gridded meteorological data set (Uppala et al., 2005), its extension (ERA-Interim) with improved resolution (Dee et al., 2011). We will examine the response to HCS crossings in non-volcanic as well as volcanic winters for both VAI time series, and evaluate the response to minima in the REF, and estimate the changes in amplitude of the response with both changing levels of stratospheric volcanic aerosol and changing REF. The VAI time series are calculated from the ERA-40 data from 1957 to 2002, and scaled to match the ERA-Interim data from 1989 to 2010 by fitting in the period of overlap. The times of HCS crossings are from the list by Svalgaard (2011), and the REF time series are from satellite measurements at geosynchronous orbit. The LANL flux (in units of electrons (1.8–3.5 MeV) cm2 s1 sr1 kev1) was used
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as the primary time series to determine minima in the REF, but to fill data gaps use has been made of GOES (2011) measurements scaled to the same units. 2. Results Fig. 1 is a set of superposed epoch analyses of the responses to HCS crossings of the 500 h Pa VAI (top and third rows) and the 300 h Pa VAI (second and bottom rows) for 8 groupings of northern hemisphere extended winters (November through March) between 1957 and 1995. The latitude range is 20–80°N, and the groups are designated by the year of the beginning November and ending March, e.g., 57–63 is for the November through March months for the 6 winters between November 1957 and March 1963. The winters during periods of high SAC, i.e., 1963–1966 (Agung), 1982–1986 (El Chicon), and 1993–1995 (Pinatubo) show the minima in the VAI within a day or two of the HCS crossings, as observed in previous work, and more clearly at 300 h Pa than at 500 h Pa. Only at 500 h Pa for 1993–1995 is the minimum for these high ‘volcanic’ years not well defined. Several of the superposed epoch analyses for all the remaining ‘medium- or low-volcanic’ years also show some minima near HCS crossings, but with amplitude comparable to or smaller than the noise level. In 1957–1963, when there was a very low SAC (Meyerott et al., 1983, their Fig. 4), no response to the HCS crossings is apparent. Note that the mean level of the 300 h Pa VAI is about twice that of the 500 h Pa VAI, and that there are different offsets in many of the panels to accommodate the variation from year to year of the mean levels. Fig. 2 shows data for 1993–2011 when satellite data on relativistic electron flux is available. The top row of panels is the 300 h Pa VAI response to key days being HCS crossings (top row); the second row is the VAI response to the key days being minima in the solar wind speed, with minimum speed less than 370 km s1; the third row is the VAI response to the key days being minima in the REF of energy > 2 MeV measured at geosynchronous orbit, with the minimum flux being less than either 0.1 cm2 s1 sr1 kev1 or 0.2 cm2 s1 sr1 kev1; and the bottom row is the LANL REF response, with key days being the REF minima. The 1993–1995 HCS result is the same as in Fig. 1. Responses to HCS crossings are not apparent for the low volcanic years 1995–2007, and responses of the VAI to SWS minima are generally not evident. However, a clear response to REF minima can be seen in the third row in the high ‘volcanic’ years 1993– 1995, with smaller responses in the ‘low-volcanic’ periods 1995–1999, 1999–2003, 2003–2007. We discuss the 2007– 2011 response below. The second row has been included because minima in the REF tend to occur near minima in the SWS. However, the REF minima are intrinsically better than the SWS minima as markers of the solar influence, presuming the REF is the effective physical link, because the REF minima can be sorted by the absolute value of the minimum flux, which depends on the prior his-
tory of the REF energization processes, whereas this is not possible with the SWS minima. In the bottom row the first panel shows that in 1993– 1995 the average REF was high, and also that the average level of its minima was high, compared to the following years. In order to retain enough key days to bring out the signal relative to the noise, it was necessary to select minima for 1993–1995 where the flux level was less than 0.2 cm2 s1 sr1 kev1 i.e., set the upper limit a factor of two higher than the 0.1 cm2 s1sr1 kev1 used for the remaining four panels. For 2007–2011 there is no VAI minimum at days 1 and 2 as in the previous periods, but for days 3–5 there is a persistent minimum which is present with HCS crossings, SWS minima and REF minima as the key days. (It is also persistent when individual winters are examined). We have no explanation for this result, but note that 2007–2011 was an exceptionally low solar activity period with low average REF. Fig. 3 is in the same format as for Fig. 2, except it is for 500 h Pa pressure level. The VAI response to the 1993– 1995 HCS crossings, in the first panel in the top row, is the same as in Fig 1, being weaker than that at 300 h Pa. In the remaining panels of the first row, a VAI response to the HCS crossings is not evident. In none of the panels of the second row is a response to minima in the SWS evident. However, following the moderate VAI response to REF in the first panel of the third row for the 1993–1995 high volcanic period, there is a weak response in the panels for 1995–1999, 1999–2003, and 2003–2007. The response for 2007–2011 is negligible on day 1, but the dip for days 3–5 found for 300 h Pa is also apparent here, for HCS crossings, SWS minima and the REF minima. Overall, the results of Fig. 3 for the 500 h Pa VAI are similar to the results in Fig. 2 for the 300 h Pa VAI. To improve the signal/noise for evaluating the effect of the SAC, Fig. 4 shows in the first panels of the upper (for 500 h Pa) row and lower (for 300 h Pa) row all 262 ‘medium–low volcanic’ events up to 1993 where the key days are HCS crossings. The second column of panels shows all the 111 high ‘volcanic’ events combined, and the third column of panels is for the low volcanic years of 1995–2007, where the key days are minima in the REF. While a precise comparison is not meaningful, the results are consistent with the hypothesis that the level of response depends on the SAC. The response for the ‘medium’ and ‘low’ volcanic’ years is about half that for the ‘volcanic’ years, and the 1957–1963 result that shows no evidence for response with a very clean stratosphere is also consistent with the hypothesis. 3. Discussion 3.1. Effects of stratospheric aerosols The largest responses of the 500 h Pa and 300 h Pa VAI at HCS crossings was found for 1963–1966, which comes
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Fig. 1. Superposed epoch analyses of VAI responses to HCS crossings (day 0), in winters (November–March) for 1957–1995. The first and second rows are for the 500 h Pa and 300 h Pa VAI, respectively, for winters 1957–1963 (‘non-volcanic’); 1963–1966 (‘high volcanic’); 1966–1970 (‘medium-volcanic’); and 1970–1976 (‘low-volcanic’). The third and fourth rows are for 500 h Pa and 300 h Pa, respectively, for winters 1976–1982 (‘low-volcanic’); 1982–1986 (‘high-volcanic’); 1986–1993 (‘low-volcanic’): and 1993–1995 (‘high-volcanic’). The VAI values were calculated for the latitude range 20–80°N.
immediately after the evident lack of response in 1957– 1963, with the very low levels of SAC prior to the eruption of Mt. Agung. The decrease of amplitude of the effect during the next one or two decades is associated with the slow clearing out of the stratosphere, delayed by some small further eruptions (Taal and Redoubt, 1965; Fernadina, 1968; Helka, 1970). This decadal-long decay parallels the yearby-year decay of the temperature decrease at HCS crossings (3°C in 1965–1967 winters) in the Arctic at 500 h Pa found by Misumi (1983) and reviewed by Tinsley (2000). At 300 h Pa the next largest response to the 1963–1966 HCS crossings occurred in 1982–1986, following the eruption of El Chicon, with a comparable response in 1993– 1995, following the eruption of Pinatubo, with responses weak or absent in 1986-1993. These responses are similar to those shown in Kirkland et al., 1996, Fig. 2) for a 500 h Pa VAI time series derived from the National Mete-
orological Center (NMC) gridded data. The present work uses 300 h Pa and 500 h Pa time series derived from the ERA-40 and ERA-Interim gridded data sets as noted. While the NMC and ERA data sets are broadly similar, they differ in details, especially for small values of the VAI derived from them. This is evidently due to different assimilative models into which the intermittent raw meteorological data is incorporated. In one sense our results agree with those of Prikryl et al. (2009a) who showed, using a VAI time series derived from the ERA-40 data, that significant VAI responses at HCS crossings could be found for ‘non-volcanic’ winters. However, our results do not support their result that the amplitude of the response in ‘non-volcanic’ winters is comparable to that in ‘volcanic’ winters, as required by their alternate mechanism (Prikryl et al., 2009b) for tropospheric vorticity responses to solar wind changes. We note
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Fig. 2. Superposed epoch analyses for 300 h Pa VAI for winters 1993–2011, where key days in the top row are HCS crossings; in the second row are minima in the solar wind speed; and in the third row are minima in the relativistic electron flux. In the fourth row the key days are the same REF minima as in the third row, but the time series analyzed is the LANL REF (without GOES supplement). The labels for volcanic activity for each column are given at the top.
that only about 60% of the dates of HCS crossings they used (Prikryl et al., 2009a, Table 1) are the same as those of Svalgaard (2011) that were used for HCS crossings in the present work. According to Prikryl (personal communication, 2011) the reason for the changes was that the dates were adjusted to better align with the solar wind variations as judged by spacecraft measurements of interplanetary magnetic fields; solar wind speed variations, coronal holes, and optical measurements of the solar corona, with possible or observed dates of coronal mass ejections removed. Comparing the lists, it can be seen that there were a greater percentage of differences with Svalgaard’s list in ‘non-volcanic’ winters than in ‘volcanic’ winters, and this may partly explain the differences between the magnitude of the VAI responses of Prikryl et al. (2009a) in ‘non-volcanic’ winters as compared to the present results. Prikryl et al. (2009a) found that the VAI maxima immediately following the minima tended to be higher than the maxima immediately preceding them. This is apparent in Fig. 4 for 500 h Pa for the first and second panels in the top row, but not for the others. This could be related to the REF maxima following the minima being somewhat greater than the preceding maxima, as seen in the bottom rows of Figs. 2 and 3, but meteorological noise or physical processes in the dynamic recovery are other possibilities.
3.2. Use of REF minima preferable to HCS crossings Comparing the VAI responses to 300 h Pa HCS crossings in the top row of Fig. 2 with the VAI responses to REF minima in the third row of Fig. 2, we see that for 1995–1999, 1999–2003, and 2003–2007 there are VAI minima at day 0 or day 1 when REF minima are used as key days, but these are absent when HCS crossings are used as key days. For 2007–2011 in neither case is there a response. Again, in Fig. 3 for the 500 h Pa VAI, the use of REF minima as key days in the third row results in VAI minima on day 0 or day 1, with even a marginal VAI minimum for 2007–2011, but smaller or absent responses when HCS crossings are used as key days. These results support the hypothesis that the REF is a physical link between the space environment and the stratosphere, which, depending on the level of stratospheric aerosols, modulates Jz and ultimately affects cloud microphysics. 3.3. Threshold for the response The relationship expressed by Eq. (1) require that for the REF to modulate Jz the change in stratospheric column resistance S must be an appreciable fraction of the tropospheric column resistance T. This can be achieved with a
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Fig. 3. As for Fig. 2, but for the 500 h Pa VAI in the first three rows. The fourth row is the same as in Fig. 2.
Fig. 4. Combined superposed epoch analyses. The top row of panels is for 500 h Pa, and the bottom for 300 h Pa, with first column the combined analyses for medium-low volcanic winters 1966–1993 using HCS crossings as key days. The second column is for all high volcanic winters using HCS crossings as key days, and the third column is for the low volcanic years 1995–2007 using REF minima as key days.
high minimum level of the REF provided there is a very high SAC, as in 1993–1995 following the Pinatubo eruption (see Fig. 3, first panel on the bottom row). Alternatively, it could be achieved with a generally lower level near minimum for the REF with a lower SAC (as in the second and third panels of the bottom row of Fig. 3). A rule of thumb would be that S, at the REF minimum,
depends on the product of the SAC and the reciprocal of the REF, and that the amplitude of the day-to-day VAI response depends on the amplitude of the REF change, provided that S is non-negligible with respect to T. Thus, only when the ratio of the SAC to the minimum REF exceeds a threshold value, dependent on T, would there be the opportunity for modulation of Jz in this way,
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leading to the vorticity responses of the Wilcox effect. A more sophisticated analysis than this rule of thumb would take into account the altitude, latitude, and size distributions of the ultrafine aerosol particles, and the latitudinal and energy distributions of the REF. 3.4. Use of REF data for the 1960s through 1980s Given that the REF minima are better markers of the solar wind influence than HCS crossings, it should be possible to obtain data on REF levels at geosynchronous orbit, or at other altitudes, from the first observations in the 1960s through the 1980s, and process the data to ensure uniformity of flux levels, and extend the analysis to the earlier period. For future research, the optimum marker would be measurements of Jz, or simultaneous measurements of conductivity and electric field, at high tropospheric altitudes several times a day, at latitudes where the precipitation of relativistic electrons is occurring. An alternative would be measurements from satellites of upward fluxes of Bremsstrahlung, and/or REF spectra, together with solar occultation SAC measurements. 3.5. Use of forecasted VAI to improve signal/noise The VAI variations are seen in a background of meteorological ‘noise’ which is actually the ‘weather’ or internal variability. It was shown by Larsen and Kelley (1977) that forecasts of the 500 h Pa VAI made a day or two prior to an HCS crossing using the North American Fine Mesh meteorological grid data had a 20% lower correlation with the actual VAI than at other times, and this was for moderate to low SAC levels. This means that the HCS-related component of vorticity was superimposed on the forecasted vorticity, thus degrading the forecast success. Consequently, forecasting the meteorological vorticity and subtracting it from the actual VAI would seem likely to improve the signal/noise for the analysis of VAI responses to external forcing. This applies to responses to REF changes; Forbush decreases of the cosmic ray flux; solar energetic particles; Vi changes in the polar caps due to the solar wind, or to changes in thunderstorm output. In general, any day-to-day influence on the atmosphere that is not included in the physical processes in the assimilative models would be partly, but not completely assimilated into the model, and a further improvement in signal/noise might be obtained by analyzing perturbations of raw data from a forecasted output, rather than using the actual output of the assimilative model. 4. Conclusions This work has strongly supported the earlier published result that the amplitude of the response of the VAI to HCS crossings, where the dates of the crossings were determined independently of the analysis, depends strongly on the stratospheric aerosol content, contrary to the finding
of Prikryl et al. (2009a). It has shown that the VAI response is greater when minima in the relativistic electron flux (>2 MeV, measured at geosynchronous orbit) are used as key days in the superposed epoch analysis, instead of HCS crossings, consistent with an hypothesis that the REF is a physical link between the space environment and the stratosphere, where it modulates the stratospheric column resistance, and modulates the ionosphere-earth current density Jz to an extent depending on the stratospheric ultrafine aerosol content. The signal/noise of the analyses could be substantially improved by evaluating differences between a forecast of the VAI and the actual VAI. The phenomenon of the VAI response to REF changes is only one of a number of similar atmospheric responses to Jz changes, that are not included in current atmospheric models, and appears to be due to effects of electrical charges on the microphysics of clouds and cyclonic storms. Acknowledgements This work was supported by NSF Grants AGS 0855351 and AGS 0836171 to the University of Texas at Dallas, and by Chinese Fundamental Research Funds for the Central Universities, and Funds of the Shanghai Committee for Science and Technology, China (10dz0581600, 10dz1200703). We are appreciative of the ECMWF in making available the ERA-40 and ERA-Interim data. References Burns, G.B., Tinsley, B.A., Frank-Kamenetsky, A.V., Bering, E.A. Interplanetary magnetic field and atmospheric electric circuit influences on ground-level pressure at Vostok. J. Geophys. Res. 112, 1–10, D04103, 2007. Burns, G.B., Tinsley, B.A., French, W.J.R., Troshichev, O.A., FrankKamenetsky, A.V. Atmospheric circuit influences on ground-level pressure in the Antarctic and Arctic. J. Geophys. Res. 113, D15112, 2008. Byrne, G.J., Benbrook, J.R., Bering, E.A., Oro´, D., Seubert, C.O., Sheldon, W.R. Observations of the stratospheric conductivity and its variations at three latitudes. J. Geophys. Res. 93, 3879–3891, 1988. Chalmers, J.A. Atmospheric Electricity, first ed Oxford, Clarendon, 1949. Dee, D.P., Uppala, S.M., Simmons, A.J., et al. The ERA-Interim reanalysis, configuration and performance of the data assimilation system. Q. J. R. Meteorol. Soc. 137, 553–597, 2011. GOES. Energetic electron flux from the NOAA Space Weather Prediction site.
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