Meteorol. Atmos. Phys. 52, 169-175 (1993) Meteorology, and Atmospheric

Physics �9 Springer-Verlag 1993 Printed in Austria

551.594.221

Department of Physics, University of Botswana, Gaborone, Botswana

Conditional Instability and Lightning Incidence in Gaborone, Botswana

E. R. Jayaratne

With 5 Figures

Received March 29, 1993

Summary

Lightning is more frequent in deep convective storms formed by conditional instability. It has been shown that conditional instability increases with the wet bulb potential temperature. The incidence of lightning in Gaborone, Botswana was monitored over a period of two years with a CGR3 flash counter. The data were compared with the measured wet bulb temperatures. The results indicate that the monthly lightning activity in Gaborone increases by an order of magnitude for every 2 ~ rise in wet bulb temperature. There is also evidence to show that, in general, the ratio of lightning incidence to rainfall is significantly reduced as the wet bulb temperature decreases. Periods of continuous rain over a few days were generally characterised by a fall in the wet bulb temperature with a corresponding decrease in lightning activity. In consistence, one such nine day period was observed where the lightning incidence was sustained right through when the wet bulb temperature did fall. However, there is some evidence to indicate that the relationship does not hold very well during unusual winter lightning activity.

1. Introduction

Although it is generally true that lightning inci- dence and rainfall rate are closely correlated, there are numerous instances where heavy rain falls over several days with little or no lightning activity. Williams et al, (1992) have pointed out that this is especially common in tropical monsoonal storms, though less likely in continental convective storms. Using radar and electrical studies, they identified two distinctly different cumulonimbus regimes in the vicinity of Darwin, Australia and concluded that, although both types produced heavy rain,

the lightning activity in the deep continental convective type storms was at least an order of magnitude greater than that in oceanic based convective storms during monsoonal periods. The total rainfall yield per cloud to ground lightning flash was found to vary from 4-8 x 10 l~ for isolated continental thunderstorms to 0.5-1 x 1012 kg for vigorous monsoon storms. Dual Dop- pler radar information showed a t 0 - 2 0 d B en- hancement in radar reflectivity in the mixed phase region of the continental storms compared with the monsoonal types.

Williams and Renno (1991, 1993) have attributed these differences to the effect of convective avail- able potential energy (CAPE) in the two types of cloud. The energy for conditional instability is provided by CAPE and can be represented on a thermodynamic diagram by the area bounded by the environmental lapse rate (ELR) and the satu- rated adiabatic lapse rate (SALR). The SALR in turn is determined by the wet bulb potential temperature T,~. Williams and Renno have shown that observations at many stations in the tropics indicate a roughly linear relationship between CAPE and T w. Mid-lati tude soundings also show a similar relationship, but the scatter is greater and they suggest that this may be due to the presence of cool dry air aloft. T,,. is a more relevant parameter to use than the dry bulb temperature, Td, because it incorporates simultaneously the effects of both temperature and humidity.

170 E.R. Jayaratne

Williams et al. (1992) identified three monsoonal periods in the 1988-89 wet season in Darwin and showed that they coincided with an order of magnitude decrease in the lightning yield together with a 1-2 ~ drop in Tw. A modest increase in Tw may lead to a significant increase in CAPE and updraught velocity, leading to a substantial increase of the mixed phase volume within a thundercloud. Lightning activity increases with the volume of the mixed phase (Price and Rind, 1993), and Williams (1990) has shown that the presence of ice particles probably plays a major role here. It is well known that thunderstorms are strongly electrified when supercooled droplets, vapour grown ice crystals and graupel or soft hail pellets coexist within the cloud (Latham, 1981; Dye et al., 1989; Williams, 1990). Laboratory experiments have shown that when ice crystals rebound offgraupel in the presence of supercooled droplets the graupel acquires a positive charge at warmer temperatures and higher cloud water contents and a negative charge at colder tem- peratures and lower water contents (Takahashi, 1978; Jayaratne et al., 1983). Using these observa- tions, Jayaratne and Saunders (1984) proposed an explanation for the tripolar electrical structure of a thundercloud. High up in the cloud at colder temperatures, graupel pellets charge negatively and fall under gravity while the positively charged ice crystals are carried up in the updraughts to form the well known positive charge centre at the top of the cloud. At warmer temperatures graupel charge positively giving rise to the commonly observed lower positive charge centre associated with the precipitation shaft close to the 0 ~ iso- therm. The negatively charged ice crystals are carried up and, together with the graupel falling from the upper reaches of the cloud, form the main negative charge centre at the charge sign reversal temperature. Most cloud to ground lightning brings down part of this negative charge but positive flashes are not uncommon. In general, positive cloud to ground lightning is thought to originate at the top of the cloud, especially when a strong wind aloft gives rise to a highly sheared anvil cloud (Brook et al., 1982). However, recent observations of intense clustered positive ground flashes below continental mid-latitude storms in the USA do not support this view (Orville et al., 1988). Williams et al. (1991) have proposed that these originate on positively charged wet hail close

to the 0 ~ isotherm of the cloud. More recent work by Saunders and Brooks (1992) shows that there is no charge separated once the graupel surface becomes wet. Jayaratne (1993) suggests that positive ground flashes may well originate on graupel in the dry growth regime between the charge sign reversal temperature and the melting level.

2. Experimental Details

The lightning incidence was monitored with a CGR3 flash counter operating with a vertical antenna as described by Mackerras and Darveniza (1986). The counter has five registers. Negative ground flashes (NGF), positive ground flashes (PGF) and intracloud flashes (CF) are detected up to an effective horizontal range of about 14kin from the antenna. More distant flashes up to a range of about 42 km are recorded on a distant flash (DF) register. If the flash occurs within about 2 kin, an overload condition is triggered, resulting in an OL register count. Readings were recorded at least once a day and, very often, after each individual storm. Hourly readings of dry and wet bulb temperatures and daily rainfall records were obtained from the Gaborone Meteorological Services Department. The antenna was installed at the same site. Data obtained over a continuous period of 24 months between January 1991 and December 1992 were used in the analysis.

3. Results and Analysis

The sum of the counts recorded on the first four registers NGF, PGF, CF and DF gives the total lightning flashes within an effective radius of 42 kin. Figure 1 shows the total monthly lightning flash count as a function of the monthly mean maximum wet bulb temperature in Gaborone over the two year period January 1991 to December 1992. The two wayward points near the extreme left of the graph are entirely due to two unusual winter storms that occurred on 6 June 1991 and 28 August 1992. The surface air temperature on these two days was about 15 ~ colder than during the peak lightning months. Ignoring these two points, it is clear that the lightning activity increases by over four orders of magnitude when Tw increases from about 15 ~ to 23 ~ that is an order of magnitude enhancement for every 2 ~

Periods of continuous rain over several days is

Conditional Instability and Lightning Incidence in Gaborone, Botswana t71

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Fig. t. Monthly lightning counts as a function of monthly mean maximum wet bulb temperature, T w, for Gaborone during the two years 1991 and 1992

relatively rare on the inland plateau of Southern Africa. There were three such sustained spells during the period of observation. The dates and the corresponding accumulated rainfall figures are as follows: (A) January 2-9, 1991; 168ram (B) January 23-31, 1991; 78 mm (C) March 14-19, 1991; 92 ram. The lightning activity showed con- siderable variance over these periods. Figure 2 shows the daily readings of the maximum hourly wet bulb temperature, rainfall, in ram, and total cloud to ground lightning flashes detected within 14kin for the period January 2-11, 1991 which includes case A. Lightning activity and rainfall was sparse throughout the latter half of 1990, until an intense local thunderstorm occurred on the night of 2 January, 1991 between 22-23 h LST. Although only 4.5 mm of rain fell at the antenna site, the centre of the storm passed approximately 5-10kin to the west where 20-50ram of rain was recorded. The counter registered 3574 lightning flashes, of which no less than 1027 were within 14kin, 830 of them striking the ground. The maximum T w during the day was 22.5 ~ No rain was recorded on the 3rd of January and light rain was accompanied by a few flashes of lightning at

about 16 h LST on the 4th. Subsequently, a second intense storm struck on the night of the 5th with 100ram of rain and 2252 flashes of which 352 hit the ground within 14kin of the antenna. The heavy rain continued until the 9th, but as seen in Fig. 2, the lightning activity had ceased after the 7th. Observations indicated that Tw remained fairly constant until the 7th when it dropped abruptly by about 3 ~ probably aided by the continued overcast conditions.

Figure 3 shows the second spell of disturbed weather (case B). In contrast with case A, it is clear that both the rainfall and the lightning activity were sustained throughout. Lightning was ob- served even on the last days (30th and 31st) when the rainfall was tapering off. An important differ- ence between cases A and B is the variation of T,,. Whereas in Fig. 2 it shows a sharp drop in the middle of the rainy period, in Fig. 3 it maintains its value right up to the end of the spell.

The pattern is not so obvious in case C (Fig. 4). This spell occurred during the tail end of the lightning season in March. Although heavy rain continued on for 6 days, the lightning ceased completely after the third day. Tw shows a drop

172

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E. R. Jayaratne

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Conditional Instability and Lightning Incidence in Gaborone, Botswana 173

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of about 2 ~ but recovers by the fifth day. It appears that an increase is not immediately reflec- ted by an increase in lightning activity. Figure 5 shows the daily maximum dry bulb temperatures,

T d, during this spell. It is interesting to note that T a dropped by over 8 ~ at the onset of this rainy period, being the first significant sharp fall after the hot season. This spell was characterised by continuous rain over several hours at a time. The total lightning counts for the period are shown in Table 1 and will be discussed in terms of the microphysical properties of the cloud in the next section.

4. Discussion

Figure 1 confirms that the lightning activity in Gaborone increases sharply with Tw. This is significant as Gaborone is a semi-tropical location and most of the previous observations have been well within the tropics. Further evidence comes from cases A and B, in Figs. 2 and 3 respectively, where there is a strong positive correlation between the daily cloud to ground lightning incidence and maximum hourly T w. The third rainy spell, that occurred in March 1991, does not show an obvious correlation. However, it should be noted that during this spell the dry bulb temperature, Ta, fell by over 8 ~ to its lowest value after several hot

174 E.R. Jayaratne

summer months (Fig. 5). Consequently, the relative humidity on the 15th and 16th was higher than normal. Low ground temperatures give rise to lower updraught velocities and shallower cloud depths. If the cloud top temperature was warmer than the charge sign reversal temperature, the negative charge centre would be located at the top of the cloud. The cloud would then exhibit a dipolar charge structure instead of the well known tripolar distribution. Under such circumstances, ground flashes are more likely to bring down positive charge from the base of the dipole. Table 1 shows the daily lightning activity during this spell. Note the unusually high ratio of positive ground flashes to all ground flashes on the 16th of March. This ratio, averaged over the past two years in Gaborone was 0.04 (Devan et al., 1991). On the 16th of March the ratio was 0.65, well over the norm. Although there was no way of checking this, it is suggestive that the positive lightning originated on charged graupel close to the 0 ~ isotherm in the lower regions of the cloud. Further- more, a shallow cloud has the two main charge centres vertically closer together and may enhance the relative frequency of intracloud flashes over that of ground flashes. The mean value of the ratio of all ground flashes to intracloud flashes, GF/CF, over the entire lightning season was 0.80. It is interesting to note from Table 1 that the ratio showed a sharp fall on the 16th, just when the positive cloud to ground lightning incidence increased.

In case C, Tw falls by about 2 ~ after the onset of the rain, but recovers by the fifth day. The increase is not accompanied by a correspond- ing increase in lightning activity. This implies a hysteresis effect, suggesting that the lightning takes some time to "pick-up" after the CAPE has been exhausted. Looking at Fig. 1, a similar effect is evident in the annual cycle. Note that the lightning activity for a given mean monthly wet bulb temperature is generally greater during the first half of the season (September-January) than in the second half (February-May). This is not unexpected as a similar behaviour has been ob- served at other locations (Williams, 1992).

5. Conclusions

In spite of its semi-tropical location, the Gaborone results indicate that a modest 2~ rise in the monthly mean maximum wet bulb temperature

enhances the lightning activity by an order of magnitude during the summer months. Discrep- ancies were almost always confined to unusual winter storms. Of the three prolonged wet spells that occurred during the period of observation, the first two were in the peak summer month of January. In the first, the lightning activity was sustained as long as the wet bulb temperature remained high, while a slight drop of about 2-3 ~ was accompanied by a total absence of cloud to ground lightning. In the second spell, although the rain continued on for 9 successive days, the wet bulb temperature did not fall significantly and the lightning activity was sustained up to the very last day. In the third spell, that occurred during the tail end of the lightning season in March, the correla- tion was not obvious.

It appears that the relationship between light- ning incidence and wet bulb temperature in Gabo- rone follows the pattern observed in the tropics during the hot summer months only. The relation- ship does not appear to hold true during the rare winter storms probably due to the presence of cold dry air aloft as has been suggested by Williams and Renno (1991).

Acknowledgements

This work received financial support through a grant from the University of Botswana Research and Publications Committee, I am grateful to the Botswana Meteorological Services Department, Gaborone, for providing the daily temperature and rainfall data and allowing the use of their premises for the lightning measurements. The CGR3 counter was gifted by Dr. Dave Mackerras of the University of Queensland, Australia, Dr. V. Ramachandran was of invalu- able assistance with the instrumentation. Finally, I would like to thank Prof. Earle Williams of the Massachausettes Insti- tute of Technology for stimulating an interest in the subject through several interesting discussions and correspondence.

References

Brook, M., Nakano, M., Krehbiel, P., Takeuti, T., 1992: The electrical structure of the Hokuriku winter thunderstorms. J. Geophys. Res., 87, 1207-1215.

Devan, K. R. S., Jayaratne, E. R., Ramachandran, V., 1991: Characteristics of lightning in Gaborone, Botswana. Third ANSTI Seminar in Physics, University of Botswana, Gaborone, Botswana.

Dye, J. E., Winn, W. P., Jones, J. J., Breed, D. W., 1989: The electrification of New Mexico thunderstorms 1: Relation- ship between precipitation development and the onset of electrification. J. Geophys. Res., 94, 8643-8656.

Jayaratne, E. R., 1993: The heat balance of a riming graupel pellet and the charge separation during ice-ice collisions. J. Atmos. Sci. (in press).

Conditional Instability and Lightning Incidence in Gaborone, Botswana 175

Jayaratne, E. R., Saunders, C. P. R., Hallett, J., 1983: Laboratory studies of the charging of soft-hail during ice crystal interactions. Quart. J. Roy. Meteor. Soc., 109, 609-630,

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Author's address: E. R. Jayaratne, Department of Physics, University of Botswana, Private Bag 0022, Gaborone, Botswana.