The model’s standard temperature profile uses the coefficients shown in Table 3.5 for equations (3.18) to (3.20). The values were chosen to approximate to the 1976 US standard atmosphere. TS ZTP ZST TR dT dz TP dT dz ST dT dz Value 15°C 11km 20km -6.5x10-3Km-1 0Km-1 1.5x10-3Km-1
Table 3.5 Coefficients used in equations (3.18) to (3.20) for the standard temperature profile of the model.
As pressure is derived from the hypsometric equation (3.21), changes in atmospheric temperature will alter the air pressure profile accordingly, although the surface value is still defined. A graph of the temperature profile based on these coefficients is shown in Figure 3.15.
0 5 10 15 20 25 30 35 40 45 50 -60 -50 -40 -30 -20 -10 0 10 20 Air temperature (°C) H e ig h t (k m )
Figure 3.15 Assumed air temperature profile for the model, using equations (3.18) to (3.20) and the coefficients
In order to investigate the sensitivity of RC, JC and PG at 1m to atmospheric temperature,
these parameters were calculated using the standard temperature profile described by the coefficients in Table 3.5, with an aerosol and ionisation rate profile using the coefficients in Table 3.4. The temperature of the entire atmospheric column was then varied about this standard temperature profile (with aerosol and ionisation rate profiles kept constant) by changing TS, with associated changes in RC, JC and PG at 1m calculated as a percentage
of the values found using the standard temperature profile. Unlike variation caused by aerosol and ionisation rate, the variation in RC, JC and PG at 1m is linear with atmospheric
temperature change between TS of -10°C to 30°C, the gradients of which are shown in
Table 3.6.
RC JC PG at 1m
Sensitivity 0.41%K-1 -0.43%K-1 0.27%K-1
Table 3.6 Percentage sensitivity of RC, JC and PG at 1m to atmospheric temperature change
between 0-50km.
Temperature will affect RC through changes in the air density profile which will affect ion
mobility and ion production rates, as well as changes in ion-aerosol attachment and ion- ion recombination coefficients. The percentage sensitivities to atmospheric temperature change were calculated for a uniform change in atmospheric temperature, so these values should be considered as maximum sensitivities. For example, the diurnal range of air
temperature due to solar heating of the land surface may have an upper limit of approximately 20°C at a mid-latitude site in summer . However, this temperature variation will not penetrate into the entire depth of the atmosphere, rather only the lowest few hundred metres (at most) corresponding to the net vertical heat flux (Stull, 2000). Even if the temperature of the lowest 1km of the atmosphere changed by 20°C, this would only correspond to a change in RC, JC and PG at 1m of 2%, -2% and 4% respectively. These
changes are small compared to those of a moderate change in boundary-layer aerosol number concentration over a similar boundary layer height (Figure 3.12 and Figure 3.13).
3.8 Discussion
The RC model assumes a zero ion and aerosol flux, with ions and aerosols existing in
steady-state for each horizontal layer. Aerosol particles are considered neutral, spherical and monodisperse. Any dynamical source of local space charge that would result when JC
flows through a conductivity gradient is assumed to have a negligible effect on the overall conductivity profile and therefore RC. For layer thicknesses (i.e. vertical resolution) of
order 10m or larger, and fair-weather conditions (i.e. no sharp aerosol concentration boundaries, local space charge or significant vertical mixing) these assumptions are expected to be valid. From these assumptions it is possible to calculate the total conductivity profile from the surface to 50km. As 95% of RC occurs in the lowest 13km
(Hoppel et al. 1986), this region below 50km is expected to contain all the RC, which is
found by integration of the resistivity profile. Once RC is known, JC and PG can be
calculated for a given value of VI. The model ionisation rate and conductivity profiles are in
good agreement with observed profiles, once a suitable aerosol profile is assumed for the latter. This model allows the influence of factors such as the aerosol profile on the surface measurements of JC, PG and σT to be investigated, and the relative contribution of local
and global effects on these atmospheric electrical observations a the surface.
The model is sensitive to changes in the main input parameters; aerosol number concentration, ionisation rate and air temperature. The relative sensitivity of the model outputs (RC, JC and PG at 1m) is not fixed, but is dependent not only on the size of the
change in input but also the scale heights. This is due to differences in the relative importance of changes in local and columnar resistance (ρ and RC) to the three output
parameters, and the relative importance of RC change with height, with nearly half of total
RC residing in the lowest 1km. It is primarily the aerosol number concentration, followed by
ionisation rate that controls RC, with atmospheric temperature change being of negligible
The next chapter describes instrumentation used to measure atmospheric electrical parameters, including a new type of sensor to directly measure JC at the surface.
4 Instrumentation
for
atmospheric
electrical
research
4.1 Overview
The three main atmospheric electrical parameters used for the study of atmospheric electricity at the RUAO are the potential gradient (PG), air-Earth conduction current density (JC) and total air conductivity (σT). Details of all the instruments used to make
these measurements will be given in this chapter. Of these instruments, two were developed by the University of Reading Department of Meteorology before the commencement of this study. These were the Passive Wire (PW) for PG measurement, and the Programmable Ion Mobility Spectrometer (PIMS) for measurement of σ and small ion properties. The JCI 131 Electrostatic Field Mill (FM) is a commercially available instrument which measures the PG and was installed at the start of this study in Autumn 2004. The FM and PIMS will be used to determine the atmospheric electrical climatology of the Reading University Atmospheric Observatory (RUAO) site, with shorter periods selected for case studies. The instruments for direct measurement of the air-Earth current density were designed and developed as part of this study and will consequently be discussed in particular detail here. Like the FM and PIMS, the aim is to use the JC
instrumentation to produce a dataset capable of indicating the electrical climatology and providing data for case studies, such as investigation of the validity of Ohms Law, which requires simultaneous measurement of all three parameters (JC, PG and σT).