Parameterizing total storm conduction currents in the Community Earth System Model

Abstract

Electrified clouds are known to play a major role in the Global Electric Circuit. These clouds produce upward currents which maintain the potential difference between Earth's surface and the upper atmosphere. In this study, model output from two simulations of the Community Earth System Model (CESM) are compared with conduction currents and other data derived from the Tropical Rainfall Measuring Mission (TRMM) satellite, including both the Lightning Imaging Sensor and Precipitation Radar. The intention is to determine CESM's skill at representing these microphysical and dynamical properties of clouds. Then, these cloud properties are used to develop a model parameterization to compute conduction currents from electrified clouds. Specifically, we evaluate the ability of global mean convective mass flux, ice water path, and convective precipitation to represent conduction current sources. Parameterizations using these variables yield derived global mean currents that agree well with the geographical patterns of TRMM currents. In addition, comparing the diurnal variations of modeled global mean current to the observed diurnal variations of electric potential gradient, root‐mean‐square (RMS) errors range between 6.5% and 8.1%, but the maximum occurs 4 to 6 h early in all three variables. Output currents derived from the model variables generally match well to the currents derived from TRMM, and the total global current estimates agree well with past studies. This suggests that cloud parameters are well suited for representing the global distribution and strength of currents in a global model framework.

Relationship between the global electric circuit and electrified cloud parameters at diurnal, seasonal, and interannual timescales

“…This pattern is consistent for 20 and 40 dBZ (Table ) with the calculations including total volumes of greater than or equal to the specified reflectivity thresholds. The volume of 30 dBZ found in the altitude range of −5°C to −35°C has been shown to have a good relationship to the flash rate in the PFs [ Deierling et al ., ; Liu et al ., ] and could be used as one proxy for the rate of charge separation [ Liu et al ., ; Kalb et al ., ]. Figure f shows the variation of rainfall from both thunderstorms and electrified shower clouds combined.…”

A TRMM/GPM retrieval of the total mean generator current for the global electric circuit

“…RMS errors for the proxies from the literature range from 20% [ Whipple , ] to 3.3% [ Liu et al , : thunderstorm rainfall] while maximum differences range from 33% to 6.4%. Global currents used by the Frontiers in Earth System Dynamics (FESD) Electrical Connections and Consequences within the Earth System (ECCWES) project [ Kalb et al , ; Lucas et al , ] have RMS difference between 6 and 7% and maximum differences around 10%.…”

Toward a Realistic Representation of Global Electric Circuit Generators in Models of Atmospheric Dynamics

“…Note that other studies may use different isotherms to approximate the upper boundary of the mixed phase region (e.g., Kalb et al, identified the upper boundary with the 50 °C isotherm) owing to the variation of the lowest temperature across different clouds. However, we will show below that the simulated diurnal variation of the IP is not very sensitive to the entire factor involving and (see section ), hence it should not be very sensitive to the choice of the upper boundary either.…”

“…Lucas et al () and Kalb et al () employed the model CESM (Community Earth System Model) to discuss the related problem of parameterizing contributions to the GEC from electrified clouds on the basis of their dynamical and microphysical parameters calculated in climate models (note that such calculations are based on special schemes and parameterizations, since large‐scale models are unable to resolve individual clouds and small‐scale processes). The parameters considered by Kalb et al () include convective mass flux, ice water path, and convective precipitation in the mixed phase region (where both ice particles and supercooled liquid water droplets are present, which is required for noninductive charging), assumed to be bounded by the 0 °C and 50 °C isotherms. Of these three parameters convective mass flux was found to provide the best characterization of the current distribution estimated from the TRMM (Tropical Rainfall Measuring Mission) satellite data.…”