  Measuring radio emission from air showers provides excellent opportunities to
directly measure all air shower properties, including the shower development.
To exploit this in large-scale experiments, a simple and analytic
parameterization of the distribution of the radio signal at ground level is
needed. Data taken with the Low-Frequency Array (LOFAR) show a complex
two-dimensional pattern of pulse powers, which is sensitive to the shower
geometry. Earlier parameterizations of the lateral signal distribution have
proven insufficient to describe these data. In this article, we present a
parameterization derived from air-shower simulations. We are able to fit the
two-dimensional distribution with a double Gaussian, requiring five fit
parameters. All parameters show strong correlations with air shower properties,
such as the energy of the shower, the arrival direction, and the shower
maximum. We successfully apply the parameterization to data taken with LOFAR
and discuss implications for air shower experiments.
