A model of scalar structure function parameters in the entraining, convective boundary layer is developed based on a top-down and bottom-up diffusion approach. The behavior of the structure function parameters is obtained from the large eddy simulations of the scalar variance budget equations given by Moeng and Wyngaard. The conventional convective scaling formalism is augmented with an additional scaling parameter, Rc, which is the ratio of the entrainment flux of the scalar variable, C, to the surface flux. The model is compared to atmospheric measurements of the structure function parameters for temperature (CT2) and humidity (CQ2). Two types of comparisons are done: average profiles from several well known measurement campaigns (e.g., Minnesota and AMTEX) and individual profiles from twenty soundings by a light aircraft. The model appears to fit the CQ2 data better than it fits the CT2 data, particularly for the average profiles. There is a tendency for the model to underestimate the structure function parameters in the upper mixed-layer, particularly for CT2.