Turning a weather forecast into pavement and/or bridge deck forecast is a straightforward, yet complex mathematical procedure made more difficult due to the highly variable nature of many of the influencing weather and pavement factors. These factors include the albedo, heat capacity, conductance, texture, and emissivity of the pavement and its substrates, the solar and longwave radiation received at the top surface of the pavement, shading effects by surrounding trees and terrain, the atmospheric temperature, humidity, wind speed, and precipitation, as well as the profound effects of winter maintenance activities and traffic. In order to address these complex processes, Meridian developed the highly sophisticated HiCAPS™ model (for Highway Condition Analysis and Prediction System).

HiCAPS™ forecasts pavement and bridge deck temperatures using what is commonly referred to as the unsteady heat flow equation, combined with sophisticated parameterizations for representing heat and moisture exchanges between the road, the atmosphere, and pavement substrate. A key distinction that sets HiCAPS™ apart from other models in the industry is the coupling between the mass and energy balances in the model. In simple terms this means that when moisture (as snow, rain, frost, dew) is deposited onto the road it also transfers energy to or from the road, and that evaporation or sublimation of moisture from the road requires the road to have an adequate amount of energy available to support those processes. This coupling has key implications for the model’s ability to support the more advanced features of tools such as Maintenance Decision Support Systems (MDSS) in a scientifically sound manner.

HiCAPS™ has a very sophisticated scheme for handling latent heat fluxes (heat exchanges related to changes in the state of water). This includes modeling of heat exchanges associated with evaporation, sublimation, condensation, frost formation, and phase changes (e.g., water freezing on the road surface) – naturally or chemically induced, as well as making actual predictions of the depth of water, ice, frost, and snow upon the roadway. This capability is essential to effectively handle the impact of chemical treatments and HiCAPS™ sophisticated handling of these processes makes it uniquely adaptable to MDSS modules.

Click to enlarge - HiCAPS™ processes and features

HiCAPS™ was built to be a robust model, capable of filtering through the occasionally erratic observations that inevitably come from in-pavement sensors. The continuous data assimilation system of HiCAPS™ allows it to continue to make reliable forecasts in the complete absence of RWIS observations (recent side-by-side tests indicate less than a 1°F loss of forecast accuracy in the absence of RWIS observations). This important feature permits HiCAPS™ to provide accurate road and bridge forecasts even at locations within the state where an RWIS Environmental Sensor Station (ESS) is not available locally.

Given perfect input data, a model of this type is capable of yielding very accurate forecasts of pavement and/or bridge deck temperatures. Research indicates that the accuracy of pavement temperature models is most sensitive to errors in forecasted weather conditions. Thus, pavement temperature and surface condition forecasts benefit most from a system that places great emphasis on providing the most accurate weather forecast possible. This is one of the reasons Meridian placed so much emphasis on the development of the MPower™ weather forecast system and highlights its involvement as part of the weather and pavement forecasting system.