The Response of an Idealized Squall Line to Increases in Atmospheric Temperature With Constant Relative Humidity

Extreme precipitation events are known to be dominated by the amount of moisture advected into the system from surrounding areas rather than local evaporation. Global climate models generally predict that relative humidity will remain largely unchanged as global mean temperature increases. This means that as the climate warms, atmospheric water vapor content will increase. Therefore, it seems reasonable to assume that in a warmer and moister climate a given storm will produce larger precipitation totals. This paper tests that assumption for an idealized squall line using the WRF (Weather Research and Forecasting) regional atmospheric model with a horizonatal resolution of 1km. It is found that storm total rainfall and storm peak rainfall scale with precipitable water when the atmospheric temperature is increased with the relative humidity held constant. However, for a given point on the ground, maximum rainfall accumulation does not exhibit the same scaling property. In fact, changes in maximum point rainfall appear to be unrelated to changes in precipitable water. This is due a strengthening of the internal circulation of the system when the atmosphere becomes warmer and moister. Increased latent heat release intensifies upward motions resulting in stronger outflow from the storm and more rapid advection of the gust front convergence zone. Therefore, although storm peak rainfall increases with increasing precipitable water, rainfall accumulation at a point on the ground does not necessarily do so as the storm passes over the point more rapidly.