Characteristics of Orographic Precipitating Systems - tests with two models

The understanding of orographic precipitation is far from complete and often the lack of predictability has severe social impacts.

In order to gain deeper insight into this phenomenon, we have analyzed the atmospheric water cycle for orographic flows. Three efficiency measures characterizing orographic precipitating systems are identified. The global measure of the system is referred to as the drying ratio (DR); it determines the fraction of water vapor influx which turns into precipitation.The interior controlling measures, precipitation efficiency (PE) and condensation efficiency (CE), describe conversion processes.

In order to assess how the measures change as input parameters like mountain half-width and surface temperature vary, two distinct mathematical models have been used: 1) a fully nonlinear numerical model that conserves water by design and 2) a linear orographic-precipitation model (with both linear micro-physics and air flow dynamics).

Using a warm rain scheme in the numerical model, we find that DR increases for decreasing temperatures and increasing mountain width. The re-evaporation of condensated water, described by a combination of CE and DR, is strong for low temperatures, indicating low PE. Even though there are discrepancies between the two models, both show increased re-evaporation as the mountain-half width decreases.