Reducing misclassified precipitation phase in conceptual models using cloud base heights and relative humidity to adjust air temperature thresholds

Abstract

In cold region, conceptual models assigned precipitation phase, liquid (rain) or solid (snow), cause vastly different atmospheric, hydrological, and ecological responses, along with significant differences in evaporation, runoff, and infiltration fates for measured precipitation mass. A set air temperature threshold (ATT) applied to the over 30% annual precipitation events occurring with surface air temperatures between −3 and 5 °C resulted in 11.0 and 9.8% misclassified precipitation in Norway and Sweden, respectively. Surface air temperatures do not account for atmospheric properties causing precipitation phase changes as snow falls toward the ground. However, cloud base height and relative humidity (RH) measured from the surface can adjust ATT for expected hydrometeor-atmosphere interactions. Applying calibrated cloud base height ATTs or a linear RH function for Norway (Sweden) reduced misclassified precipitation by 4.3% (2.8%) and 14.6% (8.9%) misclassified precipitation, respectively. Cloud base height ATTs had lower miss-rates with low cloud bases, 100 m in Norway and 300 m in Sweden. Combining the RH method with cloud base ATT in low cloud conditions resulted in 16.1 and 10.8% reduction in misclassified precipitation in Norway and Sweden, respectively. Therefore, the conceptual model output should improve through the addition of available surface data without coupling to an atmospheric model. HIGHLIGHTS This paper lays out two new methods to decrease misclassified precipitation in conceptual surface-based models using air temperature thresholds (ATTs) to distinguish between rain and snow, common in hydrological modeling.; The cloud base height method has not been written about in scientific publications and gives another simple solution to help reduce misclassified precipitation phase in surface-based models.; The linear relative humidity ATT formula suggested in this paper simplifies earlier attempts to include relative humidity in surface-based precipitation phase determination.; The information needed for both methods is widely available in surface meteorological reports and improves surface-based models without coupling to an atmospheric model for precipitation phase.; In this paper, misclassified precipitation between −3 and 5 °C was reduced by 4.3% (2.8%) using cloud base height ATTs, 14.6% (8.9%) using a linear relative humidity formula, and 16.1% (10.8%) using a combination of both methods in Norway (Sweden), showing great potential for reducing model uncertainties when applying this work.;

Keywords

• conceptual models
• hydrological model
• lsm
• precipitation phase
• snow