Abstract
An accurate precipitation phase determination—i.e., solid versus liquid—is of paramount importance in a number of hydrological, ecological, safety and climatic applications. Precipitation phase can be determined by hydrological, meteorological or combined approaches. Meteorological approaches require atmospheric data that is not often utilized in the primarily surface based hydrological or ecological models. Many surface based models assign precipitation phase from surface temperature dependent snow fractions, which assume that atmospheric conditions acting on hydrometeors falling through the lower atmosphere are invariant. This ignores differences in phase change probability caused by air mass boundaries which can introduce a warm air layer over cold air leading to more atmospheric melt energy than expected for a given surface temperature, differences in snow grain-size or precipitation rate which increases the magnitude of latent heat exchange between the hydrometers and atmosphere required to melt the snow resulting in snow at warmer temperatures, or earth surface properties near a surface observation point heating or cooling a shallow layer of air allowing rain at cooler temperatures or snow at warmer temperatures. These and other conditions can be observed or inferred from surface observations, and should therefore be used to improve precipitation phase determination in surface models.
Highlights
The hydrological, ecological, and atmospheric responses to rain are extremely different from snow responses [1] and correct precipitation phase is, very important for applications across many scientific fields
An example of a calibrated parameter used in many hydrological models is the precipitation phase determination scheme (PPDSs) usually based on surface temperature relationships, ignoring important landscape and atmospheric interactions [17]
The use of a vertical temperature/moisture profile of the environment is preferred in meteorology because it can first determine which hydrometeors are most likely to form in clouds, and calculate any precipitation phase changes that might occur between and below the cloud level at which the hydrometeor formed
Summary
The hydrological, ecological, and atmospheric responses to rain are extremely different from snow responses [1] and correct precipitation phase is, very important for applications across many scientific fields. This paper reviews and analyses studies dealing with precipitation phase processes and methods to determine the phase with an aim to suggest approaches that will improve the separation of rain and snow in cold region hydrological models It starts with a basic overview of fundamental meteorological knowledge to build a baseline understanding of the physical changes between hydrometeors and air in the lower atmosphere. This first section builds to explain terrain/landscape, precipitation rate, and frontal boundary effects on the atmospheric temperature/moisture with height which has an impact on energy needed to drive phase changes in precipitation. The advantages and disadvantages of these PPDS methods are discussed along with a proposed formula to simplify surface air temperature–relative humidity PPDS schemes
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