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

In cold region, conceptual models assigned precipitation phase, liquid (rain) or solid (snow), cause vastly different atmospheric, hydrological, and ecological responses, along with signi ﬁ cant differences in evaporation, runoff, and in ﬁ ltration 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 (cid:1) 3 and 5 (cid:3) C resulted in 11.0 and 9.8% misclassi ﬁ ed 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 misclassi ﬁ ed precipitation by 4.3% (2.8%) and 14.6% (8.9%) misclassi ﬁ ed 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 misclassi ﬁ ed 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. in surface-based precipitation phase determination. (cid:129) In this paper, misclassi ﬁ ed precipitation between (cid:1) 3 and 5 (cid:3) 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.


INTRODUCTION
The fate of precipitation impacting the ground is very different between liquid (rain) and solid (snow) (Jennings et al. ). This uncertainty makes precipitation phase one of the three most essential parameters in cold region hydrological models (Kongoli & Bland ). Areas such as the US Pacific North-West are reliant on mountain snowmelt for drinking water, agriculture, and recreation (Vano et al. ) to supplement water supplies in the drier summer months. A projected 30% decrease in wintertime snow-dominated areas in the Western United States, coinciding with a 2-month reduction in annual snowfall duration by the mid-21st century will change the timing of, and decrease snowmelt water contributions (Klos et al. ). Therefore, the accuracy of a precipitation phase determination scheme (PPDS) in a model is essential in both short-term (e.g., rain on snow flood forecasting), and longterm (e.g., magnitude and duration of summer baseflow levels) water resource forecasting (Harpold et al. b).
Despite criticism of being too simple (Daly et al. ; Harpold et al. a), a set air temperature threshold (ATT) is often used for the PPDS in hydrological models.
These PPDS play a crucial role in water resource management and safety decisions. An advantage of using ATT is that it is a widely available parameter. However, it does not take into account the properties of the atmosphere.
These properties drive sensible and latent energy exchanges (microphysics) between hydrometeors and the air they fall through.
In general, depending on air temperature, humidity, and the depth of a warmer than freezing layer, sensible and latent heat exchanges between snow and the atmosphere can cause phase change from dry snow to wet snow to slush to rain (Fassnacht et al. ; Thériault & Stewart ). The equations describing microphysics are calculation intensive. They require detailed information, including initial hydrometeor phase (Harder & Pomeroy ), atmospheric properties through which the hydrometeors fall, and hydrometeor shape and size (Harder & Pomeroy ).

Stewart () had a relatively simple microphysics solution
using five equations which approximated: (1) heat exchange resulting from phase change (sublimation 2594 j/G, evaporation 2260 j/G, melting 334 j/G which cool the atmosphere, and condensation 2260 j/G, ice condensation 2594 j/G, and freezing 334 j/G which warm the atmosphere), (2) condensation adding mass to hydrometeors, (3) latent heat flux resulting from condensation, (4) heat exchange due to collision coalescence or accretion of ice and liquid, and (5) latent heat of fusion resulting from melting and freezing when particles contact each other. More complexed microphysical schemes are found in, e.g., Lundquist et al. () and Thériault & Stewart (). The atmospheric information required for even simple microphysics schemes cannot be recreated from surface data alone. Therefore, attempts to account for microphysics in surface-based hydrological models are seldom attempted.
Cloud base height can be used to adjust ATT for microphysics, even if the required atmospheric measurements to calculate a simple microphysics scheme are not available.
In clouds, evaporation and sublimation can be considered non-factors. This is due to the high vapor densities surrounding the hydrometeors (Harder & Pomeroy ). Below the cloud level, lower vapor density around hydrometeors will increase evaporation and sublimation (Harder & Pomeroy ). This increased latent heat flux will help cool the air and decrease the potential sensible heat flux (or melt energy) and, therefore, explains some theory on why cloud base height values should affect optimal ATTs.
Taking this a step further, lower relative humidity (RH) measured at the surface could indicate a drier environment. Precipitation formation through the Bergeron process can favor ice condensation (snow) over liquid (rain) due to the vapor pressure over ice being less than vapor pressure over water. This leads to more initial snow phase in cold clouds, especially when the air is saturated for ice formation but unsaturated for rain. In a drier environment, latent heat fluxes are more significant than in moist environments allowing evaporation and sublimation heat exchanges to cool the atmosphere. These heat exchanges can offset some melt.

METHOD
In this study, only meteorological observations reporting current precipitation were analyzed.
Initial processing of datasets consisted of first removing all observations containing a WMO weather code that did not identify precipitation presently occurring. Next, all observations, with air temperatures cooler than À3 C and warmer than 5 C, were removed from the datasets. This was due to 0.12 and 0.30% precipitation classified as rain, mixed, or frozen occurring at temperatures cooler than À3 C, while 0.20 and 0.36% snow, mixed, or frozen precipitation occurred in air temperatures warmer than 5 C in Norway and Sweden, respectively.
Mixed precipitation observations were then removed from the À3 to 5 C datasets. This is due to (1)  Next, all observations, with cloud base height above 1,000 m reporting category, were separated from the primary datasets. This was for two reasons: (1)   (2) rain predicted in air temperatures warmer than the ATT when the observation reported snow.
Next, cloud base height ATTs were tested as an indicator of RH or if they perform better than a linear RH based ATT formula (T RH ). T RH (Equation (1) The sum of misclassified precipitation for a PPDS using cloud base height ATTs, or T RH , is compared with the sum of misclassified precipitation from a PPDS using country thresholds 1.2 and 0.9 C for Norway and Sweden, respectively.        Cloud base height and T RH method results using full dataset Using all precipitation observations, except mixed phase and freezing events, occurring in air temperatures between À3 and 5 C, the use of cloud base height ATTs reduced misclassified precipitation, but that reduction was less than 1/3 of the reduction found using T RH (Table 3).

Long-term snow bias from PPDS methods
The use of a set ATT or cloud base height ATT results in the least change in percentage snow assigned by the compared PPDS ( Figure 6). The methods resulting in the greatest decrease in misclassified precipitation (Figure 4) result in the greatest imbalances in the assigned percentage of snow events. The use of WB 0.0 C resulted in a À9.5% and À6.9% mean change in snow events, whereas T RH resulted in a 10.6 and 11.7% mean increase in snow events assigned to precipitation in this climatological study (Figure 6).

DISCUSSION
The techniques of using T RH or adjusting ATT for cloud base heights between 0 and 1,500 m could be incorporated into conceptual models to improve precipitation phase determination from the standard, often criticized ( Table 3).
The use of cloud base height ATTs reduced more misclas-  J. M. Feiccabrino | Improved precipitation phase in conceptual models using cloud height and relative humidity Hydrology Research | in press | 2021 In Sweden, a drier environment than Norway, cloud base height ATTs indicate precipitation phase better than T RH through a greater depth of unsaturated layer (Figure 3).
This could be due to the differences in latent and sensible heat fluxes between a hydrometeor and the environment in a moist/low cloud base height environment compared with a dry/high cloud base height environment.
In a mountain environment often saturated (in cloud), Marks et al. () found WB ≈ DP ≈ AT, and a DP 0 C to be a better indicator of precipitation phase than ATTs, often calibrated warmer than 0 C. Swedish cloud base height ATTs near the ground start closer to 0 C than Norwegian cloud base height ATTs ( Sweden results using cloud base height ATTs, and T RH depended on the physiographic group, but generally improved ( Figure 5) with some exceptions in the coastal environments. Perhaps a future addition of onshore or offshore influence could help in these coastal areas.
Since assigning a precipitation phase is a crucial first step in cold region hydrological models (Kongoli & Bland ), improvements in PPDS skill should result in better information driving water resource management decisions. Here, using all observations with or without a RH or cloud base height, misclassified precipitation between À3 and 5 C was reduced from 10.62% (9.86%) to 10.16% (9.58%) using cloud base height ATTs, 9.07% (8.98%) using T RH and 8.91% (8.79%) using a combination of T RH and cloud base height ATT methods in Norway (Sweden) ( Table 3). This indicates that either method or both combined could be used to reduce precipitation phase determination errors in conceptual surfacebased models. There is also potential to improve future studies by filling data gaps (removed observations) and expand station selection options by using simulated RH values (averaged winter grid-square RH) in a reanalysis step as was done by  3. Improvements to precipitation phase determination from traditional ATT by applying T RH and cloud base height ATT should yield fairly universal improvements across diverse landscapes because they indirectly adjust for the atmospheric properties that control phase change before precipitation reaches the ground.
4. Short of adding atmospheric datum, the PPDS in conceptual hydrological models could be improved by adding surface observational data that act as proxies for the atmospheric conditions a hydrometeor must interact with as it falls to the ground.

DATA AVAILABILITY STATEMENT
All meteorological RAW data used in this study is public information available at no charge through the Norwegian Meteorological Institute and the Swedish Meteorological and Hydrological Institute.