The IMPROVE-1 Storm of 1–2 February 2001. Part III: Sensitivity of a Mesoscale Model Simulation to the Representation of Snow Particle Types and Testing of a Bulk Microphysical Scheme with Snow Habit Prediction

Abstract

A mesoscale model simulation of a wide cold-frontal rainband observed in the Pacific Northwest during the Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE-1) field study was used to test the sensitivity of the model-produced precipitation to varied representations of snow particles in a bulk microphysical scheme. Tests of sensitivity to snow habit type, by using empirical relationships for mass and velocity versus diameter, demonstrated the defectiveness of the conventional assumption of snow particles as constant density spheres. More realistic empirical massdiameter relationships result in increased numbers of particles and shift the snow size distribution toward larger particles, leading to increased depositional growth of snow and decreased cloud water production. Use of realistic empirical mass-diameter relationships generally increased precipitation at the surface as the rainband interacted with the orography, with more limited increases occurring offshore. Changes in both the mass-diameter and velocity-diameter relationships significantly redistributed precipitation either windward or leeward when the rainband interacted with the mountain barrier.A method of predicting snow particle habit in a bulk microphysical scheme, and using predicted habit to dynamically determine snow properties in the scheme, was developed and tested. The scheme performed well at predicting the habits present (or not present) in aircraft observations of the rainband. Use of the scheme resulted in little change in the precipitation rate at the ground for the rainband offshore, but significantly increased precipitation when the rainband interacted with the windward slope of the Olympic Mountains. The study demonstrates the promise of the habit prediction approach to treating snow in bulk microphysical schemes.

The role of precipitation size distributions in km‐scale NWP simulations of intense precipitation: evaluation of cloud properties and surface precipitation

“…This supports that similar findings for single case-studies of stratiform precipitation (e.g. Thompson et al, 2004;Colle et al, 2005;Woods et al, 2007) can be generalized to a broad range of synoptic conditions and can cautiously be interpreted as model improvements. Table 5 and Figure 8(a) and (b) provide an overview of the domain average and maximum 24-hour (0000-2400 UTC) accumulated surface precipitation for all experiments, and as derived from combined radar and rain-gauge information for the stratiform composite (blue symbols).…”

“…From the higher precipitation rates in Figure 12, the more efficient removal of excess water vapour in ExpGS also likely enhances precipitation efficiency and hence surface precipitation. Woods et al (2007) found a similar impact of lower relative humidity with respect to ice and enhanced surface precipitation in a cold front simulation, when using non-spherical instead of spherical snowflakes. Similar to our findings for a broad range of stratiform cases, Colle et al (2005) also found a 10 to 15% increase in surface precipitation for frontal stratiform simulations when a temperature-dependent N 0S was applied as compared to using a fixed N 0S .…”

“…Both of these modifications have been shown before to enhance the depositional growth of snow at high altitudes in case-studies. Thompson et al (2004) have shown this for the temperature-dependent N 0S in an idealized simulation of orographic precipitation, and Woods et al (2007) found this for non-spherical snow particles in a real-case simulation of a cold-frontal rain band. Figure 6 shows the impact of these modifications to the snow size distribution on the theoretical snow depositional growth as a function of temperature.…”

“…It should be mentioned that, while the modified representation of the snow size distribution might be a more realistic approach compared to the original assumptions for constant-density spheres in Lin et al (1983), it does not fully account for the variation of vapour diffusion with the shape of snow and graupel. Woods et al (2007) for instance showed a pronounced impact on the three-dimensional precipitation structure applying such a habit-prediction for a cold frontal rain band, in which the empirical constants a m and b m were varied in accordance with particle vapour diffusion differences for temperature and moisture regimes. An overview of the size distribution assumptions applied in all experiments is given in Table 2.…”

“…For frontal stratiform precipitation events, surface precipitation was sensitive to the representation of the snow size distribution and fall velocity (Thompson et al, 2004;Colle et al, 2005;Serrafin and Ferretti, 2007;Woods et al, 2007). For convective precipitation events, the size and nature of the largest rimed ice species (either hail or graupel) have been shown to have a significant impact on surface precipitation (Gilmore et al, 2004;van den Heever and Cotton, 2004;Cohen and McCaul, 2006;Morrison and Milbrandt, 2011).…”