Search Results
Abstract
Subgrid-scale turbulence in numerical weather prediction models is typically handled by a PBL parameterization. These schemes attempt to represent turbulent mixing processes occurring below the resolvable scale of the model grid in the vertical direction, and they act upon temperature, moisture, and momentum within the boundary layer. This study varies the PBL mixing strength within 4-km WRF simulations of a 26–29 January 2015 snowstorm to assess the sensitivity of baroclinic cyclones to eddy diffusivity intensity. The bulk critical Richardson number for unstable regimes is varied between 0.0 and 0.25 within the YSU PBL scheme as a way of directly altering the depth and magnitude of subgrid-scale turbulent mixing. Results suggest that varying the bulk critical Richardson number is similar to selecting a different PBL parameterization. Differences in boundary layer moisture availability, arising from reduced entrainment of dry, free tropospheric air, lead to variations in the magnitude of latent heat release above the warm frontal region, producing stronger upper-tropospheric downstream ridging in simulations with less PBL mixing. The more amplified flow pattern impedes the northeastward propagation of the surface cyclone and results in a westward shift of precipitation. In addition, trajectory analysis indicates that ascending parcels in the less-mixing simulations condense more water vapor and terminate at a higher potential temperature level than do ascending parcels in the more-mixing simulations, suggesting stronger latent heat release when PBL mixing is reduced. These results suggest that spread within ensemble forecast systems may be improved by perturbing PBL mixing parameters that are not well constrained.
Abstract
Subgrid-scale turbulence in numerical weather prediction models is typically handled by a PBL parameterization. These schemes attempt to represent turbulent mixing processes occurring below the resolvable scale of the model grid in the vertical direction, and they act upon temperature, moisture, and momentum within the boundary layer. This study varies the PBL mixing strength within 4-km WRF simulations of a 26–29 January 2015 snowstorm to assess the sensitivity of baroclinic cyclones to eddy diffusivity intensity. The bulk critical Richardson number for unstable regimes is varied between 0.0 and 0.25 within the YSU PBL scheme as a way of directly altering the depth and magnitude of subgrid-scale turbulent mixing. Results suggest that varying the bulk critical Richardson number is similar to selecting a different PBL parameterization. Differences in boundary layer moisture availability, arising from reduced entrainment of dry, free tropospheric air, lead to variations in the magnitude of latent heat release above the warm frontal region, producing stronger upper-tropospheric downstream ridging in simulations with less PBL mixing. The more amplified flow pattern impedes the northeastward propagation of the surface cyclone and results in a westward shift of precipitation. In addition, trajectory analysis indicates that ascending parcels in the less-mixing simulations condense more water vapor and terminate at a higher potential temperature level than do ascending parcels in the more-mixing simulations, suggesting stronger latent heat release when PBL mixing is reduced. These results suggest that spread within ensemble forecast systems may be improved by perturbing PBL mixing parameters that are not well constrained.
Abstract
Numerical simulations are performed with the Weather Research and Forecasting Model to elucidate the diabatic effects of ice phase microphysical processes on the dynamics of two slow-moving summer cyclones that affected the United Kingdom during the summer of 2012. The first case is representative of a typical midlatitude storm for the time of year, while the second case is unusually deep. Sensitivity tests are performed with 5-km horizontal grid spacing and at lead times between 1 and 2 days using three different microphysics schemes, one of which is a new scheme whose development was informed by the latest in situ observations of midlatitude weather systems. The effects of latent heating and cooling associated with deposition growth, sublimation, and melting of ice are assessed in terms of the impact on both the synoptic scale and the frontal scale. The results show that, of these diabatic processes, deposition growth was the most important in both cases, affecting the depth and position of each of the low pressure systems and influencing the spatial distribution of the frontal precipitation. Cooling associated with sublimation and melting also played a role in determining the cyclone depth, but mainly in the more intense cyclone case. The effects of ice crystal habit and secondary ice production are also explored in the simulations, based on insight from in situ observations. However in these two cases, the ability to predict changes in crystal habit did not significantly impact the storm evolution, and the authors found no obvious need to parameterize secondary ice crystal production at the model resolutions considered.
Abstract
Numerical simulations are performed with the Weather Research and Forecasting Model to elucidate the diabatic effects of ice phase microphysical processes on the dynamics of two slow-moving summer cyclones that affected the United Kingdom during the summer of 2012. The first case is representative of a typical midlatitude storm for the time of year, while the second case is unusually deep. Sensitivity tests are performed with 5-km horizontal grid spacing and at lead times between 1 and 2 days using three different microphysics schemes, one of which is a new scheme whose development was informed by the latest in situ observations of midlatitude weather systems. The effects of latent heating and cooling associated with deposition growth, sublimation, and melting of ice are assessed in terms of the impact on both the synoptic scale and the frontal scale. The results show that, of these diabatic processes, deposition growth was the most important in both cases, affecting the depth and position of each of the low pressure systems and influencing the spatial distribution of the frontal precipitation. Cooling associated with sublimation and melting also played a role in determining the cyclone depth, but mainly in the more intense cyclone case. The effects of ice crystal habit and secondary ice production are also explored in the simulations, based on insight from in situ observations. However in these two cases, the ability to predict changes in crystal habit did not significantly impact the storm evolution, and the authors found no obvious need to parameterize secondary ice crystal production at the model resolutions considered.
Diabatic Heating and Cooling Rates Derived from In Situ Microphysics Measurements: A Case Study of a Wintertime U.K. Cold Front
Abstract
In situ measurements associated with the passage of a kata cold front over the United Kingdom on 29 November 2011 are used to initialize a Lagrangian parcel model for the purpose of calculating rates of diabatic heating and cooling associated with the phase changes of water within the cloud system. The parcel model calculations are performed with both bin-resolved and bulk treatments of microphysical processes. The in situ data from this case study reveal droplet number concentrations up to 100 cm−3, with planar ice crystals detected at cloud top, as well as columnar crystals produced by rime splinter ejection within the prefrontal warm sector. The results show that in terms of magnitude, the most significant rates of diabatic heating and cooling are produced by condensation growth of liquid water within the convective updrafts at the leading edge of the front. The peak temperature tendencies associated with condensation are typically found to be at least an order of magnitude larger than those associated with the ice phase, although the cooling effect from sublimation and melting occurs over a wide region. The parcel model framework is used in conjunction with the observations to assess the suitability of existing bulk microphysical treatments, of the kind used in operational weather forecast models. It is found that the assumption of spherical ice crystals (with diameters equal to the maximum dimension of those sampled), along with the use of negative exponential functions to describe ice particle size distributions, can lead to an overestimation of local diabatic heating and cooling rates by a factor of 2 or more.
Abstract
In situ measurements associated with the passage of a kata cold front over the United Kingdom on 29 November 2011 are used to initialize a Lagrangian parcel model for the purpose of calculating rates of diabatic heating and cooling associated with the phase changes of water within the cloud system. The parcel model calculations are performed with both bin-resolved and bulk treatments of microphysical processes. The in situ data from this case study reveal droplet number concentrations up to 100 cm−3, with planar ice crystals detected at cloud top, as well as columnar crystals produced by rime splinter ejection within the prefrontal warm sector. The results show that in terms of magnitude, the most significant rates of diabatic heating and cooling are produced by condensation growth of liquid water within the convective updrafts at the leading edge of the front. The peak temperature tendencies associated with condensation are typically found to be at least an order of magnitude larger than those associated with the ice phase, although the cooling effect from sublimation and melting occurs over a wide region. The parcel model framework is used in conjunction with the observations to assess the suitability of existing bulk microphysical treatments, of the kind used in operational weather forecast models. It is found that the assumption of spherical ice crystals (with diameters equal to the maximum dimension of those sampled), along with the use of negative exponential functions to describe ice particle size distributions, can lead to an overestimation of local diabatic heating and cooling rates by a factor of 2 or more.
