Timm Uhlmann
AtmSci 950 HW2 Report
Sensitivity of Bow Echo Simulations to Microphysical Parameterizations
By Rebecca D. Adams-Selin
1) Introduction
Computer models often use bulk microphysical schemes to parameterize the overall
precipitation and organization of weather systems such as squall lines and supercells. Further as
shown in the past by numerous authors cited within this paper, and as we have been learning in
class all semester, small changes to these parameterizations can have a large impact upon the
overall model result. The inclusion of ice brought more realistic stratiform rainfall to multiple
squall line simulations and additional cooling and melting brought on faster moving systems and
more intense convection. Of interest to this paper as well, the difference in hail to graupel brought
changes to previous study simulations. For squall lines, hail brings increasing fall speeds, a weaker
cold pool, and a slower system. Similarly, supercells with graupel received stronger updrafts due
to increased latent heat from deposition, ultimately increasing convective strength, hail size, and
slowing the system down thanks to the weaker cold pool. Previous studies utilizing hail-only and
graupel-only schemes were compared, with the latter having significantly decreased precipitation.
Studies that omitted graupel completely and used more widespread snow parameters resulted in
more widespread, lighter precipitation. Lastly, previous studies showed that a decrease in mean
graupel size ultimately led to a faster development of the bow echo from linear systems. The
previous studies mentioned all either dealt with idealized cases, were more focused on “long-term
statistical verification”, or were focused upon the dynamics of that particular case. Motivated by
the importance of ice parameterization, this study utilized eight different microphysical schemes
revolving around the graupel parameter and their impact on “1) timing of new bowing, 2)
maximum 10-m wind gust speed, 3) system propagation speed, 4) convective and stratiform areal
coverage, and 5) total accumulated precipitation” (pp. 1189). To aid in comparison to a real event
as well as between simulations, the Weather Research and Forecasting model was utilized to
simulate the different changes to a bow echo case that occurred over Oklahoma.
2) Methodology
a) Model Setup
As mentioned previously, the Weather Forecasting and Research Model was chosen for
this study for its ability to simulate mesoscale phenomenon. The boundary conditions for
the simulation were from the National Center for Environmental Prediction Global
Forecast System Final Analysis set a 1 x 1 degree horizontal and 6 hour temporal
resolution. Parameterization schemes other than microphysics include the Mellor–
Yamada– Janjic boundary layer scheme, the Noah land surface model, the Rapid Radiative
Transfer Model longwave radiation scheme, and the Goddard shortwave radiation scheme.
The model was initialized at 1200 UTC on 12 March 2003, and run for 36 h to 0000 UTC
14 March 2003.
b) Microphysical Parameterizations
The meat of this paper, the eight different microphysical schemes utilized for the
simulations were Kessler, Ferrier, WRF single-moment five-class , WRF double-moment
five-class, Lin, WRF single-moment six-class, WRF double-moment six-class, and
Morrison. As discussed in the introduction, the variation of ice parameters can greatly
influence the overall impact of the simulation with the Kessler scheme being the least
complex as it does not consider frozen hydrometeors and the Morrison as the most complex
as it considers four double-moment classes for hydrometeor mixing ratio and total number
concentration. On Table 1 is the summary provided in the paper of all eight schemes. Four
of the schemes don’t consider graupel at all: Kessler, Ferrier, WSM5, and WDM5, while
the other four vary in graupel density, fall speed, and intercept. In general, including larger
graupel particles and higher fall speeds will increase the overall downward mass flux while
simultaneously decreasing the overall melting and evaporation through the lower levels,
thus decreasing the overall cooling and strength of the cold pool for the storm.
c) Convective Area Determination
A single radar reflectivity algorithm was utilized in this study to keep results consistent
across the board for all eight parameterizations. These were compared to the composite
reflectivity values of the Oklahoma City Radar NWS 88D for the storm and taking the
maximum value found at any given point in the column. All values below 40 dBZ (15-40)
were considered to be stratiform while those above as convective. Radar data was then fit
to a 3 km resolution using the nearest neighbor interpolation allowing for direct comparison
between simulated and observed radar values.
3) Key Findings
The schemes with the smaller and slower falling graupel hydrometeors had much stronger cooling
rates in the lower regions of the storm thanks to the overall increase in surface area and time until
impact with the ground increasing the opportunities for melting and evaporative cooling. In terms
of the meteorological variables studied, the bowing time was strongly affected as a result of the
increased strength for the cold pool. In the case of the WRF single moment scheme, which could
be adjusted from more graupel like to more hail-like particles, the latter produced a bow 105
minutes later than the former. The stronger cold pools also brought an increase in wind strength at
10-m (73%). On the other hand, the heavier and faster falling particles produced much heavier rain
rates, up to six times the amount of the slower and smaller graupel simulations. This is perhaps to
be expected from the greater overall downwards mass flux. The difference between the more
graupel-like and more hail-like hydrometeors then leads to a great disparity in the accumulated
precipitation coverage and making this the most sensitive variable considered. Found in the paper
they conclude: “Changing the graupel parameter to be more graupel like or hail like had a larger
effect than even switching between single- and double-moment rain or graupel distributions, or
removing graupel altogether” (pp. 1206).
The impact on convective and stratiform regions were also found in the radar analysis between
observed and simulated. For the WSM6 configurations used to specifically test for the graupel
sensitivities they found variances of 65% (127%) for stratiform and convective areas respectively.
4) Future Work
One of the more evidenced opportunities for future work will be changing the algorithm used to
simulate radar reflectivity. There are many schemes out there to provide the simulation and the
choice of algorithm will greatly impact the end result when comparing to the observed cases, and
perhaps also between simulations as the size of particle (in this case simulated graupel and rain) is
a large factor in radar returns. Doing this would be a matter of finding another radar algorithm and
using it to compare with the results of this study as well as to the observed values.
Additionally, as mentioned in the paper, currently there is little in the way of field or observed data
for the parameters selected in schemes for intercept and fall speed. This would not be as feasible
for a numerical simulation as more observed data would be needed in this case to adjust the
parameters.
Bibliography
Rebecca D. Adams-Selin, Susan C. van den Heever, and Richard H. Johnson, 2013: Sensitivity
of Bow-Echo Simulation to Microphysical Parameterizations. Wea. Forecasting, 28, 1188–1209.