**P15.3** AN EVALUATION OF MESOSCALE MODEL SIMULATIONS OF AN\
OSCILLATING DRYLINE

Mark R. Conder^\*^ and Richard E. Peterson

Department of Geosciences, Texas Tech University, Lubbock, Texas

1.  **INTRODUCTION**

One of the many interesting forecasting "problems" facing the
operational meteorologist is determining the evolution of the high
plains dryline. Forecasts of the afternoon dryline position are
frequently critical to the success of a convective initiation forecast.
Numerous previous studies have already established the dryline's role of
providing a near-surface boundary, enhancing the lift and convergence
necessary for the development of thunderstorms. The quiescent dryline
may be described as one that where the influence from a translating
synoptic scale system is absent (Schaefer, 1986). Although the
synoptically quiescent dryline does not generate as much severe
convection as its dynamically-forced cousin, it is not uncommon to see
isolated severe weather associated with it. As the horizontal
resolutions of operational models increase (e.g. NCEP ETA from 22 to 12
km in 2001), forecasts of the dryline are becoming increasingly
accurate. The quiescent dryline exists in a synoptic environment
characterized by weak (but still meeting some minimum requirements)
flow. This implies that the model PBL schemes will have paramount
effects on the dryline forecast. For example, the daytime eastward
motion of the dryline is accomplished through vertical turbulent mixing
of the PBL (Hane, 2003). Previous model studies have shown that
topography and soil moisture gradients are two very important controls
that can affect the evolution of this process (e.g. Grasso et al.,
2001). For this study, simulations were conducted of an oscillating
southern plains dryline from 14-18 April 2002. Minimal thunderstorm
activity occurred during this episode, so that storm-scale feedback to
the environment was minimized. Two mesoscale models were employed, the
PSU-NCAR MM5 and the CSU-ATMET RAMS. Near-surface in-situ measurements
of the observed dryline are provided by 30 stations of the West Texas
Mesonet. A comparison of model output to the observations is performed
through a spatial analysis of dryline position and the calculation of
mean bias and rmse of the water vapor mixing ratio.

2.  **THE DRYLINE OF 14-18 APRIL 2001**

The evolution of synoptic-scale features associated with this dryline
episode is fairly typical of early-season patterns. A fast-moving
shortwave moved through the central U.S. on 13-14 and had sent a shallow
cold front into the region.

\* Mark R. Conder, Texas Tech University,

Department of Geosciences, Lubbock, TX

79409-2101; email: mark.conder@ttu.edu

As southwesterly 500 hPa flow became established across the west-central
U.S., a low level lee-side trough and southerly jet at 850 hPa
developed. This enabled low-level moisture to be drawn northward from
the Gulf of Mexico, first across south and central Texas and then up
onto the Caprock. Finally, by the 19^th^, another shallow cold front
dropped into the region, scouring out the deeper moisture. Figure 2
shows an overview of the synoptic features. The surface dryline
oscillated across the domain during the period. In general the dryline
began to move eastward in the morning, stalling during the late
afternoon and early evening, and then reversing during the night.
However, each oscillation of the dryline exhibited differences in speed,
timing, direction, and distance traveled before stalling. Figure 3
depicts the evolution of the dryline. The motion of the dryline can also
be readily seen via a time series of wind and dewpoint shown in Figure
4.

2.  **MODEL SIMULATIONS**

Both the MM5 and RAMS models are generally classified as
three-dimensional, non-hydrostatic primitive equation models. They can
be configured with a wide variety of options and resolution depending on
the phenomena undergoing investigation. Both have been successfully used
with mesoscale resolution to simulate drylines in several
investigations. For this study, the model runs were configured to be as
similar as possible. Table 1 shows some of the configuration model
details for the study.

Table 1: Short list of configuration options used in\
the model simulations.

  ------------------------- ---------------------------------------------
  **Parameter**             **Specification**

  Number of grids           2

  Grid resolution           18 km (grid 1), 6 km (grid 2)

  Dimensions                47 by 47 (grid 1 and 2)

  Grid center point         24.0 lat, -101.7 lon

  Initialization            NCEP ETA model initialization (40 km AWIPS
                            Grid 212)

  Simulation\               60 hours
  duration                  

  Output                    Lowest sigma-levels every hour

  Lateral boundary\         NCEP ETA model every 12 hours
  nudging                   

  PBL Scheme                MM5: MRF (1^st^ order closure)\
                            RAMS: Mello-Yamada TKE K

  Elevation data            USGS 30-sec (inner grid)

  Land use data             USGS 1-km
  ------------------------- ---------------------------------------------

The model domains were configured with two considerations in mind.
Foremost was the horizontal resolution required to accurately model the
dryline. A 6-km grid spacing (inner grid) was chosen. Typically, between
four and six grid points are required to resolve a meteorological
feature. Thus, a 6-km grid can resolve features approximately 24 to 36
km or greater in horizontal dimension. Secondly, for verification, the
model output was to be compared with an OA grid created from
observations from the surface network. The \~20 km average spacing of
the West Texas Mesonet compares favorably to the model resolution. The
model domains are shown in Figure 1.

![](media/image1.png){width="2.513888888888889in"
height="2.4680555555555554in"}

![](media/image2.png){width="2.4923611111111112in"
height="2.2868055555555555in"}

Figure 1. a) Domains of the MM5 and RAMS model configuration showing the
coarse and nested grids. The domains are centered near the Reese Center
West Texas Mesonet station. b) The topography of the inner grid is
contoured and shaded every 100 meters. The Caprock Escarpment is
approximately located along the 900 meter contour and the large valley
on the north side of the inner grid is Palo Duro canyon.

3.  **ANALYSIS OF MODEL RESULTS**

Figures 5 and 6 respectively depict the modeled dryline for the MM5 and
RAMS simulations. First of all, the simulations show that the
simulations do create a moisture gradient along with a wind shift that
is similar to observed dryline behavior. These figures also show that
the accuracy of the location of the 8.5 g kg^-1^ contour generally
decreases with each iteration, particularly in the case of the RAMS
model. While both models depict the diurnal oscillation on the 15-16th,
the MM5 begins with the dryline located too far east (figure 5a), while
the RAMS initial location is more accurate. The MM5 also retreats the
dryline too early in the evening. The RAMS timing is better, however it
begins to diverge from the observed dryline by greatly underestimating
the extent of the westward retreat (figures 3b and 6b). During the day
of the 16^th^, both models advance the dryline too quickly eastward
(figures 5c and 6c). The MM5 begins the dryline's retreat several hours
too early, but is roughly accurate with the dryline's position by the
morning of the 17^th^ (Figure 5d). The RAMS simulation however, does not
retreat the dryline back into the experimental domain. A plot of the
mixing ratio trace from the Reese Site (approximate center of the
domain) shown in Figure 4 can be compared to the model forecasts. Figure
7a shows that the MM5 forecast looks similar in a broad sense. However,
one can notice that in general the change in mixing ratio occurs much
quicker in the observed data. The RAMS forecast falters fairly quickly,
underestimating even the first moisture return. By examining the model
forecasts of the u-component of the 10 meter wind in figure 8, one
reason for the RAMS failure to forecast the westward motion of the
retreating dryline is due to the lack of an easterly component to the
forecast wind.

4.  **ACKNOWLDGEMENTS**

The authors wish to thank the Wes Burgett of the West Texas Mesonet for
providing the surface data. The authors also thank Tim Doggett, Air
Worldwide Corp., and David Ovens, Univ. of Wash., for their assistance
with converting the model output to GEMPAK format.

This research was funded in part by the U.S. Army Department of Defense
contract #DAAD-13-00C-0048.

5.  **REFERENCES**

Grasso, Lewis D., 2000: A numerical simulation of\
dryline sensitivity to soil moisture. *Mon. Wea.\
Rev.,* **128**, 2816-2834.

Hane, Carl E., 2004: Quiescent and synoptically-active\
drylines: A comparison based upon case studies.\
*Meteor. Atmos. Physics.,***86-3**,195-211.

Schaefer, Joseph T., 1986: The dryline. *In Mesoscale\
meteorology and forecasting* (Ray, P. S., ed),\
American Meteorological Society, Boston, MA, pp\
549-572.

![](media/image3.png){width="6.492361111111111in"
height="5.944444444444445in"}

Figure 2. Objectively analyzed constant pressure maps for a) 04-14-2002
12 GMT at 500 hPa, b) 04-14-2002 12 GMT at 850 hPa, c) 04-17-2002 12 GMT
at 500 hPa, and d) 04-14-2002 12 GMT at 850 hPa. 500 hPa geopotential
heights are contoured every 40 meters. 850 hPa heights are contoured
every 30 meters. Station plot depicts the wind in knots, temperature and
dewpoint in degrees Celsius, the 500 hPa heights in decameters and the
850 hPa heights in meters minus 1000. Units of absolute vorticity
shading on the 500 hPa maps are 10^-5^s^-1^. Dewpoints greater than 6
degrees Celsius are shaded on the 850 hPa maps.

![](media/image4.png){width="2.863888888888889in"
height="3.2805555555555554in"}
![](media/image5.png){width="2.863888888888889in"
height="3.270138888888889in"}

![](media/image6.png){width="2.863888888888889in"
height="3.2597222222222224in"}
![](media/image7.png){width="2.863888888888889in"
height="3.2597222222222224in"}

> Figure 3. Plots of the 8.5 gkg^-1^ mixing ratio contour for the
> oscillating dryline. a) 16 -- 19 GMT 15 April,\
> b). 04 -- 11 GMT 16 April, c) 14 -- 18 GMT 16 April, and d) 11 -- 16Z
> 17 April.

![](media/image8.png){width="2.863888888888889in"
height="3.2597222222222224in"}
![](media/image9.png){width="2.863888888888889in"
height="3.2805555555555554in"}

![](media/image10.png){width="2.863888888888889in"
height="3.2597222222222224in"}
![](media/image11.png){width="2.863888888888889in"
height="3.238888888888889in"}

> Figure 3 continued. Plots of the 8.5 gkg^-1^ mixing ratio contour for
> the oscillating dryline. e) 19 -- 23 GMT 17\
> April, f). 00 -- 08 GMT 18 April, g) 13 -- 20 GMT 18 April, and h) 21
> GMT 18 April -- 06Z\
> 19 April.

![](media/image12.png){width="5.686805555555556in"
height="3.957638888888889in"}

> Figure 4. Time series plots of dewpoint temperature and wind direction
> from the West Texas\
> Mesonet site at Reese Center -- near the center of the domain. The
> four dryline\
> oscillations can be seen in the trace. Note the approximate
> mirror-image appearance to\
> the two graphs, with veering winds associated with decreasing dewpoint
> and vise versa.

![](media/image13.png){width="2.942361111111111in"
height="3.3090277777777777in"} ![](media/image14.png){width="2.95in"
height="3.2569444444444446in"}

![](media/image15.png){width="2.95in" height="3.2569444444444446in"}
![](media/image16.png){width="2.95in" height="3.270138888888889in"}

> Figure 5. Plots of the 8.5 gkg^-1^ mixing ratio contour from the MM5
> output for: a) 16 -- 19 GMT 15 April,\
> b). 23 -- 06 GMT 15-16 April, c) 08 -- 13 GMT 16 April, and d) 08 --
> 15 GMT 17 April.

![](media/image17.png){width="2.95in" height="3.317361111111111in"}
![](media/image18.png){width="2.95in" height="3.3305555555555557in"}

![](media/image19.png){width="2.95in" height="3.3305555555555557in"}
![](media/image20.png){width="2.95in" height="3.3430555555555554in"}

> Figure 6. Plots of the 8.5 gkg^-1^ mixing ratio contour from the RAMS
> output for: a) 16 -- 19 GMT 15 April,\
> b). 04 -- 11 GMT 16 April, c) 14 -- 18 GMT 16 April, and d) 11 -- 16Z
> 17 April.

![](media/image21.png){width="5.0in" height="2.5in"}

![](media/image22.png){width="5.0in" height="2.5in"}

> Figure 7. Plots of the mixing ratio (gkg^-1^) from the a) MM5 output,
> b) RAMS output, interpolated to the\
> Reese Center Mesonet site (refer to figure 4 for comparison).

![](media/image23.png){width="5.0in" height="2.5in"}

![](media/image24.png){width="5.0in" height="2.5in"}

> Figure 8. Plots of the u-component of the surface wind (m s^-1^) from
> the a) MM5 output, b) RAMS output,\
> interpolated to the Reese Center Mesonet site (refer to figure 4 for
> comparison).