**AGLMIX**

**Description/Purpose:** This tool will return values using either a
layer or a height defined by an above ground layer (AGL). For
temperatures, the tool finds a value at a specified height AGL and
brings it down to the surface using the desired lapse rate. For
dewpoints, an average mixing ratio from the surface to the desired
height is calculated. A new dewpoint value is calculated from this
average mixing ratio. The intention of this tool is to help mitigate the
effects of sloping terrain, and also help enhance temperature forecasts
by easily allowing terrain effects to be placed in the grids. Since this
tool uses GFE topography, it will actually have higher resolution
terrain details than the model, as coarser model values are fitted to
the terrain. To help illustrate the purpose of tool the following image
shows the height in dm of the H85 level across the area.

![presagl.PNG](media/image1.png){width="6.0in"
height="5.541666666666667in"}

Notice that the height of this level varies from 80 dm (800 m) at KHLC
to 20 dm (200m) at KYMA. If you were to assign a uniform temperature at
this level, say 10C this would be the following result of "mixing down"
temperatures from a constant pressure level, assuming a dry adiabatic
lapse rate of 9.8 C/km.

KYMA

sfct = (9.8 (C/km) \* .2 km) + 10.0 C = 11.96C (53.53 F)

KHLC

sfct = (9.8 (C/km) \* .8 km) + 10.0 C = 17.84C (64.11 F)

Not only due large differences in temperatures occur using the constant
pressure level, but this level means two different things for each
location with respect to mixing. At YMA this level is very near the
ground, and in most cases would easily be mixed to this point. At HLC
this level is significantly higher, and may never be reached if other
conditions inhibit deep mixing. This tool will take some of these
problems out of the equation, as 1 km AGL may be nearly H7 at KYMA, H75
at KGLD and H85 at KHLC, yet running one tool will show the values from
all these pressure levels. For dewpoints the problem is even more
amplified as using one pressure level may be very shallow in the west,
but very deep in the east.

**Grids Edited :** T and Td

**Assumptions:** When sounding intersections fall in between model
levels, all variables (except pressure) are assumed to be linear. If
pressure needs to be calculated, the hypsometric equation is used. Also
assumes moist adiabatic lapse rate is constant at 6.5 C/Km.

**Running the Tool:** With either a T or Td selected, right click in the
grid editor and select AGLMIX. The following GUI will then appear :

![AGL.PNG](media/image2.png){width="5.824324146981628in"
height="6.035621172353456in"}

[Height :]{.underline} This variable sets the height that the tool will
use for its calculations. For instance if 2KM is selected when running
tool on a T grid, then the tool will first figure out the value of T at
2 km AGL in the model data at each point.

[User Height :]{.underline} If the USER ENTERED option is selected, you
can select a value between 100 and 5000 m to use as a mixing height.
This value is in meters NOT kilometers.

[Adiabatic Lapse Rate :]{.underline} Selecting DRY will use 9.8 C/km to
bring temperatures down to the surface, MOIST will use 6.5 C/km and
SUPER uses 11 C /km. This variable does nothing when calculating
dewpoints.

[Model:]{.underline} Chooses a model to use for calculations.

[Model Version?]{.underline} : Selects the desired model version.

**Known Issues/Bugs:**

Tool sometimes will create small pixels of bad data, when very shallow
mix depths are used. This is particularly true over the mountains.
Recent improvements in the interpolation scheme have improved this
greatly but it does happen every once and a while.

A constant mixing height is not always representative, as values can
range significantly. For example, on either side of a dryline large
differences in mixing height can occur. This is not a problem with the
tool, but forecasters should realize this limitation.

Although no formal studies of verifying this method have been done, but
from personal use of the tool it seems to do well at forecasting
afternoon temperatures but is generally a few degrees cool on the actual
maximum value. This may be a function of a slight super adiabatic layer
forming in the afternoon, or just model errors.

Determining an optimal depth for mixing an afternoon Td is still very
uncertain. Local research would suggest that using a height based on the
maximum 3 hourly temperature provides the best result, but converting
this to an AGL height operationally is difficult. Also several other
factors go into the dewpoint temperature, and horizontal advection may
totally overwhelm vertical mixing.

Some operational models, especially the NAM can have very bad surface
dewpoint values. While this tool tries to correct these fields, the
surface data does get into the averaging and could skew the results. Be
aware that the tool may give values that are too high if models are
greatly overestimating the dewpoint before mixing takes place. A future
upgrade to this is to allow forecasters to override the surface values
with something more representative.