copy of http://www.drjack.info/BLIP/INFO/parameters.html
BLIPMAP Parameters and Description
RASP BLIPMAP Prediction Parameters and Description
RASP = Regional Atmospheric Soaring Predictions
BLIPMAP = Boundary Layer Information Prediction MAP
NB: The atmospheric Boundary
Layer (BL) is the vertical region above the surface within which air has been mixed by thermal or windshear eddies, i.e. the region where
glider pilots normally fly.
Most forecasts require only a few of the parameters presented below,
so those new to BLIPMAPs should first read the
Basic Parameters webpage
for an introduction to those parameters most important when forecasting
thermal soaring conditions.
THERMAL PARAMETER FORECASTS:
-
Surface Temperature
-
The temperature at a height of 2m above ground level. This can
be compared to observed surface temperatures as an indication of model
simulation accuracy; e.g. if observed surface temperatures are
significantly below those forecast, then soaring conditions will be
poorer than forecast. This parameter is obtained directly from
WRF model output and not from a BLIPMAP computation.
-
Surface Sun
-
The solar radiation reaching the surface. A fraction of this
radiation goes into heating the air (see the "Surface Heating"
parameter, to which this parameter can be compared). This
parameter is obtained directly from WRF model output and not from a
BLIPMAP computation.
-
Normalized Surface Sun
-
The "Surface Sun" parameter normalized (divided) by the amount of
solar radiation which would reach the surface in a dry
atmosphere (i.e. in the absence of clouds and water vapor), expressed
as a percentage. This parameter indicates the degree of
cloudiness, i.e. where clouds limit the sunlight reaching the
surface.
-
Surface Heating
-
Heat transferred into the atmosphere due to solar heating of the
ground, creating thermals. This parameter is an important
determinant of thermals strength (as is the BL depth). This
parameter is obtained directly from WRF model output and not from a
BLIPMAP computation. MoreInfo
-
Boundary Layer Depth
-
Depth of the layer mixed by thermals or (vertical) wind shear. This
parameter can be useful in determining which flight direction allows
better thermalling conditions when average surface elevations vary
greatly in differing directions. (But the same cautions mentioned under
"Height of BL Top" also apply.) It is also an important
determinant of thermals strength (as is the Surface Heating).
MoreInfo
-
Height of Boundary Layer Top
-
Height of the top of the mixing layer, which for thermal convection is
the average top of a dry thermal. Over flat terrain, maximum
thermalling heights will be lower due to the glider descent rate and
other factors. In the presence of clouds (which release
additional buoyancy aloft, creating "cloudsuck") the updraft top will be above this
forecast, but the maximum thermalling height will then be limited by
the cloud base (see the "Cloud prediction parameters" section
below). Further, when the mixing results from shear turbulence
rather than thermal mixing this parameter is not useful for glider
flying. NB: this BL Top is not the height where the "Thermal Index" (TI)
is zero, which is a criteria used by many simple determinations of the BL top - instead,
the RASP BL Top uses a more sophisticated BL Top criteria based on turbulent fluxes.
MoreInfo
-
Height of Critical Updraft Strength (Hcrit)
-
This parameter estimates the height at which the average dry
updraft strength drops below 225 fpm and is expected to give better
quantitative numbers for the maximum cloudless thermalling
height than the BL Top height given above, especially when
mixing results from vertical wind shear rather than thermals.
(Note: the present assumptions tend to underpredict the
max. thermalling height for dry consitions.) In the presence of clouds the maximum
thermalling height may instead be limited by the cloud base (see the
"Cloud prediction parameters" section below).
Being for "dry" thermals, this parameter omits the effect of "cloudsuck".
MoreInfo
-
Thermal Height Uncertainty
-
This parameter estimates the uncertainty (variability) of the BL Top
height prediction which can result from meteorological
variations. Specifically, it gives the expected increase of a
BL Top height based on a Thermal Index (TI) = 0 criteria should the actual surface temperature be 4 °F warmer than
forecast. Larger values indicate greater uncertainty/variability
and thus better thermalling over local "hot spots" or small-scale
topography not resolved by the model. But larger values
also indicate greater sensitivity to error in the predicted
surface temperature, so actual conditions have a greater likelihood of
differing from those predicted. Small values often result from the presence
of a stable (inversion) layer capping and limiting thermal growth.
This parameter is most easily utilized through relative values, i.e. by first
determining a "typical" value for a location and subsequently noting whether
predictions for a given day are for more/less uncertainty than is typical.
MoreInfo
-
Thermal Updraft Velocity (W*)
-
Average dry thermal updraft strength near mid-BL height.
Subtract glider descent rate to get average vario reading for
cloudless thermals. Updraft strengths will be stronger
than this forecast if convective clouds are present, since cloud
condensation adds buoyancy aloft (i.e. this negects "cloudsuck"). W* depends upon
both the surface heating and the BL depth. MoreInfo
-
Buoyancy/Shear Ratio (B/S)
-
Dry thermals may be broken up by vertical wind shear (i.e. wind changing with height) and unworkable if
B/S ratio is 5 or less. [Though hang-gliders can soar with
smaller B/S values than can sailplanes.] If convective clouds are
present, the actual B/S ratio will be larger than calculated
here due to the neglect of "cloudsuck". [This parameter is truncated at 20 for plotting.]
MoreInfo
-
Thermal Updraft Velocity & B/S Ratio
-
A composite plot displaying the Thermal Updraft Velocity contours in
colors overlaid by a stipple representing the Buoyancy/Shear
Ratio. The stipple is heavy for B/S Ratios 0-4 and light for B/S
Ratios 4-7. The intent is to mark regions where a small B/S
Ratio will make thermals difficult (or impossible) to work, though
that depends upon pilot skill and circling radius.
WIND PARAMETER FORECASTS:
-
Surface Wind
-
The speed and direction of the wind at 10m above the
ground. Speed is depicted by different colors and direction by
streamlines. This parameter is obtained directly from WRF model output and not from
a BLIPMAP computation.
-
Boundary Layer Average Wind
-
The speed and direction of the vector-averaged wind in the BL. This
prediction can be misleading if there is a large change in wind
direction through the BL (for a complex wind profile, any single number is not
an adequate descriptor!). MoreInfo
-
Wind at the Boundary Layer Top
-
The speed and direction of the wind at the top of the BL. Speed
is depicted by different colors and direction by streamlines.
-
Boundary Layer Wind Shear
-
The vertical change in wind through the BL, specifically the magnitude
of the vector wind difference between the top and bottom of the
BL. Note that this represents vertical wind shear and
does not indicate so-called "shear lines" (which are
horizontal changes of wind speed/direction). MoreInfo
-
BL Max. Up/Down Motion (BL Convergence)
-
Maximum grid-area-averaged extensive upward or downward motion
within the BL as created by horizontal wind convergence.
Positive convergence is associated with local small-scale convergence
lines (often called "shear lines" by pilots, which are
horizontal changes of wind speed/direction) - however, the
actual size of such features is much smaller than can be resolved by
the model so only stronger ones will be forecast and their predictions
are subject to much error. If CAPE is also large, thunderstorms
can be triggered. Negative convergence (divergence) produces
subsiding vertical motion, creating low-level inversions which limit
thermalling heights. This parameter can be noisy, so users
should be wary. For a grid resolution of 12km or better
convergence lines created by terrain are commonly predicted -
sea-breeze predictions can also be found for strong cases, though they
are best resolved by smaller-resolution grids.
MoreInfo
CLOUD PARAMETER FORECASTS:
-
Cumulus Potential
-
This evaluates the potential for small, non-extensive "puffy cloud"
formation in the BL, being the height difference between the
surface-based LCL (see below) and the BL top. Small cumulus
clouds are (simply) predicted when the parameter positive, but it is
quite possible that the threshold value is actually greater than zero
for your location so empirical evaluation is advised. Clouds can also occur with
negative values if the air is lifted up the indicated vertical
distance by flow up a small-scale ridge not resolved by the model's
smoothed topography. MoreInfo
-
Cumulus Cloudbase (Sfc. LCL)
-
This height estimates the cloudbase for small, non-extensive "puffy"
clouds in the BL, if such exist i.e. if the Cumulus Potential
parameter (above) is positive or greater than the threshold Cumulus
Potential empirically determined for your site. The surface
LCL (Lifting Condensation Level) is the level to which humid air must ascend before it cools enough
to reach a dew point temperature based on the surface mixing ratio and
is therefore relevant only to small clouds - unlike the below BL-based
CL which uses a BL-averaged humidity. However, this parameter has a theoretical
difficulty (see "MoreInfo" link below) and quite
possibly that the actual cloudbase will be higher than given
here - so perhaps this should be considered a minimum possible
cloudbase. MoreInfo
-
Cumulus Cloudbase where CuPotential>0
-
Combining the previous two parameters, this depicts the Cumulus
Cloudbase only at locations where the Cumulus Potential parameter is
positive. This single plot can be used, instead of needing to
look at both the Cumulus Potential and Cumulus Cloudbase plots,
if the threshold Cumulus Potential empirically determined for
your site approximately equals the theoretical value of zero. For
locations where the actual threshold is greater than zero, as is often the
case, this depiction will over-estimate the extent of the cumulus region.
-
OvercastDevelopment Potential
-
This evaluates the potential for extensive cloud formation
(OvercastDevelopment) at the BL top, being the height difference between
the BL CL (see below) and the BL top. Extensive clouds and
likely OD are predicted when the parameter is positive,
with OD being increasingly more likely with higher
positive values. OD can also occur with negative
values if the air is lifted up the indicated vertical distance by flow
up a small-scale ridge not resolved by the model's smoothed topography.
[This parameter is truncated at
-10,000 for plotting.] MoreInfo
-
OvercastDevelopment Cloudbase (BL CL)
-
This height estimates the cloudbase for extensive BL clouds
(OvercastDevelopment), if such exist, i.e. if the OvercastDevelopment
Potential parameter (above) is positive. The BL CL (Condensation
Level) is based
upon the humidity averaged through the BL and is therefore relevant
only to extensive clouds (OvercastDevelopment) - unlike the above
surface-based LCL which uses a surface humidity. [This parameter
is truncated at 22,000 for plotting.] MoreInfo
-
OvercastDevelopment Cloudbase where ODpotential>0
-
Combining the previous two parameters, this depicts the OvercastDevelopment (OD)
Cloudbase only at locations where the OD Potential parameter is
positive. This single plot can be used, instead of needing to
look at both the OD Potential and OD Cloudbase plots,
if the threshold OD Potential empirically determined for
your site approximately equals the theoretical value of zero.
-
BL Explicitly-predicted CloudWater
-
This parameter is primarily for DrJack's use. It predicts the
cloud base of extensive clouds based on model-predicted formation of
cloud water, giving the lowest height at which the predicted cloud
water density is above a criterion value within the BL. In
theory it should be useful predicting OvercastDevelopment (OD) within
the BL since it predicts extensive cloudiness, i.e. when BL
clouds are predicted to extend over a full model gridcell
volume. However, the criterion to be used to indicate the
presence of clouds is problematical since no single value reliably
differentiates between "mist" and "cloud" concentrations. This
parameter has not yet been verified again actual conditions -
comparision to flight observations will be needed to evaluate its
usefulness.
-
BL Cloud Cover
-
This parameter provides an additional means of evaluating the
formation of clouds within the BL and might be used either in
conjunction with or instead of the other cloud prediction
parameters. It assumes a very simple relationship between cloud
cover percentage and the maximum relative humidity within the
BL. The cloud base height is not predicted, but is expected to
be below the BL Top height. DrJack does not have a lot of
faith in this prediction, since the formula used is so simple, and
expects its predictions to be very approximate - but other
meteorologists have used it and it is better than nothing.
Note: Since The the "BL Cloud Cover", "Cumulus
Potential", and "BL Extensive CloudBase" are based upon fundamentally
different model predictions -- respectively the predicted maximum
moisture in the BL, the predicted surface moisture, and an explicit
cloud-water prediction -- they can yield somewhat differing
predictions, e.g. the "Cumulus Potential" can predict puffy cloud
formation when the "BL Cloud Cover" is zero or vice versa.
-
Surface Dew Point Temperature
-
The dew point temperature at a height of 2m above ground level.
This can be compared to observed surface dew point temperatures as an
indication of model simulation accuracy; e.g. if observed surface dew
point temperatures are significantly below those forecast, then BL
cloud formation will be poorer than forecast. This parameter is
obtained directly from WRF model output and not from a BLIPMAP
computation.
-
CAPE
-
Convective Available Potential Energy indicates the atmospheric
stability affecting deep convective cloud formation above the
BL. A higher value indicates greater potential instability,
larger updraft velocities within deep convective clouds, and greater
potential for thunderstorm development (since a trigger is needed
to release that potential). Note that
thunderstorms may develop in regions of high CAPE and then get
transported downwind to regions of lower CAPE. Also, locations
where both convergence and CAPE values are high can be subject to
explosive thunderstorm development.
MoreInfo
WAVE/UPPER-LEVEL FORECASTS:
-
Vertical Velocity at 850/700/500mb
-
Vertical velocity at a constant pressure level, plus wind
speed/direction barbs. [850/700/500mb presure levels are
approximately at 5000/8000/19000 ftMSL or 1500/2500/5800 mMSL.]
Such upward motions can result from mountain wave or BL
convergence. A white dashed straight-line represents the
location of the slice used for the "Vertical Velocity Slice through Vertical Velocity Maximum"
parameter since these parameters are intended to be used in
conjunction. These parameters are obtained directly from WRF model
output and not from a BLIPMAP computation.
-
Vertical Velocity Slice through Vertical Velocity Maximum
-
A vertical slice depicting vertical velocity (colors) and potential
temperature (lines), intended to help analyze occurrences of strong
upward motion. The slice is taken through the location of the
maximum vertical velocity found at a height of approximately 5000
ftAGL over a domain which excludes the outer edge of the domain (the
value of that maximum and its location is given in a subtitle of the
plot). The slice parallells the wind direction at that height
and is depicted by a white dashed line on the "Vertical Velocity at 850/700/500mb" paramter plots
(with left-right on the slice always being left-right on the plan
view). Mt. wave predictions are best made using resultions no
larger than 4km, since a coarser grid generally does not resolve the
waves accurately. Mountain waves tend to occur above the surface
and tilt upwind with height, whereas BL convergences are surface-based
and vertically oriented, ala this
example containing both mt. wave and convergence upward
motion. These parameters are obtained directly from WRF model
output and not from a BLIPMAP computation.
INFORMATION
FORECAST DESCRIPTION:
BLIPMAPs predict thermal soaring conditions
resulting from surface heating of the Boundary Layer (BL), the
scientific term for the turbulent atmospheric region mixed by
surface-based thermals (so thermal tops occur at the top of the
BL). The RASP BLIPMAP program uses numerical weather model
predictions to provide parameters suited to the needs of soaring
pilots and presents them in graphical format. Relative
differences, both in location and in time, are expected to be more
reliable indicators of soaring differences than are the precise
numerical values.
A sequence of forecasts, all for the same
validation time given on the regional BLIPMAP webpage link, is
produced during the day as new observational data becomes available,
each updated forecast having a shorter forecast periodbetween
the latest observation and validation times.
[Model initialization (observation analysis) time + Forecast Period = Validation (forecast) time]
At the top of each
forecast plot is the name of the parameter, the validation date and
time, and the forecast period. The regional RASP pages
give forecasts for the "Current" day and also provide the previous
forecast for each time (which can be for either the current day or the
preceding day). Archived forecasts from previous days (for a
single time only) can be viewed using the RASP
Archive Viewer.
The parameters are averages over grid squares as
forecast by the WRF (Weather Research and Forecasting) model.
Several model grids are forecast for each location, progressing from
coarsest to finest resolution with the area covered decreasing for a
finer resolution grid. Because model solutions from different
grids must be hammered together at their boundaries, due to inevitable
mis-matching between the grids, forecasts near the grid boundary are
less accurate than those nearer the center of the domain - a dashed
white line indicate the frame outside of which forecasts are strongly
affected by boundary condition errors.
The parameter values are represented by color hues
which increase in "warmness" as the value increases in
magnitude. A screen magnifying tool, such as the freeware Super
Magnify for Windows machines or Xzoom
for X11/Linux/Unix machines, helps when discrimination between
adjacent contours is difficult; alternatively, many browsers are
capable of increasing/decreasing the size of an image, and Firefox
users can install the Image
Zoom plug-in extension to add that capability.
Geographic outlines are depicted on each BLIPMAP in
white. The model topography is plotted as black contours, to
assist in location identification but also to emphasize the smoothed
nature of the model topography. The BLIPMAP does not predict
thermal lift created by small-scale terrain features which are not
resolved by the model topography, which often give localized updrafts
significantly stronger than those over the surrounding smoother
terrain.
Notes:
As with all weather products, users should check
the date on each map for currency. Small anomalous diamonds,
the size of an individual model gridpoint, may appear in the plots,
particularly for more sensitive parameters such as convergence or cloud
parameters; these result from numerical noise and should be
disregarded.
RASP BLIPMAPs are still in development and there will
likely be problems, changes, and tweaks. Opinions on
factors affecting its usability are solicited.