The optimization steps presented here are only guidelines and should not be substituted
for a thorough evaluation of your particular site. IRIS provides many of the tools needed to
perform such an evaluation.
Table 18 Mean Velocity Shear Correction
The radial velocity can be expressed in terms of the u and v components of the wind (the vertical velocity
is ignored here because the observations are typically near horizontal:
where:
V
R
= ucosθsinφ + Vsinθsinφ
V
r
is the radial velocity
u is the x-component of the wind (East)
v is the y-component of the wind (North)
θ is the azimuth defined as positive CCW from the x-axis (East)
φ is the elevation defined as positive upward
Let the wind be represented by a mean wind [ u
o
, v
o
] and a perturbation wind [ u', v' ], then
V
R
= u
o
+ u′ cosθsinφ+ v
o
+ v′ sinθsinφ
The azimuth derivative is then,
αV
R
αθ
+ − u
o
sinθsinφ + v
o
cosθsinφ +
α
αθ
u′sinθsinφ + v′cosθsinφ
where the first term is the apparent azimuth shear caused by the mean wind. Thus, when the azimuth
derivative is computed, the correction term is applied as follows:
▵ V
R
▵ θ
+ u
o
sinθsinφ − v
o
cosθsinφ =
α
αθ
u′cosθsinφ + v′sinθsin φ
The values of u
o
and v
o
are obtained from the VVP algorithm. In terms of the azimuth #beta# measured in
the traditional radar sense (clockwise from north) the correction is:
θ = 90 − β
u
o
cosβsinφ − v
o
sinβsinφ
The SHEAR product is first produced from tasks that are optimized for close-range, high-
resolution measurement of radial velocity.
The SHEAR products are then fed to the WARN product, which checks to see if the strength
and size of the wind shear regions exceed a threshold value in the protected areas.
Additional criteria, such as reflectivity aloft from a higher scan or a requirement to see the
shear on two adjacent scans, can be added to the WARN product to reduce the false alarm
rate if this is a problem for the particular location.
Chapter 5 – Configuring and Scheduling Optional IRIS Products
RESTRICTED 183
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