Downdraft velocity estimator for a microburst precursor detection system

A weather surveillance apparatus utilizes a set of beams in an elevation angular sector, one beam being offset from the other by a predetermined offset angle. Radar signal returns in each beam are processed to establish an average doppler frequency shift for the signals in the respective beams. An average of the averages and a difference of the averages are determined which are utilized to establish horizontal and vertical wind velocities. These velocities are further processed to determine whether a microburst precursor exists and the location, magnitude, time to impact, and track of any resulting windshear.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to the prediction of weather 
disturbances and, more particularly, to the prediction of weather 
disturbances that give rise to microburst wind shear conditions at low 
altitudes over the earth's surface which are hazardous to aircraft during 
takeoff and landing. 
2. Description of the Prior Art 
A microburst is a powerful downward blast of air, usually associated with a 
thunderstorm or rain, followed by wind shear, a violent horizontal burst 
of air in all directions at low altitudes. Wind shear, which is extremely 
hazardous during aircraft takeoffs and landings, occurs over a relatively 
small region and typically lasts 5 to 15 minutes. 
A system for providing an early warning of wind shear conditions is 
disclosed by W. L. Rubin et al in U.S. patent application Ser. No. 
07/683,356 entitled "Microburst Precursor Detection Utilizing Microwave 
Radar" and is assigned to the assignee of the present invention. In 
accordance with this prior art, early warning of wind shear conditions is 
provided by detecting the vertical wind downdraft during its descent 
before it reaches ground level and establishes the conditions that 
generate wind shear. Vertical wind downdraft is determined by extracting 
four weather parameters from received signals of a scanning single beam or 
vertically stacked multiple beam microwave doppler radar system which 
illuminates a preselected altitude range for a predetermined distance 
about an airport. The number of beams of the doppler radar system and 
their beamwidths are designed to provide coverage over the preselected 
altitude range in a manner that establishes a vertical and horizontal 
limit for each range cell of the doppler radar system for all slant ranges 
that are less than a predetermined distance. The horizontal limit is 
selected to insure that a vertical wind downdraft column completely fills 
the beam, while the vertical limit is selected to restrict the effects of 
wind velocity gradients within a range cell. The received radar signals 
are processed to establish mean radial velocity, spectral width and 
skewness of the precipitation doppler velocity spectrum, and precipitation 
reflectivity in each range-azimuth cell in an illuminated 
azimuth-elevation sector. This data is then utilized to establish 
hydrometeor (precipitation) vertical velocity, horizontal wind velocity, 
and spatial location and extent of these parameters. The vertical wind 
velocity, spatial extent, and reflectivity are then compared to 
meteorological characteristics of storm generated microburst precursors: a 
vertical wind downdraft velocity of at least five meters per second, a 
vertical wind downdraft column between 1.5 and 3.0 kilometers in 
diameters, and an increase in precipitation reflectivity of 0-20 dB over 
that of the surrounding regions within the vertical wind downdraft. All of 
these criteria are utilized to confirm that a microburst generating 
downdraft has been initiated. 
The doppler velocity spectrum within an elevated antenna beam is 
established by combining the radial component of vertical rain drop 
velocity, which is a function of the sine of the beam elevation angle, 
with the radial component of horizontal rain drop velocity, which is a 
function of the cosine of the beam elevation angle, over the beamwidth of 
each elevated beam. This velocity spectrum is unique for each combination 
of average vertical and horizontal hydrometeor velocities within each 
range azimuth cell. The measured doppler spectrum parameters in each 
range-azimuth cell in each beam within the illuminated altitude region are 
stored on successive radar scans to establish a four dimensional map. 
Measured doppler spectral parameters include mean doppler velocity, 
doppler spectrum width, doppler spectrum asymmetry, and total doppler 
spectral power in the radar echo. These measured parameters of 
hydrometeors immersed in a microburst downdraft provide the basic 
information from which microburst precursor vertical and horizontal wind 
velocity can be estimated. When it is determined from these maps that a 
vertical wind column of between 1.5 and 3.0 kilometers diameter, having a 
vertical wind velocity which exceeds five meters per seconds and exhibits 
a precipitation reflectivity that is 0-20 dB above the surrounding areas 
has been detected, a microburst warning is generated. Since time for the 
vertical downdraft to descend to the earth's surface from the maps data 
area is in the order of five minutes, this warning will precede the 
occurrence of subsequent surface microburst wind shear by a time that is 
adequate to divert or to delay an aircraft takeoff or landing. 
The aforementioned patent application includes a microburst downdraft 
verification mode, wherein a paired set of measurements of (.beta., 
V.sub.RAD) in each range-azimuth cell, V.sub.RAD being the mean radial 
doppler velocity and .beta. the mean doppler spectral skewness, are 
processed to provide a paired estimate of (V.sub.V, V.sub.H), V.sub.V 
being the mean vertical velocity, V.sub.H the mean horizontal velocity of 
raindrops in the range-azimuth cell, and a paired set of measurements 
(.sigma., V.sub.RAD), .sigma. being the mean doppler spectral width, are 
processed to provide a complementary paired estimate (V.sub.V, V.sub.H) in 
the range-azimuth cell. 
Two difficulties exist in this prior art system. First, since the magnitude 
of the spectral skewness .beta. is small, an accurate estimate of this 
parameter requires a very large number of data samples (received radar 
pulses for processing). Second, to obtain the paired set (V.sub.V, 
V.sub.H) from the paired set (.sigma., V.sub.RAD) the system assumes that 
the raindrop turbulence is 1.0 meter per second. Actual raindrop 
turbulence deviating from this assumed value produce errors in the paired 
set (V.sub.V, V.sub.H). Though meteorological data indicates that raindrop 
turbulence is generally in the order of 1.0 meter per second, it is 
desirable to eliminate the dependence upon this assumption and provide a 
more accurate paired set (V.sub.V, V.sub.H) when the raindrop turbulence 
differs from this value. 
SUMMARY OF THE INVENTION 
In accordance with the present invention a preselected altitude range about 
an airport is scanned by one or more antenna beam combinations. Each 
combination contains two beams, the peaks of which are offset by a 
preselected elevation angle. One beam of the two beam combination, 
hereinafter referred to as the second beam, possesses a narrower elevation 
beam width than the first beam in the combination. During the down draft 
verification and track mode, microwave energy is transmitted via the first 
beam and radar returns received via both beams. The received signals are 
processed to obtain the reflectivity of the rain and to provide the 
average of the radial velocities detected in each beam. The two averages 
are then further processed to establish the difference therebetween and 
the average value of the two radial velocity averages. These values, the 
difference between the average radial velocity in each beam and the 
average of these two averages, are then utilized with predetermined 
functions of radial velocity difference versus the average of the averages 
of radial velocity for preselected values of horizontal wind and vertical 
rain velocities, to establish the horizontal wind and vertical rain 
velocities. Vertical rain velocity in still air is obtained with the 
utilization of the reflectivity in the Joss-Waldvogel relationship and 
subtracted from the established vertical rain velocity to derive the 
vertical wind velocity. The horizontal and vertical wind velocities are 
then further processed, as in the prior art discussed above, which is 
incorporated herein by reference, to determine whether a microburst 
precursor has been detected. This processing requires fewer data samples 
than that required by the prior art by a factor of approximately 100 to 
obtain an accurate estimate of the paired set (V.sub.V, V.sub.H). Further, 
the invention is substantially independent of raindrop turbulence. 
The invention will be more fully explained in the following detailed 
description with references to the drawings herein provided.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A major cause of aircraft landing/take-off accidents is a particular form 
of wind shear, referred to as a microburst. The term microburst, coined to 
connote an aviation hazard, is a powerful downward blast of moist air 
which causes a violent horizontal burst of air in all directions near 
ground level. This violent horizontal burst creates a horizontal wind 
velocity differential across its center. A microburst is said to have 
occurred when this wind shear is greater than or equal to 10 meters per 
second across a surface region approximately 4 Km in diameter below 500 
meters above ground level (AGL). At low altitudes ground radar detection 
of wind shear is limited by ground returns known as clutter and by the 
fact that in many cases much of the (radar echo producing) moisture in the 
downdraft evaporates before it reaches the ground. Typical clutter levels 
and airport surveillance radar antenna rotation rates at urban airports 
limit wind shear detection to microbursts having precipitation 
reflectivities in the order of 10-20 dBz or greater. If clutter were not 
present, or attenuated when feasible through signal processing means, 
noise limitations determine detectable signal levels and an order of 
magnitude increase in sensitivity would be realized. The degree that 
clutter can be attenuated through signal filtering is dependent on antenna 
rotation rates and azimuth beamwidth. More rapid antenna rotation or 
narrower antenna beamwidths produce higher levels of modulation of ground 
clutter making it more difficult to reduce clutter through input signal 
filtering. Two types of microbursts are known: dry and wet. Dry 
microbursts generally occur in dry climates whereat heavy rain aloft, 
which initiates the events that cause severe ground wind shear conditions, 
mostly evaporates before reaching the ground. Dry microburst wind shear, 
due to the low level of entrained moisture at the ground level, exhibit 
reflectivities well below 20 dBz. Wet microburst wind shear generally 
occurs in regions of heavy rain and only partially evaporates before 
reaching the ground. Such microburst wind shear normally exhibit 
reflectivities well in excess of the 20 dBz level. Thus ground clutter 
inhibits the detection of dry microburst wind shear as well as wet 
microburst wind shear by radar systems operating with near ground level 
radar beams. 
Refer now to FIG. 1. A microburst is caused by a strong vertical downdraft, 
having a horizontal diameter D that is between 1.5 and 3 kilometers, which 
originates at high altitudes. The disturbance diameter increases as the 
downdraft approaches the earth's surface and establishes a horizontal wind 
velocity differential V=V2-(-V1) near the surface, that is at least 10 
meters per second (20 kts) and may be between 60 kts and 100 kts, over a 
distance W of at most 4 Km. (When W is greater than 4 Km, a macroburst is 
said to have occurred, a condition which is less dangerous for aircraft 
landing or taking off.) Although a downdraft is one of several 
meteorologically detectable phenomena, which are collectively referred to 
as microburst precursors, the downdraft is the least unambiguous precursor 
of follow-on surface microburst wind shear. 
Landing aircraft AC entering a microburst wind shear region first 
experiences an increase in head wind which causes the aircraft AC to fly 
above the glide slope GL. The pilot may attempt to return to the glide 
slope GL by reducing air speed and angle of attack. As the aircraft AC 
continues through the microburst, it encounters a strong downdraft which 
forces it downward while it moves horizontally and then a tail wind 
resulting in a loss of lift. As the aircraft AC falls beneath the glide 
slope GL, the pilot must now increase power and angle of attack to bring 
the aircraft AC back to the glide slope GL. Since the aircraft requires a 
finite time to respond to the control commands, a crash may occur when it 
is too close to the ground to recover. 
Microburst precursors occur between 1 and 8 Km above ground level (AGL) 
about 5-15 minutes prior to the onset of low altitude wind shear. Diagrams 
depicting the formation of a typical wet microburst are shown in FIGS. 2a, 
2b, and 2c. In the first stage 10 a core 11 of densely packed water, with 
a concomitant high reflectivity, is formed at an altitude of between 3 and 
8 Km AGL. Coinciding with the formation of the core 11 is an inflow of air 
12 at or above the core 11. When instability causes the high reflectivity 
core 11 to descend, it causes an additional convergence of air 13 behind 
its descent and, in many cases, air rotation 14 of the descending column. 
The falling high reflectivity core 11 also pushes moisture laden air below 
it downward, resulting in a strong downdraft which accelerates as air 
cooling takes place due to moisture evaporation. This high reflectivity 
core may reach the surface coincident with or after wind shear has been 
initiated. The strong downdraft establishes an air divergence 15 at the 
surface, giving rise to the wind velocity differential V. 
Thus weather phenomena aloft provide detectable precursors from which 
microbursts at the surface may be predicted with sufficient lead time to 
prevent an aircraft disaster during landing or takeoff. Precursors 
associated with the descending downdraft include: a descending 
reflectivity core, horizontal wind convergence aloft, and horizontal 
rotation of the downdraft column. These precursors are indirect signatures 
of the vertical wind downdraft, which is the direct cause of surface 
microburst wind shear. Since a descending high reflectivity core together 
with wind convergence and rotation are only indirect signatures of the 
vertical wind downdraft, they are less reliable than direct measurement of 
the vertical wind velocity as indicators of an impending microburst. 
Descending high reflectivity cores, coupled with substantial horizontal 
wind convergence and rotation, have been observed without the occurrence 
of subsequent microburst; and microbursts have also occurred in their 
absence. Consequently, unambiguous prediction of a microburst requires 
direct knowledge of a vertically descending downdraft having a 
reflectivity greater than 15 dBz that is typically at least equal to or 
greater than the surrounding region, and a vertical wind velocity greater 
than 5 meters per second within a column having an aloft diameter between 
1.5 and 3.0 Km. As the moist downdraft descends, evaporation in the column 
causes cooling and induces an acceleration which can increase the vertical 
wind velocity up to 25 meters per second. The presence of all three 
factors establishes a definite precursor of an imminent microburst. 
Consequently, an early warning system for the prediction of a surface 
microburst must be able to detect vertical downdrafts at altitudes 1-3 Km. 
This may be accomplished with a squinted set of antenna beams, each 
coupled to a doppler radar system, oriented for high elevation angle 
scanning or a multiplicity of stacked sets of such antenna beams, each set 
oriented to scan an assigned elevation sector, as shown in FIG. 3. In each 
beam set a first (1a, 2a, 3a), or primary beam, is utilized to transmit 
radar signals and to receive radar signal returns, while a second (1b, 2b, 
3b), or added beam, having a beam peak offset by a preselected elevation 
angle from the beam peak of the first is utilized only to receive radar 
signal returns. The second beam in each set may have an elevation 
beamwidth that is 3.degree. to 5.degree. narrower than that of the primary 
beam. In a stacked beam system, the number of beam sets and the beam 
widths of the primary beams are selected to provide coverage over a 
desired altitude range above ground level (AGL) in a region around an 
airport. Once the elevation coverage and the number of beams to provide 
this coverage is selected, an elevation beam width for each primary beam 
is established which provides approximately the same percentage spread of 
vertical wind velocity as measured in each primary elevation beam. 
The primary elevation beam width for each beam in the beam configuration 
shown in FIG. 3 would be selected in accordance with the following 
relationship: 
##EQU1## 
where .THETA..sub.i are the successive elevation angles defining the 
primary beam elevation crossovers. Though only three beam sets are shown 
in FIG. 3, this is not restrictive and a greater or lesser number may be 
chosen to optimize coverage at a system location. 
Refer now to FIG. 4 wherein a block diagram of a preferred embodiment of 
the invention for operation with a set of antenna beams is shown. A 
transmitter 5 generates a radar signal which is coupled through a 
circulator 6 to an antenna and beam selector 7 wherefrom it is radiated 
via the primary beam of the selected beam set, as for example beam a of 
beam set 1 (beam 1a). Return signals are received on both beams a and b of 
the selected set and respectively coupled to coherent receivers 8 and 9. 
It should be recognized that the beam sets are rotationally selected to 
provide a continuous elevation sector coverage as the antenna is rotated 
azimuthally by an azimuth drive mechanism, not shown. The coherent 
receivers 8, 9 respectively provide two output signals, designated I and 
Q, to digital filter banks 16a and 17a in mean velocity estimators 16, 17. 
Each filter in the filter banks 16a, 17a processes the I and Q signals 
resulting from a predetermined number of received radar returns in manner 
well known in the art and provides a coded signal representative of the 
center frequency of the filter when a signal having a doppler frequency 
corresponding to the center frequency of the filter is detected. These 
coded signals are coupled to averaging processors 16b, 17b, respectively, 
wherein the average doppler frequency of the radar returns in beams 1a and 
1b are computed. Signals representative of these two average are coupled 
to a differencing processor 21 and to an averaging processor 22 which 
respectively provide the difference between the average doppler velocity 
and the average of the average doppler velocity in beams 1a and 1b. Since 
the elevation angle difference between the two beams is small, the 
horizontal and vertical velocities which combine to establish the detected 
radial velocities are equal in beams 1a and 1b. Those skilled in the art 
can readily verify that the difference .DELTA. between the average radial 
velocities and the average .SIGMA. of the average radial velocities may be 
represented as: 
EQU .DELTA.=.delta.[V.sub.VR COS (.THETA.-.delta./2)-V.sub.HW SIN 
(.THETA.-.delta./2)] (1) 
EQU .SIGMA.=V.sub.VR SIN (.THETA.-.delta./2)+V.sub.HW COS 
(.THETA.-.delta./2)(2) 
where .THETA. is the elevation angle of the first beam 1a peak, .delta. is 
the offset elevation angle of the second beam 1b peak from the first beam 
1a peak, V.sub.VR is the vertical rain velocity, and V.sub.HW is the 
horizontal wind velocity. Since .THETA. and .delta. are known, these 
equations may be solved to uniquely obtain the values of V.sub.VR and 
V.sub.HW. 
A carpet plot graphically illustrating solutions of the above equations 
when .THETA. equals 40.degree. and .delta. equals 3.degree. is shown in 
FIG. 5. The solid curves on the graph represent constant horizontal wind 
velocity and the dotted curves represent constant vertical rain velocity. 
Each set of .DELTA., .SIGMA. values provides a corresponding unique set of 
V.sub.VR, V.sub.HW values. 
Each set of .DELTA., .SIGMA. values computed by the differencing 21 and 
averaging 22 processors is coupled to a processor 23 which simultaneously 
solves equations (1) and (2) for the vertical rain velocity and the 
horizontal wind velocity. This processor may be a memory having memory 
cells addressed by the .DELTA., .SIGMA. value set, each memory cell 
containing the solution for the addressing values. The processor 23 may 
also be a computer programmed to simultaneously solve equations (1) and 
(2) directly. 
The I and Q values from receivers 8, 9 are also coupled to a reflectivity 
processor 25 wherein the mean power received by each beam is determined in 
a manner similar to the manner described in the aforementioned patent 
application. These mean powers are averaged and the average utilized to 
establish the reflectivity of the rain as the described in the referenced 
patent application. The reflectivity so determined is coupled to vertical 
rain velocity in still air estimator 27 wherein the Joss-Waldvogel 
relationship is utilized to obtain an estimate of the vertical rain 
velocity in still air. This estimate is coupled to a differencing network 
28, wherein it is substracted from the vertical rain velocity V.sub.VR, 
coupled to the differencing network from the processor 23 to obtain the 
vertical wind velocity V.sub.VW. 
This vertical wind velocity is compared to a threshold downdraft velocity 
V.sub.wt, which is representative of a minimum downdraft velocity of a 
microburst, in a comparator 29 wherefrom a signal is coupled to enable a 
gate 32 through which the address of the range bins in which the downdraft 
velocity exceeds the threshold is passed to a precursor region identifier 
33. These addresses are provided by search region determinator 34 which 
processes antenna position and range gate location signals respectively 
received from the beam selector 7 and receiver 8. 
The range bin addresses are stored in the region identifier 33 wherefrom 
they are coupled to gate 35 and to a spatial extent tester 36, wherein the 
spatial extent of the downdraft velocity exceeding the threshold is 
determined and compared to a stored spatial extent of a microburst. Should 
the comparison determine that the spatial extent of the downdraft velocity 
exceeding the threshold is comparable to that of a microburst, gate 35 is 
activated and the values of vertical and horizontal wind velocity, 
reflectivity, and location are provided at the output terminals of the 
gate 35. 
The horizontal and vertical wind velocities, reflectivity, and location 
coordinates coupled through gate 35 are provided to the wind shear 
predictor 37, a block diagram of which is shown in FIG. 6. The 
reflectivity R provided by processor 25 is coupled through the gate 35 to 
a microburst predictor 39 wherein a prediction of a wet or dry microburst, 
based upon the magnitude of the reflectivity, is made. A dry microburst is 
predicted should R be between 15-25 dBz and a wet microburst is predicted 
should R be above 25 dBz. 
Horizontal wind velocity provided by the processor 23, the vertical wind 
velocity provided by the differencing network 28, and the coordinates of 
the region in which the downdraft exceeds the threshold are coupled to a 
microburst surface location predictor 41 which utilizes this data in a 
conventional manner to predict the surface location of the microburst 
impact. The vertical wind velocity is also coupled to a time to impact 
predictor 42 which, in a conventional manner, predicts the time that the 
microburst will impact the surface and to a wind shear magnitude predictor 
43. 
Wet/dry microburst predictor 39 and surface location predictor 41 each 
couple data to a wind shear tracker 44 which also receives radar data from 
a radar receiver (not shown) that is coupled to a doppler radar beam which 
provides coverage near ground level, beam 4 in FIG. 3. The wet/dry 
microburst data, the predicted microburst impact location, and the data 
provided by the receiver coupled to beam 4 are utilized to track the wind 
shear along the surface and provide predictions of subsequent wind shear 
locations. 
Dry microburst surface wind shear contains a very small amount of moisture, 
since most of the original moisture content aloft evaporates before the 
downdraft reaches the surface. As a result it is very difficult to detect 
a dry microburst wind shear during its earliest occurrence at ground level 
because of ground clutter, without the predicted microburst downdraft 
impact location and wind shear magnitude. Using information from the 
wet/dry microburst predictor, the time-to-impact predictor, and the 
microburst surface location predictor, the receiver coupled to beam 4 
searches the range-azimuth bins covering the predicted surface impact area 
on each scan to pick up the first indications of wind shear resulting from 
the downdraft reaching the ground. After initial detection of the wind 
shear, beam 4 derived information provides up-to-date information with 
respect to the location and magnitude of microburst wind shear. This 
information is provided until the wind shear magnitude attenuates to the 
point at which it is no longer a treat. 
While the invention has been described in its preferred embodiments, it is 
to be understood that the words which have been used are words of 
description rather than of limitation and that changes within the purview 
of the appended claims may be made without departure from the true scope 
and spirit of the invention in its broader aspects.