Receive Dual Polarization Upgrade
During a two-week period, beginning January 30, 2012, the Doppler radar at your National Weather Service Forecast Office will undergo an upgrade to incorporate new technology. For these two weeks, radar data will be unavailable from NWS Kansas City/Pleasant Hill! Surrounding radars include: Omaha, NE,Des Moines, IA, Quad Cities, IA, St. Louis, MO, Paducah, KY, Springfield, MO, Wichita, KS, and Topeka, KS.
|Current NWS Doppler radars transmit and receive pulses of radio waves in a horizontal orientation. As a result, the radar only measures the horizontal dimensions of targets (e.g. cloud and precipitation droplets). Dual-polarimetric radar transmits and receives pulses in both a horizontal and vertical orientation. Therefore, the radar measures both the horizontal and vertical dimensions of targets. Since the radar receives energy from horizontal and vertical pulses, we can obtain better estimates of the size, shape, and variety of targets. It is expected that this will result in significant improvements in the estimation of precipitation rates, the ability to discriminate between precipitation types (e.g. hail vs. rain), and the identification of non-meteorological returns, such as chaff, ground clutter, and smoke plumes from wildfires that are not uncommonly detected by weather radar systems such as WSR-88D.|
|Current NWS Doppler Radar||Dual-Pol Radar|
The full benefit of dual-pol radar, however, will not be fully realized until NWS forecasters and research meteorologists develop real-time expertise.
|A radio wave is a set of oscillating electric and magnetic fields, oriented 90 degrees to each other. Polarization of the wave is the direction, or orientation, of the electric field.
|The electric field is oriented horizontally, along the x-axis (blue). The magnetic field is oriented vertically along the y-axis (white).|
|The electric field is oriented vertically, along the y-axis (orange). The magnetic field is oriented horizontally along the x-axis (white).|
How do weather radars work?
The word radar is actually an acronym for radio detection and ranging. Therefore, as implied by the name, energy transmitted by weather radars is essentially a radio wave with a frequency at the high end of the radio spectrum. That is, if you could tune your radio up the frequency dial to a location well beyond the upper limit where radio stations broadcast, you would eventually enter the frequency regime used by weather radars. Corresponding to any radio wave frequency is a uniquely defined distance from one wave crest to the next, referred to as the wavelength. For NEXRAD, the frequency and wavelength are typically around 3000 Megahertz and 10 cm (4 inches), respectively.Weather radars transmit short pulses of radio waves at a rate of approximately 1000 pulses per second, with each pulse lasting only about a millionth of a second. After each pulse, there is a short time period during which the radar is not transmitting. Rather, during this time the radar is listening for a “reflected signal” from the cloud. This reflected signal is the result of the energy from the transmitted pulse interacting with cloud (cloud water and cloud ice) and precipitation (snow, ice pellets, hail, and rain) particles. A small portion of the power is then returned to the radar, received by the antenna, and analyzed by the radar signal processor to determine an estimate of the rain or snow rate.Doppler radars have the added capability of being able to measure a frequency shift that is introduced into the reflected signal by the motion of the cloud and precipitation particles. This frequency shift is then used to determine wind speed.Here, we discuss topics that will hopefully lead to a better understanding of polarimetric radar design, operation, and capabilities.
The radio wave frequency used by weather radars is specifically chosen for its ability to “interact” with cloud and precipitation particles. Radio waves with lower frequencies, such as those transmitted by radio stations, tend to pass through clouds. Therefore, they are not well suited for weather radar applications.Other than the Doppler frequency shift, there are essentially two processes by which the cloud and precipitation particles interact with the radar’s power and phase (position of the wave crest). The first is backscattering. The second is a propagation effect.
Together, an understanding of backscattering and propagation effects is essential to interpret polarimetric radar signatures.
As noted earlier, the reflected power returned to the radar is related to the size, shape, and ice density of each cloud and precipitation particle that it illuminates. What do we mean by illuminate? Well, think of the radar as a big flash light. If you are standing in a dark room with a flashlight, the closer you are to a wall, the brighter the beam is and the smaller the area that it illuminates. However, as you back away from the wall, the weaker the returned power and the larger the area that is illuminated. Such is the case with radars. As you get further from the cloud, the width of the radar beam broadens, the power becomes more diffuse, and the number of cloud and precipitation particles “illuminated” by the radar becomes larger.Given the wide variety of precipitation types, shapes, and sizes of cloud and precipitation particles, it is easy to see how the interpretation of reflected radar signal can become quite complicated. Listed below are just a few of the complicating factors:
Combined, these problems sometimes make radar precipitation estimation very difficult. Using a new pulse transmission scheme, polarimetric radars are designed to eliminate many of these problems.
Technically speaking, a radio wave is a series of oscillating electric and magnetic fields. Of course, you can’t actually see the oscillating electric and magnetic fields, but the radar can detect and interpret them just as your car radio can detect and interpret those transmitted at the slightly lower frequencies. If, however, we could see them, they would look something like the waves depicted below in Fig. 1.
Figure 1a: Example of the structure of a horizontally polarized radio wave. The electric field wave crest is oriented in the horizontal direction (blue in this figure). The magnetic field wave crest is oriented in the vertical direction (white in this figure).
Figure 1b: Example of the structure of a vertically polarized radio wave. The electric field wave crest is oriented in the vertical direction (red in this figure). The magnetic field wave crest is oriented in the horizontal direction (white in this figure).
As can be seen in Fig. 1, the electric and magnetic fields are oriented at 90 degree angles to each other. This concept is important for understanding what is meant by polarization. That is, the polarization of the radio wave is defined as the direction of orientation of the electric field wave crest. Thus, in Fig 1a, the polarization is horizontal since the electric field wave crest (shown in blue) is aligned along the horizontal axis. In Fig. 1b, the polarization is vertical since the electric field wave crest (shown in red) is aligned along the vertical axis.Polarimetric radars gain additional information about the precipitation characteristics of clouds by essentially controlling the polarization of the energy that is transmitted and received.
Most weather radars, including NEXRAD, transmit and receive radio waves with a single, horizontal polarization. That is, the direction of the electric field wave crest is aligned along the horizontal axis. Polarimetric radars, on the other hand, transmit and receive both horizontal and vertical polarizations. Although there are many different ways to mix the horizontal and vertical pulses together into a transmission scheme, the most common method is to alternate between horizontal and vertical polarizations with each successive pulse. That is, first horizontal, then vertical, then horizontal, then vertical, etc. And, of course, after each transmitted pulse there is a short listening period during which the radar receives and interprets reflected signals from the cloud.Since polarimetric radars transmit and receive two polarizations of radio waves, they are sometimes referred to as dual-polarization radars. The difference between non-polarimetric and polarimetric radars is illustrated below:
NEXRAD (non-polarimetric) Radar
Figure 2a: Non-polarimetric radars, such as NEXRAD, transmit and receive only horizontal polarization radio wave pulses. Therefore, they measure only the horizontal dimension of cloud and precipitation particles.
Figure 2b: Polarimetric radars transmit and receive both horizontal and vertical polarization radio wave pulses. Therefore, they measure both the horizontal and vertical dimensions of cloud and precipitation particles. This additional information leads to improved radar estimation of precipitation type and rate.
Engineers at the National Severe Storms Laboratory are currently working to develop a polarimetric NEXRAD Doppler radar. Polarimetric technology should be available for installation into the national radar network in 5-10 years. It should be noted that a polarimetric upgrade does not replace Doppler technology. Rather, it complements it. Doppler capabilities give information on cloud motion. Polarimetric capabilities give information on precipitation type and rate.
All weather radars, including NEXRAD, measure horizontal reflectivity. That is, they measure the reflected power returned from the radar’s horizontal pulses. Polarimetric radars, on the other hand, measure the reflected power returned from both horizontal and vertical pulses. By comparing these reflected power returns in different ways (ratios, correlations, etc.), we are able to obtain information on the size, shape, and ice density of cloud and precipitation particles.Some of the fundamental variables measured by polarimetric radars, and a short description of each, are listed below:
While other variables that are commonly referred to as “cross-polar terms” are also measured, they are largely unexplored. Indeed, there is yet much to be learned about polarimetric radar signals.
Since the power returned to the radar is such a complicated function of the size, shape, and ice density of each cloud and precipitation particle, any information that we can gain about the average characteristics of the precipitation help us to better determine rain rates, snow rates, or even possibly the size of hail.Rainfall ExampleAs a simple example of how a polarimetric radar can give additional information on precipitation type and rate, let us examine a hypothetical rain event. When compared to snow, for example, rain is a very simple precipitation type. Well, suppose it were raining and you somehow had the magical ability to suddenly stop the rain from falling, go outside to grab a cubic meter of air (including the suspended rain drops), and then take it all inside for examination. Once inside, you start removing the individual rain drops, examining each of their sizes, and adding up the total water content to get an estimate of the rain rate. For the sake of this discussion, lets assume that the first time you do this you find a few very big drops, no small ones, and get a rain rate of 0.5 inch per hour. You wait 15 minutes and then repeat the experiment. But this time you find a large number of very small drops and no big ones. But, much to your surprise, you again get a rain rate of 0.5 inch per hour!How can this be? The number and average size of the rain drops has changed dramatically but the rain rate has not. It is because, in the first sample, the rain water was concentrated in a very small number of large drops and, in the second sample, the rainwater was concentrated in a very large number of small drops. Yet, since the reflected power returned to the radar is heavily weighted towards the largest drops (discussed in an earlier section), the power returned to the radar from the first sample might be as much as 10 times greater than the power returned to the radar from the second sample! As you can see, if you are strictly using the returned power to estimate rain rate, you might end up with either a significant overestimation or a significant underestimation of the rain rate. It would all depend on the dominant drop size. This can be a severe limitation of non-polarimetric radars.Now suppose we have a polarimetric radar. So far, we have been assuming that rain drops are spherical in shape. In reality, that is only true for the very smallest drops. For bigger drops, drag forces as they fall through the atmosphere causes a flattening effect that results in an almost “hamburger bun” type appearance for the very big drops. If we had a polarimetric radar, we would measure differential reflectivity. That is, we would first transmit and receive a horizontal pulse of energy. This would give us an indication of the horizontal dimension of the drop. We would then transmit and receive a vertical pulse of energy. This would give us an indication of the vertical dimension of the drop. Combined, we could use this information to get a measure of the average drop shape and, in turn, dominant drop size. This could be used to refine the radar rain rate estimate.Of course, understanding polarimetric radar power returned from oddly shaped snow, ice crystals, hail, and regions that contain mixtures of precipitation types can get quite complicated. But in each case, polarimetric radars allow us to obtain more information on the overall precipitation structure of the cloud.
Radars won’t tell you if it is going to rain tomorrow. However, once a cloud does develop and precipitation starts falling, they can be used to examine storm structure and estimate rain and snow rates.The improvements associated with polarimetric radars comes from their ability to provide previously unavailable information on cloud and precipitation particle size, shape, and ice density. With this in mind, just a few of the potential applications of polarimetric radar data are listed below.
Techniques are also being developed to use mathematical functions to weight the relative importance of the polarimetric variables as they relate to identifying each cloud (cloud water and cloud ice) and precipitation (snow, ice pellets, hail, and rain) particle type. For example, differential reflectivity may do a better job identifying one particle type, whereas specific differential phase may do a better job identifying another. By combining the weights for each variable, a “classification” of the dominant particle type can be determined for each portion of the cloud. This information can be used to improve predictions from short-term computer forecast models.
In addition to providing information on cloud and precipitation particle size, shape, and ice density, polarimetric radar variables also exhibit unique signatures for many non-meteorological scatterers. Examples would be birds and insects. Though radar measurements of birds and insects may not at first appear to be of interest to meteorologists, there are indeed applications. For example, the motion of the brids and insects may affect the measured Doppler winds, occasionally making interpretation of wind fields difficult. Radar measurements of birds and insects may also be of interest to other commercial and scientific disciplines. For example, birds are hazardous to aircraft. Therefore, radar measurements of birds might interest the aviation industry. Radar measurements of bugs might interest entomologists who study, for example, crop damage resulting from bug migrations. For weather radars, of course, birds and bugs are generally thought of as a data contamination. Fortunately, their unique polarimetric signatures generally make them easily identifiable in the data. This is not always the case for non-polarimetric radars.Another problem that frequently plagues radar measurements is the presence of anomalous propagation (commonly referred to as AP). AP refers to a “ground return contamination” that sometimes occurs in the radar data when a warm layer of air forms above a cold layer of air. This phenomena, which is called an inversion layer, essentially bends the radar beam back towards the ground resulting in a ground return contamination that makes it very difficult, at times, to distinguish the location and intensity of clouds and precipitation. Polarimetric radar signatures also aid in the elimination of AP.