參考自https://www.radartutorial.eu/15.weather/wr20.en.html
雙偏振雷達
使用雙偏振是區分冰雹和雨滴的一種方法。雷達發射和接收線性極化電磁波訊號，並在水平和垂直偏振之間快速切換，在單個發射脈衝之間或在脈衝組之間交替。現代雙偏振雷達通常同時傳輸兩個偏振方向。
示意圖：偏振雷達測量降水粒子的示意圖。（水平和垂直方向上，分別測量粒子）
圖 2：下落的雨滴取代了空氣。在其側面，流線被壓縮併產生拉力（類似於飛機機翼的空氣動力學）。這會將水滴拉得更寬。落差越大，這種壓縮力就越強，這種力也就越強。
偏振雷達產品
差分反射率Zdr Differential Reflectivity
水平和垂直方向的反射率之差即為差分反射率ZDR。在一箇中到大雨中，雨滴在自由下落中，很大很平，形成扁平的球體。這就是水平極化時回波更強的原因。
The two receiving signals are called ZH and ZV and from these, the differential reflectivity ZDR is calculated. In a medium to heavy rain the raindrops are large and flatten during their free fall, forming flattened spheroids. This, in turn, is the cause of a stronger echo when the polarization is horizontal.
圖 3：雨滴越大，形狀越扁平，ZDR 越大
固體冰的介電常數僅為水的20%左右，而且其顆粒形狀對冰雹的影響比對雨的影響要小得多。冰雹顆粒下落時會翻滾，因此 ZDR 會很小。因此，冰雹是透過高 ZH 值和低 ZDR 值來識別的。如果甚至存在小於 1（或具有負分貝值）的線性 ZDR 值，則這是冰雹的典型訊號。 （只有這些才能最終“直立”倒下！）The dielectric constant of solid ice is only about 20% of that of water and the particle shape, therefore, has a much smaller effect in hail than in rain. Hail particles tumble when falling, so ZDR will be small. So hail is identified by a high ZH and low ZDR value. If there are even linear ZDR values smaller than one (or with a negative decibel value), this is a typical sign for hailstones. (Only these can finally fall down “upright”!)
利用偏振雷達，可以在一定程度上測量水滴的大小。墜落時，水凝物會稍微變平。水凝物的高度和寬度之比稍微取決於它們的大小。但更重要的是反射率的大小。每立方米的水量超過一定值時，水滴必須具有一定的尺寸。如果差分反射率與此匹配，則結果是有意義的。With a polarimetric radar, the size of the water droplets can be measured to a certain degree. When falling, the hydrometeors flatten out a little. The ratio of height and width of hydrometeors is slightly dependent on their size. But more important is the size of the reflectivity. Above a certain amount of water per cubic meter, the water drops must have a certain size. If the differential reflectivity matches this, the result is meaningful.
線性退極化比Linear depolarization ratio
在水平極化輻射的情況下，垂直極化通道的接收器工作。它接收變成垂直平面的水平偏振輻射部分（“去偏振”）。水平輻射和垂直偏振接收的反射率與純水平偏振傳輸和接收的反射率的對數比稱為線性去偏振比（LDR）。
In the case of radiating in horizontal polarization, nevertheless, the receiver of the vertically polarized channel works. It receives the part of the horizontally polarized radiation turned into the vertical plane (“depolarized”). The logarithmic ratio of reflectivity for horizontal radiation and vertically polarized reception to that for purely horizontally polarized transmission and reception is called Linear Depolarization Ratio (LDR).
對於球形粒子，LDR 理論上會接近負無窮大，但實際上只會達到低至 -40 dB 的值。For spherical particles, LDR will theoretically approach to negative infinity, but in practice will only reach values down to −40 dB.
不幸的是，據我所知，沒有雙偏振雷達可以直接測量冰雹粒徑。它只能根據差分反射率 ZDR 以及可測量的水平和垂直偏振接收訊號之間的相位差來確定它是冰雹。透過比較迄今為止用該雷達測量的反射率以及迄今為止真實降水型別下降的經驗，雷達可以確定是否考慮有冰雹！無論是許多小霰子還是少量大冰雹，雷達都無法測量。Unfortunately, no dual polarized radar known to me can directly measure a hail particle size. It can only determine that it is hail from the differential reflectivity ZDR and the measurable phase differences between the horizontally and vertically polarized received signal. From the comparison of the reflectivities measured so far with exactly this radar and the experiences made so far over then real precipitation types have fallen, the radar can determine whether it concerns hail or not! Whether it is then many small graupels or few large hailstones, the radar cannot measure.
差分相位Differential Phase (φDP)
φDP 是脈衝體積中水平和垂直偏振脈衝的 2 路衰減差異的度量。它定義為：
φDP = φHH − φVV (2)
其中： φHH、φVV 是雷達和解析度體積之間總往返路程的累積差分相移。
The differential phase φDP is a measure of the difference in 2-way attenuation for the horizontally and vertically polarized pulses in a pulse volume. It is defined as:
Where: φHH, φVV are the cumulative differential phase shift for the total round trip between radar and resolution volume.
當水平和垂直偏振雷達脈衝透過給定目標時，兩個脈衝不僅會衰減，甚至會減慢，從而改變每個脈衝的相移。在非球形水凝物中，水平偏振波的路徑比垂直偏振波的路徑稍長。差分相位 φDP 顯示水平和垂直偏振脈衝之間的相位差。這提供了有關雷達目標的形狀和密度的資訊。When the horizontally and vertically polarized radar pulses pass a given target, both pulses are not only attenuated but even slowed down, changing the phase shift of each pulse. Within nonspherical hydrometeors, the path for the horizontally polarized wave is slightly longer than for the vertically polarized wave. The differential phase φDP shows the difference in phase between the horizontally and vertically polarized pulses. This provides information about the shape and density of radar targets.
差分相位 φDP 用於區分非氣象回波和氣象回波。為此，我們計算差分相位的均方根誤差。大的偏差值顯示不是體積目標，而是點目標。
The differential phase φDP is used to distinguish between non-meteorological and meteorological echoes. For this purpose, we calculate the root of the mean square deviation of the differential phase. Large deviations are then not volume targets but point targets.
差分傳播相移率 Kdp，Specific Differential Phase (KDP)
然而，φDP 的值沿著每個徑向（即每個脈衝週期內）累積，這使得解釋變得困難。因此，計算出一個特定的差分相位，它描述了 φDP 沿傳播路徑的變化。 KDP 也可以描述為相位梯度：
However, the values of φDP are accumulated along each radial, i.e., within each pulse period, which makes interpretation difficult. Therefore, a specific differential phase is calculated, which describes the change of φDP along the propagation path. KDP can also be described as a gradient of phase:
(3)
與 ZDR 一樣，KDP 也取決於水凝物的粒徑及其形狀和方向。因此，它是區分雨滴與冰雹和雨夾雪的另一個有價值的量。
Like ZDR, KDP also depends on the particle size of hydrometeors and their shape and orientation. It is thus another valuable quantity to distinguish raindrops from hail and sleet.
互相關係數Co-polar Cross-correlation Coefficient (ρHV)
互相關係數 ρHV 是水平和垂直極化回波訊號之間的統計相關性。它是對給定解析度細胞中水汽凝結形式多樣性的衡量，並且可以很好地指示存在混合降水型別（例如雨和雪）的區域。The co-polar cross-correlation coefficient ρHV is a statistical correlation between the horizontally and vertically polarized echo signals. It is a measure of the diversity of hydrometeor forms in a given resolution cell and can be a good indicator of areas where there is a mixture of precipitation types, such as from rain and snow.
使用兩個或多個脈衝週期為解析度單元測量的散射振幅的平均值（SHH、SVV）用於計算。
ρHV 計算如下：
The mean values of the scattering amplitudes (SHH, SVV) measured for the resolution cell from two or more pulse periods are used for the calculation.
ρHV is calculated as follows:
(4)