a new method in determining potential region of precipitation enhancement above coastal land

J. Ocean Univ. China (Oceanic and Coastal Sea Research) DOI 10.1007/s11802-010-1747-4 ISSN 1672-5182, 2010 9 (4): 309-316 http://www.ouc.edu.cn/xbywb/ E-mail:[email protected]

A New Method in Determining Potential Region of Precipitation Enhancement Above Coastal Land

WANG Yilin*, and WU Wei

Shandong Provincial Meteorological Institute, Jinan 250031, P. R. China

(Received March 15, 2010; revised April 26, 2010; accepted July 7, 2010) © Ocean University of China, Science Press and Springer-Verlag Berlin Heidelberg 2010

Abstract To ensure the effectiveness of the operation of artificial precipitation enhancement, a potential region for the operation should be determined in advance. As cloud microphysical measurements needed for the determination of the potential region of cloud seeding are not available before the operation of routine precipitation enhancement, a new method based on the growth process of ice crystal is put forward for determining the potential region using the numerical weather prediction model output. The ice supersaturation, accumulated water vapor within minus temperature layer (≥9 mm), and upward water vapor transportation are adopted as criteria to determine the potential time, height and region of cloud seeding, and the real-time radar images are applied to make decisions on the seeding commanding. The criteria and Doppler radar images are studied in a case of precipitation enhancement characterized by sig-nificant water vapor supply from the north part of a tropical cyclone in the northwest Pacific, which shows that the ocean plays a crucial role in the advection transportation of water vapor to the potential region of the coastal area. The study presents a new method to determine the potential region of precipitation enhancement using macro-physical quantities under ice crystal growth environment. The method possesses a clear physical significance and can be readily applied with the required and easily predicted parameters.

Key words precipitation enhancement; criteria; seeding; potential region; new method

1 Introduction In the last 50 years, better understanding has been ob-

tained in physical mechanisms of natural precipitation and much progress has been made in theoretical research of precipitation enhancement (Gao et al., 2005; Wang and Li, 1989; Ma et al., 2007; Xiao et al., 2004). Hundreds of scientific research programs have been carried out in dif-ferent countries (Bruintjes, 1999), while the scale of field experiment is greatly enlarged (Lei et al., 2008; Huang et al., 2003). The conclusions and simulation results of the cloud seeding effects on cloud microphysical proc-esses have been verified in field experiments (Gabriel, 1981; Guo et al., 1999; Wang, 2000). Precipitation en-hancement has been carried out in most parts of China using about 12104 anti-aircraft guns or rockets in opera-tions. Cloud seeding operation in the northern part of China has been developed based on the theory of ice- wa-ter conversion. Drought and water shortage in such a re-gion have gradually forced the artificial precipitation en-hancement to become a routine work. As the determina-tion of a potential region is the first critical step in pre-cipitation enhancement (Hong and Zhou, 2006), several kinds of cloud rainfall models have been developed based

* Corresponding author. Tel: 0086-531-81603657

E-mail: [email protected]

on a number of cloud physical measurements by airplane and numerical simulations, and different sets of indices and criteria have been presented for determining the cloud seeding potential region (Hu, 2001). The airplane measurements have revealed that only a little amount of super-cooled water exists in cold stratus, therefore, the precipitation enhancement would be extremely limited should only super-cooled water be utilized (Liu et al., 2006). Numerical simulations have shown that the water vapor in ice supersaturation region can be converted by ice crystals into raindrops (Hu et al., 1983). In recent years, the contribution of cloud water vapor to the potential of cold cloud precipitation has been recognized gradually.

Because of the high frequency and broad coverage of operational precipitation enhancement, direct micro-physical measurements are impossible to be carried out before every operation, and it is difficult to determine the potential region by real-time cloud parameter measure-ments. Degree of ice supersaturation, static water vapor amount, and dynamic water vapor transportation can be used, to some extent, as indicators of potential regions, and they can easily be calculated by numerical weather prediction models. Therefore, for the operational precipi-tation enhancement, new criteria are put forward in this paper for the determination of potential regions (Wang et al., 2002). Ahead of operations, the values of the above mentioned parameters are predicted by numerical models, and predicted potential regions of precipitation enhance-

WANG et al. / J. Ocean Univ. China (Oceanic and Coastal Sea Research) 2010 9: 309-316

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ment are then determined, which is more convenient for operational work. In this paper, three criteria for potential region determination, i.e. ice supersaturation, accumulated water vapor within minus temperature layer (≥9 mm), and water vapor upward transportation, are first studied. The determined potential region airplanes, and ground-based tools are used to carry out the precipitation enhancement operation according to the rainfall distribution shown by real-time radar echoes. Considering the effectiveness of the precipitation enhancement operation, generally we choose larger regions with larger rainfall as the potential areas of precipitation enhancement. To this end, in a pe-riod of one month, we tested the potential regions of pre-cipitation enhancement using rainfall. Specific pro-

cedures are: for three times a day (8 h, 14 h, 20 h) the in-dexes of potential regions were calculated using the nu-merical models and then precipitation enhancement po-tential regions were predicted using the indexes. The fore-cast was tested with the 06–12, 12–18, 18–06 hour rainfall. During this period 47 of a total of 183 forecasts could be used for implementing precipitation enhancement and 38 forecasts were correct. The accuracy rate was 81%.

2 Ice Supersaturation According to the Bergeron process, in cold cloud, where

both ice crystal and super-cooled water exist, super-cooled water drops may increase the size of ice crystals through

Fig.1 Distribution of ice supersaturation (shaded) in the −10℃ layer between 1500 UTC September 9 and 0600 UTC September 10.


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evaporation and deposition processes, for the saturation vapor pressure over water surface (Ew) is higher than that over ice surface (Ei). The difference ΔE= Ew-Ei reaches its maxima, 0.266 hPa, at temperature of -11.5℃ derived from computation. When the supersaturation extent over ice surface reaches 12%, ice crystals get the fastest growth rate through deposition. The maximum growth rate of ice crystals on 500 hPa appears at temperature of -16.75℃, if latent heat is taken into account. The Bergeron process sets up the basis for cold cloud precipita-tion enhancement. The aim of precipitation enhancement is to increase the amount of ice crystals in cloud, and makes them grow and fall as rain drops. Therefore, to discriminate ice supersaturation is an important task before seeding.

The cold cloud detection by the PMS (Particle Meas-uring System) shows that the amount of ice crystals is usually small in cloud (<30 L-1) and a little of super-cooled water (<0.3 g m-3) exists, which indicates the potential of precipitation enhancement. It is very important for ob-taining higher cloud seeding efficiency to determine po-tential region before cloud seeding operation. However, it is impossible to carry out potential region detection be-fore each operation due to many limitations. To make it more applicable, the mesoscale weather prediction model MM5 was run for determining the potential region of cloud seeding. In the process of potential region determi-nation, the position and the area of ice supersaturation (ΔE>0) should be first determined. Observational analysis has showed that the region with ΔE>0 usually lies inside the cloud with air temperature between 0℃ and -30℃. It gradually emerges from upper layer to lower layer by rainfall and disappears after rainfall stops. These charac-teristics suggest that it is valuable to forecast the region of ΔE>0 using artificial precipitation enhancement, includ-ing its distribution, persistent interval, moving direction and disappearing moment. Therefore, with the help of numerical models, 48 h ΔE’s in 6 layers (between 0℃ and -30℃ with 5℃ interval) are predicted with 3h intervals, and the region of ΔE>0 is suitable for cloud seeding.

A case of cold front is presented as an example. From 12:00 UTC September 9 to 12:00 UTC September 10, 2008, a northeast-southwest oriented cold front moved eastward and swept Shandong province, generating moder-ate-to-heavy rainfall. During this period, precipitation en-hancement was carried out with ground-based rockets. The distributions of supersaturation region over ice surface at -10 between 1500 UTC ℃ September 9 and 0600 UTC September 10, 2008 are shown in Fig.1. The supersatura-tion region over ice surface moved from west to east to-gether with the cold front. We can determine the timing, area, altitude of cloud seeding based on the occuring time, area, and altitude of ice supersaturation region. Clearly, only one is out of the set of criteria to determine the cloud seeding region; however, it is the most important one.

3 Accumulated Water Vapor Within Negative Temperature Layer The super-cooled cloud water generally coexists with

water vapor and ice crystals in cold cloud. Based on the measurements taken in different countries, there is about 0.1 mm super-cooled cloud water contained within a unit-

area vertical column of precipitating stratus. The amount of column-accumulated super-cooled cloud water and cold layer water vapor of a cyclone occurring on April 5, 2002 have been calculated using MM5. It shows that be-tween 0 and ℃ -30 , the maximum amount of accum℃ u-lated super-cooled cloud water is about 0.5mm, while the accumulated water vapor can reach 13mm. In different stages of cyclone development, the amount of water va-por above the 0 layer is always larger than that of s℃ u-per-cooled water. Because the amount of super-cooled cloud water is small, the potential of precipitation en-hancement based on the Bergeron ice- water conversion process is small. The numerical simulation shows that the potential of precipitation enhancement also lies in ice-vapor conversion. The deposition of vapor on ice crystals not only increases the size of ice crystals but also increases the vertical velocity through the release of latent heat. Then the water vapor is increased. Therefore, the amount of accumulated water vapor in the cold cloud also shows the potential of precipitation enhancement.

Ninety eight cases of rainfall in the City of Jinan and the City of Qingdao have been studied. The study reveals that the amount of accumulated water vapor between 0 ℃and -30 increases steeply before rainfall starts, ℃ and decreases dramatically after rainfall stops. The criterion of 9 mm is a good performance indicator. Therefore, ac-cumulated water vapor within negative temperature layer (≥9 mm) is applied as the second criterion in precipitation enhancement.

The distributions of accumulated water vapor within negative temperature layer from 1500 UTC September 9 to 0600 UTC September 10, 2008 are calculated in the same way as under ice supersaturation (Fig.2). At 1500 UTC September 9, the region with accumulated water vapor ≥9 mm (shaded area) was located at the northwest-ern part of the computation domain. With the maximum value greater than 13 mm, it moved continuously towards east. The distribution and the movement of this region are similar to those of ice supersaturation region (Fig.1), and the criteria can be applied together with the criteria of ice supersaturation in the determination of potential region of cloud seeding.

4 Vertical and Advective Transportation of Water Vapor In the potential region determined according to the

above two criteria, the static water vapor is too little to maintain the potential of precipitation enhancement. The continuous water vapor supply is absolutely necessary. Since the amount of water vapor decreases with altitude, the vertical movement plays an important role in trans-portation of water vapor from warm to cold layer. If a large amount of water vapor is transported to the potential region for precipitation enhancement, it will efficiently replenish the water vapor consumed by deposition, and


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Fig.2 Distribution of accumulated water vapor in minus temperature layer between 1500 UTC 9 and 0600 UTC September 10 (shaded area indicates accumulated water vapor ≥9 mm).

then accelerate the process of water vapor deposition. To maintain the potential of precipitation enhancement, the upward movement and advective movement of water vapor are needed for the potential region. The amount of vertical water vapor transportation can be represented by the multiplication of water vapor density by vertical ve-locity, and can be applied as the third criterion for the determination of the potential region.

The distributions of vertical water vapor transportation in different layers (with 5 intervals) ℃ were computed by the MM5 output. Fig.3 displays the distribution of verti-cal water vapor transportation in layer of -10 from ℃1500 UTC September 9 to 0600 UTC September 10, 2008. The shaded area has positive values, which means that water vapor is transported upward to the layer of -10 . At 1500 UTC September 9, the maximum upward ℃

water vapor transportation occurred in the northwestern part of the computation domain, and moved eastward. At 0300 UTC September 10, the distribution of upward wa-ter vapor transportation has two centers, and the southern one moved to the east of Shandong Peninsula at 0600 UTC September 10, and then moved out to the Yellow Sea. The upward water vapor transportation and the for-mer two criteria compose the set of criteria for potential region determination.

The synoptic weather system plays a very important role in the advection transport of water vapor. Air stream from the northwest Pacific at the north part of a tropical cyclone supplies water vapor to the region of precipita-tion enhancement area. It can be seen from Figs.1 to 4 that the synoptic scale weather system is favorable for precipitation enhancement.


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Fig.3 Distribution of vertical water vapor transportation at −10℃ layer between 1500 UTC September 9 and 0600 UTC September 10 (Shaded area indicates upward transportation).

Fig.4 Synoptic map of 850 hPa at 04 UTC September 10, 2008.

5 Determination of Cloud Seeding Area The region satisfying all the three criteria, i.e. ice su-

persaturation, accumulated water vapor within minus temperature layer (≥9 mm), and upward water vapor transportation, is determined as the potential region for precipitation enhancement operation. Among the three criteria, ice supersaturation is the most important one. No cold cloud seeding operation will be suggested if ice su-persaturation area does not exist. In Fig.5, the left panels show the distribution of potential region satisfying all the three criteria, and the right side panels are radar images at the corresponding time. At 1500 UTC September 9, the potential region was located around Beijing and Shijiaz-huang, and the precipitation echoes are scattered. At 1800 UTC September 9, as the potential region moved toward Jinan, the northeast-southwest rain band was intensified and approached Jinan with the maximum composite echo of 50 dBz. It is clearly shown in Fig.4 that the rain band indicated by radar echoes moved faster than the forecasted


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Fig.5 Numerically forecasted cloud seeding potential region (left, gray area indicating potential region) and radar composite reflectivity image (right).


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potential region. At 2100 UTC September 9, potential re-gion and rain band continued to move southeastward, while on radar images the area of 30 dBz increased and radar echo intensity decreased. At 0000 UTC September 10, the rain band continued to move southeast. In general, the forecasted potential region is coincident with rain band detected by radar. Therefore, the numerically fore-casted potential region can be used as an alerting signal for operation preparation, and precipitation enhancement operation can be implemented according to the rain bands shown by radar images.

6 Determination of Operation Altitude Because ice supersaturation is favorable for ice crystal

growth, the vertical distribution of ice supersaturation region is a good indication of the vertical distribution of potential seeding area. This provides a method for deter-

mining cloud seeding altitude. Fig.6a is a vertical cross-

section diagram of ΔE along 36˚30΄E at 2100 UTC Sep-tember 9, 2008. In the layer of -10℃, there exists ice supersaturation region between 114.5˚E and 117.5˚E. The high ice supersaturation region lies between 116˚E and 117˚E and within the altitude of the layer of -6 to -15℃, defined as the most suitable altitude for cloud seeding. Fig.6b shows the distribution of the difference between temperature and dew temperature (T–Td). The region of T–Td <=2℃ is quasi-saturated. If the operation region is contained in the quasi-saturated region at the same level, it is suitable for ice crystal growth and reduction of rain-drop evaporation. As shown in Fig.6b, the quasi- saturation layer between 115˚E and 117˚E is deep and reaches the layer of -25℃. Considering this fact and shooting distance of a rocket, the layer of -10 to -15℃ is suitable for op-eration. Then airplanes can be arranged for seeding at this altitude, and the elevation of the rocket can be determined.

Fig.6 Distribution on cross-section of 36˚30΄E at 2100 UTC September 9, 2008 (a) Ice supersaturation and (b) T-Td.

7 Operation The precipitation enhancement operation was carried

out in the mountain area to the south of the City of Jinan. According to the numerical forecast shown in Fig.4, the potential region did not reached to Jinan yet at 1800 UTC and moved over Jinan at 2100 UTC. Therefore, the op-erator on duty made a decision that the precipitation en-hancement operation should be carried out between 1800 and 2100 UTC. The movement of rain band was closely monitored. The radar composite reflectivity (Fig.7) showed that the rain band entered the range of rocket (marked by white rectangle) at 2000 UTC September 9. There were 26 ground-based precipitation enhancement rockets dis-tributed within the rectangle. Between 2000 and 2030 UTC, the rockets were fired 2 times with 93 shots in total, and good results in precipitation enhancement were ob-tained. The operation demonstrated that it is feasible to use the forecasted potential region as an alerting signal for operation preparation, and conduct the precipitation enhancement operation according to the rain band shown by radar.

Fig.7 Radar composite reflectivity image at 2000 UTC Sep-tember 10, 2008. White rectangle indicates the distribution range of rockets.

8 Conclusions As it is impossible to measure microphysical parame-

ters directly in cloud before each precipitation enhance-


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ment operation, a numerical model is applied to forecast these parameters and the potential region of precipitation enhancement.

Based on the MM5 model output, the region satisfying the 3 criteria, i.e. ice supersaturation, accumulated water vapor within negative temperature layer (≥9 mm) and upward water vapor transportation, can be determined as a potential region of precipitation enhancement. The ex-istence of forecasted potential region can be utilized as an indicator for precipitation enhancement operation.

Precipitation enhancement operation can be conducted according to the rain band distributions shown by radar images; however, operators should be aware that the movement of rain band displayed by radar images can be different from that of forecasted potential region.

Acknowledgement The authors appreciate the support from the Meteoro-

logical Science and Technology Research Project (2009- sdqz05), Shandong Meteorological Bureau.

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(Edited by Xie Jun)

a new method in determining potential region of precipitation enhancement above coastal land

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