N P Amrutha Kumari, S Balaji Kumar, J Jayalakshmi & K Krishna Reddy $,*

Transcription

1 Indian Journal of Radio & Space Physics Vol 43, February 2014, pp Raindrop size distribution variations in JAL and NILAM cyclones induced precipitation observed over Kadapa (14.47 o N, o E), a tropical semi-arid region of India N P Amrutha Kumari, S Balaji Kumar, J Jayalakshmi & K Krishna Reddy $,* Semi-arid-zonal Atmospheric Research Centre (SARC), Department of Physics, Yogi Vemana University, Kadapa , Andhra Pradesh, India $ krishna.kkreddy@gmail.com Received 16 April 2013; revised 10 June 2013; accepted 12 June 2013 Raindrop size distributions (RSD) of JAL (7 November 2010) and NILAM (29 October 2012) cyclones induced precipitation were measured with PARticle SIze and VELocity (PARSIVEL) disdrometer deployed at Kadapa (14.47 N, E), a semi-arid continental site in Andhra Pradesh. Small and mid drops below 2 mm diameter have higher concentration in JAL and NILAM cyclones. RSD characteristics stratified on the basis of rainrate showed that the concentration of small (large) drops is higher (lesser) for JAL cyclone than NILAM cyclone. The concentration of mid size drops of JAL cyclone are greater than or equal to that of NILAM cyclone. The JAL cyclone induced precipitation is associated with higher (lesser) concentration of small drops (small and mid drops) in stratiform (convective) region than that of NILAM cyclone precipitation. JAL cyclone has long duration of stratiform rainfall with smaller raindrop compared to NILAM cyclone, which had a short duration of stratiform rainfall with more number of mid and large drops. In both convective and stratiform regimes, the coefficient value of Z-R relations is higher in NILAM cyclone than JAL cyclone. The average mass weighted diameter, D m of JAL cyclone is smaller (larger) in stratiform (convective) than that of NILAM precipitation. Keywords: Raindrop size distribution (RSD), Rainrate, Mass weighted diameter, Cyclone induced precipitation, Z-R coefficient PACS Nos: jf; Qx 1 Introduction Knowledge of raindrop size distribution (RSD) is essential in determining the characteristics of precipitation. Precipitation is an integral product of RSD and is highly variable in space and time. The variability of precipitation is directly linked to the variability of RSD. Over Southern India, the major period of rainfall is October to December, particularly the eastern half of the peninsula. By October, a low pressure establishes over the central and southeast Bay of Bengal, moving southward as the season progresses. Under the impact of this low pressure area, tropical cyclones originate in the Bay of Bengal between 8 N and 14 N and influence the southern peninsula of India. The process of raindrop formation, growth, transformation and decay occur on a microphysical scale within a cyclone. Each process, such as condensation growth, evaporation or collision/coalescence, leaves a signature on RSD of a rain event. The rain parameters, like rainrate, radar reflectivity, liquid water content and rainfall accumulation can be determined with the help of RSD measurements 1. For weather radar, a relationship between radar measured reflectivity, Z (in dbz), and surface rainrate, RR (in mm h -1 ), has been traditionally derived by employing RSD measurements 1. The characteristic differences in RSD of precipitations results in significant errors in radar rainfall estimation. Moreover, detailed information about the RSD is essential in cloud microphysical processes 2, numerical weather modeling 3 and weather radar applications 4,5. This shows that a concrete effort is needed in understanding the RSD of various rain regimes. There are a number of studies carried out over the globe on RSD characteristics in terms of diurnal, seasonal variation 6,7, different precipitations 8-10 and type of precipitation Systematic analysis on the form of RSD, its temporal and rainrate dependent evolution at the surface and also aloft is essential in understanding the process of rainfall formation. However, there are few studies on RSD characteristics of cyclones/typhoons

2 58 INDIAN J RADIO & SPACE PHYS, FEBRUARY 2014 over the globe 14,15. The differences in RSD characteristics prior to and during the passage of remnants of Hurricane Helene (2000) are presented by Ulbrich & Lee 16. Typhoon systems RSD are reported by Tokay et al. 17 and Chen et al. 18. The RSD of cyclonic precipitation is studied by Radhakrishna & Rao 19. Recently, RSD variations in cyclonic and northeast monsoon thunderstorm precipitation are investigated by Kumar & Reddy 20. The earlier studies Fig. 1 Track of JAL and NILAM cyclones, observational site location (Kadapa) and Chennai IMD Doppler radar location (denoted with square boxes) are mainly focused on seasonal or intra-seasonal variation of cylone activity. However, in the present paper, an attempt is made to get the RSD variations between two cyclones (JAL and NILAM) induced precipitations using a laser disdrometer. In addition, the one minute resolution RSD of two cyclone events are categorized into stratiform and convective rainfall as well as the physical reasons for the corresponding spectral shape. 2 Data and Methodology For the present study, data obtained from a groundbased laser (PARSIVEL) disdrometer, installed at Yogi Vemana University (14.47 N, E) in Kadapa district, during the land fall of JAL and NILAM cyclone is utilized. The observational site, Kadapa, is indicated by square box in Fig. 1. The infrared cloud coverage images (obtained from the Kalpana satellite) of JAL and NILAM cyclones are shown in Fig. 2. Complete details of measurement technique along with the assumptions made in determining the size and velocity of hydrometeors from the PARSIVEL disdrometer are provided by Loffler-Mang & Joss 21, Tapidor et al. 22, Jaffrain et al. 23 and in brief by Kumar & Reddy 20. The raindrop concentration N(D), in mm 1 m 3, at an instant of time from the PARSIVEL disdrometer counts is obtained from the following equation: Fig. 2 Cloud coverage images of: (a) JAL (7 November 2010) and (b) NILAM (31 October 2012) cyclones observed from Kalpana satellite images

3 AMRUTHA KUMARI et al.: RSD VARIATIONS IN JAL AND NILAM CYCLONES OVER KADAPA 59 ( D ) N = i n 32 ij (1) j=1 A. t. Vj. Di where, n ij, is the number of drops reckoned in the size bin i and velocity bin j; A, the sampling area in m 2 ; t, time in seconds; D i, the drop diameter in mm for the size bin i; D i, the corresponding diameter interval; and V j, the fall speed in ms -1 for the velocity bin j. From the raindrop concentration N(D), the radar reflectivity factor Z in mm 6 m 3 and rainrate RR in mm h -1 are derived by: 32 6 i i i i j=1 Z ( D ) = N( D ) D D (2) π 3 RR= 4 VjN( Di ) Di Di (3) 10 i=1 j=1 The mass-weighted mean diameter D m in mm, shape parameter µ (-) and slope parameter Λ in mm 1 are briefly explained by Kumar & Reddy Comparison of disdrometer and tipping bucket rain gauge A large number of drop sizing instruments have been used in the past in the measurements of the RSD. They can be divided into different groups depending on the physical principle used: impact disdrometers, PARSIVEL disdrometer/optical disdrometers, and Doppler radar disdrometers. All these instruments are able to operate continuously and unattended. It is essential to determine the accuracy of PARSIVEL disdrometer in measuring the observed rainrates. The rainrates calculated from the RSD measured by disdrometer were compared with the values measured by a tipping bucket rain gauge as integral part of Automatic Weather Station (AWS) at the same site. The disdrometer and AWS are spatially separated about 6 m in the Yogi Vemana University Meteorological Observatory. For the present study, three months (September to November 2012) precipitation data was collected from disdrometer and AWS have been utilized to understand the accuracy of the PARSIVEL disdrometer in measuring the observed rainrates. The 5-minutes averaged rainrates calculated from the RSD measured by the disdrometer were compared with the values measured by a rain gauge at the same site. Figure 3 demonstrates the regression relation [Fig. 3(a)] and deviation [Fig. 3(b)] between the two sources of measurement. The least square fitting of the two measurements is close to The observational results are in fairly good agreement and acceptable in view of the differences in the measurement technique. 4 RSD analyses of JAL and NILAM cyclones induced precipitation 4.1 Comparison of raindrop concentration for JAL and NILAM cyclones precipitation The time series of the RSD during the passage of JAL cyclone on 7 November 2010 revealed three different segments (S1, S2 and S3) that were separated by rain intermittence of half an hour to an hour with total rain accumulation of 41.0 mm occurred for a period of 8 hours 49 minutes [Fig. 4(a)]. The raindrops below 1 mm diameter were considered as small drops, above 3 mm as large drops and those in the diameter range 1-3 mm as mid-size drops 11,20. The first segment (S1) occurred for Fig. 3 (a) Linear regression and (b) deviation comparison of rainrate (RR, mm h -1 ) measured with PARSIVEL disdrometer and tipping bucket rain gauge

4 60 INDIAN J RADIO & SPACE PHYS, FEBRUARY minutes during 06:33-07:15 hrs IST. The highest reflectivity of 27.6 dbz, and maximum rainrate 1.8 mm h -1 with a maximum drop concentration of m -3 mm -1 and rainfall accumulation of 0.5 mm was observed during this event. The concentration of small drops was more compared to mid-size and large drops during this segment. The second segment (S2) lasted over 2 hours 41 minutes from 08:01 to 11:46 hrs IST with rain gaps ranging from one minute to seventeen minutes with maximum rainrate 3.4 mm h -1, reflectivity 34.1 dbz with a maximum concentration of m -3 mm -1 and rainfall of 0.9 mm. The concentration of the second segment is in between first and third segment. In this segment (S2), relatively low concentrations ( m -3 mm -1 ) of small drops were observed. The third and the most intense regime (S3) of the cyclonic precipitation had 5 hours 26 minutes of continuous rainfall from 12:34 to 24:00 hrs IST. In this segment, maximum rainrate of 55.6 mm h -1, radar reflectivity of 46.5 dbz and drop concentration of m -3 mm -1 with a rain accumulation of 39.6 mm was observed. At the end of the third portion from 21:00 hrs IST, large concentrations of small drops were present, while an appreciable number of medium drops were also present. Interestingly, higher reflectivity and rainrate values were observed during this third segment of the cyclonic precipitation between 21:00 and 23:00 hrs IST. Relatively high concentrations of mid-size and large drops are responsible for the heavy rain and high reflectivity. The rain integral parameters values of these three segments (S1, S2 and S3) are given in Table 1. The time series of the RSD of NILAM cyclonic precipitation, occurred on 31 October 2012, revealed three different segments (S1, S2 and S3) that were separated by rain intermittence of thirty nine minutes to one hour eight minutes with total rain accumulation of 32.6 mm for a period of 8 hours 27 minutes [Fig. 4(b)]. The first segment (S1) occurred for 48 minutes between 09:28 and 10:46 hrs IST. The Table 1 Maximum values of rainrate (RR, mm h -1 ), radar reflectivity (Z, dbz) and raindrop concentration [N(D), m -3 mm -1 ] of JAL (7 November 2010) and NILAM (31 October 2012) cyclonic precipitation day in different segments (S1-S3) Parameter, unit JAL cyclonic precipitation day NILAM cyclonic precipitation day S1 S2 S3 S1 S2 S3 Duration, min RR max, mm h Z max, dbz N(D) max, m -3 mm -1 Rainfall, mm Total rainfall, mm Fig. 4 Time series of raindrop size distributions of: (a) JAL (7 November 2010) and (b) NILAM (31 October 2012) cyclones precipitation days

5 AMRUTHA KUMARI et al.: RSD VARIATIONS IN JAL AND NILAM CYCLONES OVER KADAPA 61 maximum reflectivity of dbz, maximum rain intensity of 4.06 mm h -1 with a maximum drop concentration of m -3 mm -1 and rainfall accumulation of 1.3 mm was observed during this event. The concentration of small drops was more compared to mid-size drops during this segment. The second segment (S2) occurred for 3 hours 8 minutes between 11:25 and 15:16 hrs IST with a rain gap of 11 minutes. In this segment, maximum reflectivity of dbz, maximum rain intensity of mm h -1, and maximum drop concentration of m -3 mm -1 with rainfall accumulation of 6.2 mm was observed. The third segment (S3) occurred for 4 hours 31 minutes between 16:24 and 21:34 hrs IST with rain gaps ranging one minute to eight minutes. In this segment, maximum reflectivity of dbz, maximum rain intensity of mm h -1 with a maximum drop concentration of m -3 mm -1 and rainfall accumulation of 25.1 mm were observed. The rain parameter values of these three segments (S1, S2 and S3) are given in Table Variation of RSD with rainrate The RSD of JAL (7 November 2010) and NILAM (31 October 2012) cyclone precipitations at different rainrate ranges are depicted in Fig. 5. The rainrates (RR) of JAL and NILAM cyclonic precipitations have been classified into eight categories (0.5<RR<1, 1<RR<5, 5<RR<10, 10<RR<15, 15<RR<20, 20<RR<30, 30<RR<50 and RR>50) and the RSD in each category is Fig. 5 Raindrop concentration vs drop diameter at different rainrates of JAL (7 November 2010) and NILAM (31 October 2012) cyclone precipitations

6 62 INDIAN J RADIO & SPACE PHYS, FEBRUARY 2014 averaged to get the mean RSD of each rainrate range. In the rainrate range mm h -1, 104 minutes of JAL cyclonic precipitation and 42 minutes of NILAM cyclonic precipitation are observed [Fig. 5(a)]. In this rainrate range, mid-size drops up to 2 mm have same concentration in both JAL and NILAM cyclonic precipitations. For the rainrate range 1-5 mm h -1, 289 minutes of JAL cyclonic and 168 minutes of NILAM cyclonic precipitations are observed [Fig. 5(b)]. In this rainrate range, concentration of small drops is higher in JAL cyclone than NILAM cyclone and a reverse pattern is observed for mid and large drops. In the rainrate range 5-10 mm h -1, 60 minutes of JAL cyclonic and 62 minutes of NILAM cyclonic precipitations are observed [Fig. 5(c)]. In the rainrate range mm h -1, 31 minutes of JAL cyclonic and 36 minutes of NILAM cyclonic precipitations are observed [Fig. 5(d)]. In these two rainrate ranges (5<RR<10, 10<RR<15), small and mid-size drops up to 2.2 mm diameter of JAL cyclonic precipitation are having higher concentration than NILAM cyclonic precipitation and a reversed pattern is observed for mid-size (above 2.2 mm diameter) and large drops. In the rainrate range mm h -1, 12 minutes of JAL cyclonic and 17 minutes of NILAM cyclonic precipitations are observed [Fig. 5(e)]. In this rainrate range, mid drops (above 1.2 mm) and large drops are having less concentration in JAL cyclone than NILAM cyclone. In the rainrate range mm h -1, 13 minutes of JAL cyclonic and 20 minutes of NILAM cyclonic precipitations are observed [Fig. 5(f)]. In the rainrate range mm h -1, 07 minutes of JAL cyclonic and 10 minutes of NILAM cyclonic precipitations are observed [Fig. 5(g)]. For above 50 mm h -1 rainrate range, 2 minutes of JAL cyclonic and 03 minutes of NILAM cyclonic precipitations are observed [Fig. 5(h)]. In these three rainrate ranges (20<RR<30, 30<RR<50, RR>50), higher concentration of small drops and less concentration of mid and large drops is observed for JAL cyclone than NILAM cyclone. 4.3 Variation of RSD with convective and stratiform precipitation The precipitations observed due to JAL (7 November 2010) and NILAM (31 October 2012) cyclones are classified into convective and stratiform regions using a rainrate threshold value of 20 mm h -1. Precipitation segments with rainrate below 20 mm h -1 are considered as stratiform and above 20 mm h -1 as convective type (Fig. 6). JAL cyclone is having Fig. 6 Classification of convective (C) and stratiform (S) region of JAL (7 November 2010) and NILAM (31 October 2012) cyclones precipitation

7 AMRUTHA KUMARI et al.: RSD VARIATIONS IN JAL AND NILAM CYCLONES OVER KADAPA 63 convective precipitation for a period of 22 minutes and stratiform precipitation for 783 minutes. The NILAM cyclone is associated with 500 minutes of stratiform precipitation and 33 minutes of convective precipitation. The raindrop concentration of convective precipitation is higher than stratiform precipitation in both JAL and NILAM cyclones [Fig. 7(a and b)]. In the stratiform region [Fig. 7(c)], the JAL cyclone induced precipitation is having high drop concentration up to 1.2 mm diameter than NILAM cyclone precipitation and a reverse pattern is observed for raindrops above 1.2 mm diameter. In convective region [Fig. 7(d)], small drops are associated with slightly higher concentration in JAL cyclone than NILAM cyclone; whereas mid and large drops are having less concentration in JAL cyclone than NILAM cyclone precipitation. The Z= A*R b relations of stratiform and convective regions of JAL and NILAM cyclones induced precipitations are obtained from scatter plots of radar reflectivity (Z, dbz) and rainrate (RR, dbr) (Fig. 8). The coefficient 'A' and exponent 'b' of stratiform and convective regions of JAL and NILAM cyclones are given in Table 2. The JAL cyclone is having less coefficient values in both convective and stratiform regimes than NILAM cyclone. From the Z-R scatter plots of stratiform and convective regions, it is clear that the JAL cyclone induced precipitation is having raindrops of size less than or equal to that of NILAM cyclone. 4.4 Variation of mean drop diameter (D m ), shape (µ) and slope (Λ) parameters The variation of shape (µ, -) and slope parameter (Λ, mm -1 ), mean drop diameter (D m, mm) and total drop concentration (N t, m -3 ) with rainrate of JAL and NILAM cyclones induced precipitation are shown in Fig. 9. From the figure, it is clear that the range of variability of µ, Λ, D m and N t is more for JAL cyclone than NILAM cyclone for the rainrate less than 20 mm h -1. This variation decreases with increase in rainrate and becomes more uniform above 20 mm h -1. The shape (µ, -) and slope parameter (Λ, mm -1 ) decreases with the increase in rainrate and this decrease is sharper in NILAM cyclone than JAL cyclone. The D m increases with increase in rainrate but this increase is steeper in NILAM cyclone than the JAL cyclone. In the JAL cyclone induced precipitation, the average D m value is 1 mm for the Fig. 7 Raindrop concentration vs drop diameter of convective and stratiform precipitation of JAL (7 November 2010) and NILAM (31 October 2012) cyclone precipitations

8 64 INDIAN J RADIO & SPACE PHYS, FEBRUARY 2014 Fig. 8 Radar reflectivity (Z, dbz) and rainrate (dbr=10*log 10 (RR), mm h -1 ) relations for convective and stratiform regions of JAL (7 November 2010) and NILAM (31 October 2012) cyclone Fig. 9 Variation of: (a) shape (µ, -); (b) slope parameter (Λ, mm -1 ); (c) mean drop diameter (D m, mm); and (d) total drop concentration (N t, m -3 ) with rainrate (RR, mm h -1 ) for JAL (7 November 2010) and NILAM (31 October 2012) cyclone precipitations Table 2 Z=a*R^b values of stratiform and convective regions of JAL and NILAM cyclonic precipitation Day Stratiform Convective A b R^2 A b R^2 7 Nov Oct rainrate below 20 mm h -1 and 1.6 mm for above 20 mm h -1 ; and for NILAM cyclone these values are 1.26 mm and 1.25 mm below and above 20 mm h -1 rainrate. In both the cyclones (JAL & NILAM), N t increases with the increase in rainrate with a large spread in N t in JAL cyclone than NILAM cyclone. The shape (µ, -), slope parameter (Λ, mm -1 ), mean diameter

9 AMRUTHA KUMARI et al.: RSD VARIATIONS IN JAL AND NILAM CYCLONES OVER KADAPA 65 Table 3 Mean values of µ (dimensionless), Λ in mm -1, D m in mm and N t in m -3 for the stratiform and convective regions of JAL and NILAM cyclonic precipitation Day Stratiform Convective µ, - Λ, mm -1 D m, mm N t, m -3 µ, - Λ, mm -1 D m, mm N t, m -3 7 Nov Oct Fig. 10 Raindrop concentration [N(D)] as a function of the raindrop diameter and raindrop fall velocity [V(D)] for: (a) JAL (7 November 2010) and (b) NILAM (31 October 2012) cyclone precipitations (D m, mm) and total drop concentration (N t, m -3 ) values of stratiform and convective regions of JAL cyclone induced precipitation and NILAM cyclone induced precipitation are given in Table 3. From the values of D m and N t, it is clear that in the JAL cyclone, the convective region is mainly composed of small to mid-size drops rather than large drops, whereas the stratiform region is composed of only small drops. 5 Drop size and fall velocity relation Raindrop fall velocity is useful for the measurement of rain integral parameters like radar reflectivity, liquid water content and rainrate. Fall velocity plays an important role in rain related studies in numerical simulation and remote sensing 2. Figure 10 (a and b) shows the observed drop concentration as a function of drop diameter and the fall velocity for the JAL and NILAM cyclone precipitation. For the JAL cyclone induced precipitation, small and mid drops up to 2 mm diameter have high concentration with fall velocities less than 4 ms -1, whereas for NILAM cyclone induced precipitation, small and mid drops up to 2 mm diameter have high concentration with fall velocity ranging 4-5 ms -1. There is a larger spread in the drop fall velocities of mid and large drops in the NILAM cyclonic precipitation compared to JAL cyclonic precipitation. 6 Results and Conclusions The raindrop size distribution (RSD) of JAL (7 November 2010) and NILAM (31 October 2012) cyclones induced precipitations are studied using PARSIVEL disdrometer deployed at Kadapa (14.47 N; E ), a semi-arid, temperate, plateau climate region of India. From rainrate comparison studies between disdrometer and rain gauge, the rainrate data corrected for instrumental error, matches very well with a tipping bucket rain gauge measured values at 5-minute resolution placed at the same site. In the JAL cyclone induced precipitation, the concentration of small drops is high compared to mid-size and large drops; whereas in NILAM cyclone induced precipitation, the concentration of all the drops (small, mid and large) is almost same. The maximum raindrop diameter does not exceed 4 mm even at higher rainrates in JAL cyclone induced precipitation but for NILAM cyclone induced precipitation, raindrop diameter exceeds 4 mm at higher rainrates. From the RSD characteristics, it is clear that both JAL and NILAM cyclone precipitations contain more small and mid drops up to 1.3 mm diameter at lower rainrates (< 5 mm h -1 ). The JAL cyclone precipitation is associated with lower rainrates (< 5 mm h -1 ) having longer duration than NILAM cyclone precipitation. At the lower rainrate range (<5 mm h -1 ), the small raindrops have high concentration in JAL cyclone than NILAM cyclone precipitation. Above 20 mm h -1 rainrates, small and mid drops up to 1.5 mm diameter have nearly same concentration both in JAL and NILAM cyclone precipitation. The JAL cyclone induced precipitation is associated with more stratiform events than convective events compared to that of NILAM cyclone precipitation. Raindrops below

10 66 INDIAN J RADIO & SPACE PHYS, FEBRUARY mm (1.2 mm) diameter in stratiform (convective) regions have higher concentration in JAL cyclone than NILAM cyclone precipitation. From stratiform and convective regions of the Z-R scatter plots, it is clear that the JAL cyclone induced precipitation has raindrops of size less than or equal to that of NILAM cyclone precipitation raindrops. There is a large spread in total drop concentration (N t, mm -3 ), shape parameter (µ, -), slope parameter (Λ, mm -1 ) and mass weighted diameter (D m, mm) in JAL cyclone precipitation than NILAM cyclone. A more spread in fall velocities of mid and large drops is observed for NILAM cyclonic precipitation than JAL cyclone induced precipitation. JAL cyclone has long duration of stratiform rainfall with smaller raindrop when compared to NILAM cyclone, which had a short duration of stratiform rainfall with more number of mid and large drops. It may be noted that the results presented in this study are based on one disdrometer measurement, which represents the time evolution of the raindrop size distribution over a single site. More observation sites are needed to measure the spatial variability of microphysical characteristics of tropical cyclone precipitation. Acknowledgement The authors gratefully acknowledge India Meteorological Department (IMD), Government of India for providing the JAL and NILAM cyclone track information and Kalpana satellite images. One of the authors (SBK) acknowledges the Ministry of Earth Sciences (MoES), Government of India for providing the fellowship to carry out this research work. 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