**4. Results**

## **4.1. Climatology of vertical profile of precipitation**

**Figure 2a** shows the climatology of the number of reflectivity profiles for a time span of more than 17 years, from 1998 to 2014. Approximately 27,075,680 profiles have been used to generate this contour. The most dominant profile was observed in the oceans, especially in the Indian and Pacific Oceans. In addition, a rather high profile number is also observed in some parts of Borneo. The largest number of profiles is approximately 4000, and the smallest is approximately 100. A relatively small number of profiles are observed in the oceans in the southern part of Indonesia, especially in the eastern region. Thus, the largest number of profiles observed in the two warm oceans, that is, Indian and Pacific Oceans, are consistent with the climatology of annual rainfall (**Figure 1**).

In general, the DI of radar reflectivity toward the surface is more dominant than the DD pattern. The most dominant DI was observed in the Indian Ocean, whereas DD was observed on land (**Figure 2b**). This is evident from the larger ratio of DI to DD in the oceans than on land. The largest ratio of DI to DD is approximately 6, and the smallest ratio is 0.66. This value does not change much for the extreme gradient (gradient > ± 1 dBZ/km), in which the smallest ratio is 0.64 and the largest is 5.61 (**Figure 2c**). The dominant DI pattern in the ocean, which indicates a significant raindrop growth was previously found by some investigators [15–18]. Because DI is more dominant in the oceans, the mean gradient of reflectivity in the oceans is more positive than on land (**Figure 2d**). The gradient pattern seems to strongly coincide with the rain top height. A higher RTH is more frequently observed on land than in the oceans (**Figure 2e**). The largest ratio of RTH > 5 km to RTH < 5 km is approximately 6.2 and the smallest is 0.47. This RTH ratio value is nearly equal to the ratio of DI to DD.

is slightly high over land, which means that convection over land is active. On the other hand, a slightly high humidity is observed over Papua and the Pacific Ocean during JJA. November is a rainy season in some parts of western IMC, and a slightly high relative humidity was observed over this region. In general, an inactive convection period over the IMC is associated

**Figure 2.** Climatology of rain profile (a), ratio of radar reflectivity profiles with gradient greater than 0 (DI) to profiles with gradient less than 0 (DD) (b), ratio of radar reflectivity profiles with extreme gradient greater than 1 (DI > 1) to profiles with extreme gradient less than 1 (DD < 1) (c), mean radar reflectivity gradient (d), ratio of radar reflectivity profiles with rain top height (RTH) greater than 5 km to profiles with RTH less than 5 km (e) and mean RTH (f). The

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**Figure 4** shows the monthly variation in the number of radar reflectivity profiles, ratio of DI to DD, mean VPRG and mean RTH for each rain type. Percentage of profiles is calculated by normalizing the profile in one-month bins. The largest percentage of profiles occurs in December and January, which is consistent with convection period over the IMC (**Figure 3**). The largest ratio of DI to DD was also observed during these months, followed by June, July and August. The smallest percentage of profiles was seen in August, which is the dry season over the IMC. All rain types exhibit similar percentage patterns. The gradient is more positive in January and December. On the other hand, the mean rain top height is lower during this period. Thus, mean reflectivity gradient pattern is contrary to the rain top height, which is

To see the spatial variation in the radar reflectivity profile, the spatial distribution of the ratio of DI to DD, mean of VPRG and RTH for deep, shallow and stratiform are given in **Figures 4**–**6**, respectively. For the entire dataset (without rain type classification), the ratio of DI to DD for DJF,

with prevailing easterly winds [30].

diagram was calculated in the 0.25 × 0.25° grids.

discussed above (**Figure 2**).

#### **4.2. Seasonal variation of vertical profile of precipitation**

Prior to discussing the seasonal variation of the precipitation vertical profile, in this section, the seasonal variations in low level relative humidity (RH) are first provided (**Figure 3**). Active periods of convection over the IMC are observed during DJF, which is associated with a dominant westerly wind. During this period, wet conditions (RH >70% at 850 hPa) are visible within the entire IMC region except in the eastern Pacific Ocean. During MAM, the humidity

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with a rain top height (RTH) of less than the freezing level height is considered to be shallow rain; otherwise it is assumed to be deep convective. In this work, the RTH is assumed to be at

Of the many variations in precipitation over the IMC, characteristics of the vertical profile of radar reflectivity at seasonal and diurnal scales are the main focus of this chapter. The seasons are classified into four categories, that is, December, January, February (DJF); March, April, May (MAM); June, July, August (JJA); and September, October, November (SON). Using the aforementioned database, the spatiotemporal distribution of profile number, mean gradient and RTH are contoured on 0.25 × 0.25° boxes. Mean gradient and RTH is only calculated if the

**Figure 2a** shows the climatology of the number of reflectivity profiles for a time span of more than 17 years, from 1998 to 2014. Approximately 27,075,680 profiles have been used to generate this contour. The most dominant profile was observed in the oceans, especially in the Indian and Pacific Oceans. In addition, a rather high profile number is also observed in some parts of Borneo. The largest number of profiles is approximately 4000, and the smallest is approximately 100. A relatively small number of profiles are observed in the oceans in the southern part of Indonesia, especially in the eastern region. Thus, the largest number of profiles observed in the two warm oceans, that is, Indian and Pacific Oceans, are consistent with

In general, the DI of radar reflectivity toward the surface is more dominant than the DD pattern. The most dominant DI was observed in the Indian Ocean, whereas DD was observed on land (**Figure 2b**). This is evident from the larger ratio of DI to DD in the oceans than on land. The largest ratio of DI to DD is approximately 6, and the smallest ratio is 0.66. This value does not change much for the extreme gradient (gradient > ± 1 dBZ/km), in which the smallest ratio is 0.64 and the largest is 5.61 (**Figure 2c**). The dominant DI pattern in the ocean, which indicates a significant raindrop growth was previously found by some investigators [15–18]. Because DI is more dominant in the oceans, the mean gradient of reflectivity in the oceans is more positive than on land (**Figure 2d**). The gradient pattern seems to strongly coincide with the rain top height. A higher RTH is more frequently observed on land than in the oceans (**Figure 2e**). The largest ratio of RTH > 5 km to RTH < 5 km is approximately 6.2 and the small-

Prior to discussing the seasonal variation of the precipitation vertical profile, in this section, the seasonal variations in low level relative humidity (RH) are first provided (**Figure 3**). Active periods of convection over the IMC are observed during DJF, which is associated with a dominant westerly wind. During this period, wet conditions (RH >70% at 850 hPa) are visible within the entire IMC region except in the eastern Pacific Ocean. During MAM, the humidity

est is 0.47. This RTH ratio value is nearly equal to the ratio of DI to DD.

**4.2. Seasonal variation of vertical profile of precipitation**

the highest altitude with Z ≥ 18 dBZ for at least two consecutive range bins.

number of profiles in that box is more than three.

76 Engineering and Mathematical Topics in Rainfall

**4.1. Climatology of vertical profile of precipitation**

the climatology of annual rainfall (**Figure 1**).

**4. Results**

**Figure 2.** Climatology of rain profile (a), ratio of radar reflectivity profiles with gradient greater than 0 (DI) to profiles with gradient less than 0 (DD) (b), ratio of radar reflectivity profiles with extreme gradient greater than 1 (DI > 1) to profiles with extreme gradient less than 1 (DD < 1) (c), mean radar reflectivity gradient (d), ratio of radar reflectivity profiles with rain top height (RTH) greater than 5 km to profiles with RTH less than 5 km (e) and mean RTH (f). The diagram was calculated in the 0.25 × 0.25° grids.

is slightly high over land, which means that convection over land is active. On the other hand, a slightly high humidity is observed over Papua and the Pacific Ocean during JJA. November is a rainy season in some parts of western IMC, and a slightly high relative humidity was observed over this region. In general, an inactive convection period over the IMC is associated with prevailing easterly winds [30].

**Figure 4** shows the monthly variation in the number of radar reflectivity profiles, ratio of DI to DD, mean VPRG and mean RTH for each rain type. Percentage of profiles is calculated by normalizing the profile in one-month bins. The largest percentage of profiles occurs in December and January, which is consistent with convection period over the IMC (**Figure 3**). The largest ratio of DI to DD was also observed during these months, followed by June, July and August. The smallest percentage of profiles was seen in August, which is the dry season over the IMC. All rain types exhibit similar percentage patterns. The gradient is more positive in January and December. On the other hand, the mean rain top height is lower during this period. Thus, mean reflectivity gradient pattern is contrary to the rain top height, which is discussed above (**Figure 2**).

To see the spatial variation in the radar reflectivity profile, the spatial distribution of the ratio of DI to DD, mean of VPRG and RTH for deep, shallow and stratiform are given in **Figures 4**–**6**, respectively. For the entire dataset (without rain type classification), the ratio of DI to DD for DJF,

MAM, JJA, and SON are 1.36, 1.28, 1.23, and 1.22, respectively. Deep convective has a slightly dominant DI with the ratio of DI to DD being 1.37, 1.22, 1.30, and 1.22, respectively. Moreover, the shallow convective rain has a larger ratio, in which the ratio for each season is 5.27, 4.26, 5.13, and 4.05, respectively. In contrast to convective rain, stratiform rain has a more dominant DD in which the ratio of DD to DI for DJF, MAM, JJA, and SON are 1.60, 1.71, 1.80, and 1.66, respectively. Thus, in terms of the profile number, there is an increase in DI percentage during DJF and JJA, except for stratiform rain. This is also visible in the dual peak in **Figure 4**. In general, Indonesia has three rain zones with different rainfall peaks (see **Figure 3** of [31]). The first zone includes south Sumatera to Timor island, southern Kalimantan, and Sulawesi with a DJF peak. The second zone has a rainfall peak in MAM and October, November, and December, which includes northwest Indonesia from northern Sumatra to northwestern Kalimantan. The last region includes Maluku and northern Sulawesi with a rainfall peak during June and July. Because **Figure 4** was averaged in the area covering all three rain zones, two dominant peaks are observed during November,

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The DI is more dominant in deep convective rain than DD (**Figure 5**) with the previously described ratio. This indicates significant raindrop growth. Convection generally has stronger updrafts, which can modify the drops through drop sorting and enhancement of the collision–coalescence process. Both processes will increase the concentration of medium and larger-sized drops, the former by not allowing the smaller drops to fall and the latter by consuming the smaller drops. The study of the vertical profile of DSD using wind profiler radar and Micro Rain Radar (MRR) over Sumatera proves this hypothesis [3, 23, 33]. A slightly larger downward increasing of deep convective reflectivity gathered from wind profiler radar is consistent with the low-level raindrop growth, which is inferred by the MRR

In general, the DI pattern is dominant in the ocean, particularly during wet conditions (DJF). A wetter environment may be a favorable condition for raindrop growth. During MAM, JJA and SON, the gradient becomes more negative, especially between the longitude of 100 and 120° E. High relative humidity values over large islands during this period do not cause DI of radar reflectivity toward the surface. On land, the growth of the raindrops by the collision– coalescence process was much less efficient [34]. During the active convection (DJF), westerly winds blow from the Indian Ocean. Therefore, the precipitation systems over large islands such as Sumatera and Borneo are more maritime in nature than those present during the active convective period. The propagation of clouds from brightness temperature data indicates this phenomenon. Furthermore, the precipitation over land during the inactive convection period particularly in MAM and JJA, is characterized by deeper storms (**Figure 5j** and **k**), which is common with continental rain [3]. **Figure 5j** and **k** reinforce the terrain effect on the reflectivity gradient in deep convective rain, which has been reported in some previous studies [32]. **Figure 6** shows the seasonal variation of the precipitation vertical profile for shallow convective. In general, the pattern of shallow convective is the same as that of deep convective. The DI over the ocean is also more dominant than over land. A more positive gradient over land was observed during MAM and JJA (**Figure 6f** and **g**), which is coincident with a higher rain top height (**Figure 6j** and **k**). However, the ratio of DI to DD for shallow convective is much larger than deep convective. Thus, the raindrop growth during shallow convective rains is

December, January (first peak) and June and July (second peak).

observation.

**Figure 3.** Seasonal variation of relative humidity at 850 hPa from the National Centers for atmospheric prediction (NCEP) and the National Center for Atmospheric Research (NCAR) reanalysis data.

**Figure 4.** Monthly variation of percentage of radar reflectivity profile (a), ratio of DI to DD (b), mean VPRG (c) and mean RTH (d). Symbols of S, D and SH indicate stratiform, deep and shallow convective, respectively. Left *y* axis is used for shallow convective rain.

MAM, JJA, and SON are 1.36, 1.28, 1.23, and 1.22, respectively. Deep convective has a slightly dominant DI with the ratio of DI to DD being 1.37, 1.22, 1.30, and 1.22, respectively. Moreover, the shallow convective rain has a larger ratio, in which the ratio for each season is 5.27, 4.26, 5.13, and 4.05, respectively. In contrast to convective rain, stratiform rain has a more dominant DD in which the ratio of DD to DI for DJF, MAM, JJA, and SON are 1.60, 1.71, 1.80, and 1.66, respectively. Thus, in terms of the profile number, there is an increase in DI percentage during DJF and JJA, except for stratiform rain. This is also visible in the dual peak in **Figure 4**. In general, Indonesia has three rain zones with different rainfall peaks (see **Figure 3** of [31]). The first zone includes south Sumatera to Timor island, southern Kalimantan, and Sulawesi with a DJF peak. The second zone has a rainfall peak in MAM and October, November, and December, which includes northwest Indonesia from northern Sumatra to northwestern Kalimantan. The last region includes Maluku and northern Sulawesi with a rainfall peak during June and July. Because **Figure 4** was averaged in the area covering all three rain zones, two dominant peaks are observed during November, December, January (first peak) and June and July (second peak).

The DI is more dominant in deep convective rain than DD (**Figure 5**) with the previously described ratio. This indicates significant raindrop growth. Convection generally has stronger updrafts, which can modify the drops through drop sorting and enhancement of the collision–coalescence process. Both processes will increase the concentration of medium and larger-sized drops, the former by not allowing the smaller drops to fall and the latter by consuming the smaller drops. The study of the vertical profile of DSD using wind profiler radar and Micro Rain Radar (MRR) over Sumatera proves this hypothesis [3, 23, 33]. A slightly larger downward increasing of deep convective reflectivity gathered from wind profiler radar is consistent with the low-level raindrop growth, which is inferred by the MRR observation.

In general, the DI pattern is dominant in the ocean, particularly during wet conditions (DJF). A wetter environment may be a favorable condition for raindrop growth. During MAM, JJA and SON, the gradient becomes more negative, especially between the longitude of 100 and 120° E. High relative humidity values over large islands during this period do not cause DI of radar reflectivity toward the surface. On land, the growth of the raindrops by the collision– coalescence process was much less efficient [34]. During the active convection (DJF), westerly winds blow from the Indian Ocean. Therefore, the precipitation systems over large islands such as Sumatera and Borneo are more maritime in nature than those present during the active convective period. The propagation of clouds from brightness temperature data indicates this phenomenon. Furthermore, the precipitation over land during the inactive convection period particularly in MAM and JJA, is characterized by deeper storms (**Figure 5j** and **k**), which is common with continental rain [3]. **Figure 5j** and **k** reinforce the terrain effect on the reflectivity gradient in deep convective rain, which has been reported in some previous studies [32].

**Figure 6** shows the seasonal variation of the precipitation vertical profile for shallow convective. In general, the pattern of shallow convective is the same as that of deep convective. The DI over the ocean is also more dominant than over land. A more positive gradient over land was observed during MAM and JJA (**Figure 6f** and **g**), which is coincident with a higher rain top height (**Figure 6j** and **k**). However, the ratio of DI to DD for shallow convective is much larger than deep convective. Thus, the raindrop growth during shallow convective rains is

**Figure 4.** Monthly variation of percentage of radar reflectivity profile (a), ratio of DI to DD (b), mean VPRG (c) and mean RTH (d). Symbols of S, D and SH indicate stratiform, deep and shallow convective, respectively. Left *y* axis is used for

**Figure 3.** Seasonal variation of relative humidity at 850 hPa from the National Centers for atmospheric prediction

(NCEP) and the National Center for Atmospheric Research (NCAR) reanalysis data.

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shallow convective rain.

**Figure 5.** Seasonal variation of the ratio of DI to DD (left), mean VPRG (middle) and mean RTH (c) for deep convective rains.

For stratiform, DD is more dominant than DI, which indicates a reduction of raindrop concentration toward the surface due to the evaporation process [13]. While the DD is dominant, some profiles also show a DI pattern. Moderate and heavy rain intensity during stratiform

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The spatiotemporal distribution of the reflectivity gradient for stratiform rain does not show a significant seasonal variation (**Figure 7**). The land-sea contrast is not clearly observed for this rain. Thus, the effect of terrain appears to be negligible for stratiform rain, which was previously found in another region [32]. This result supports the previous study on the characteristics of DSD over the IMC. The DSD over the IMC, particularly in Kototabang (Sumatera) and Singapore, shows much less seasonal variation than in India, especially for light (stratiform) rain [24, 33].

rain is associated with DI or less negative DD [32].

**Figure 6.** Same as **Figure 5**, but for shallow convective rain.

more significant than deep convection. In addition to the collision–coalescence process due to the convective updraft, two other processes determine raindrop growth, that is, accretion of bulk cloud water and self-collection among raindrops [35]. The updraft is generally weaker over ocean than over land [36]. The strength of the updrafts affects the height at which collisions between different sized droplets occur. A strong updraft (more common over land) will not allow the smaller drops to fall until the particles are large enough to fall. Therefore, a greater concentration of small-sized-raindrops will be observed at the surface. On the other hand, smaller raindrops can fall and monotonically grow in weaker updraft condition [36]. Therefore, the value of DI in convective rain, including shallow convective, is larger over the ocean than over land.

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**Figure 6.** Same as **Figure 5**, but for shallow convective rain.

**Figure 5.** Seasonal variation of the ratio of DI to DD (left), mean VPRG (middle) and mean RTH (c) for deep convective

more significant than deep convection. In addition to the collision–coalescence process due to the convective updraft, two other processes determine raindrop growth, that is, accretion of bulk cloud water and self-collection among raindrops [35]. The updraft is generally weaker over ocean than over land [36]. The strength of the updrafts affects the height at which collisions between different sized droplets occur. A strong updraft (more common over land) will not allow the smaller drops to fall until the particles are large enough to fall. Therefore, a greater concentration of small-sized-raindrops will be observed at the surface. On the other hand, smaller raindrops can fall and monotonically grow in weaker updraft condition [36]. Therefore, the value of DI in convective rain, including shallow convective, is larger over the

rains.

ocean than over land.

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For stratiform, DD is more dominant than DI, which indicates a reduction of raindrop concentration toward the surface due to the evaporation process [13]. While the DD is dominant, some profiles also show a DI pattern. Moderate and heavy rain intensity during stratiform rain is associated with DI or less negative DD [32].

The spatiotemporal distribution of the reflectivity gradient for stratiform rain does not show a significant seasonal variation (**Figure 7**). The land-sea contrast is not clearly observed for this rain. Thus, the effect of terrain appears to be negligible for stratiform rain, which was previously found in another region [32]. This result supports the previous study on the characteristics of DSD over the IMC. The DSD over the IMC, particularly in Kototabang (Sumatera) and Singapore, shows much less seasonal variation than in India, especially for light (stratiform) rain [24, 33].

**Figure 7.** Same as **Figure 5**, but for stratiform rain.

#### **4.3. Diurnal variation of radar reflectivity gradient**

The diurnal cycle of precipitation is a prominent mode over the IMC. There have been many studies on the diurnal variation of surface rainfall over the IMC [3, 5, 7, 11], and the focus here is on the diurnal variation of precipitation vertical profile.

The large number of shallow convective profiles is also widely observed in the Indian Ocean, especially in the southern region between 01 and 09 LT. From 13 LT, the number of convective profiles on land increases and reaches a peak between 16 and 18 LT. For shallow convective rain, the largest number of profiles on land is observed between 13 and 15 LT. On the other hand, the peak of stratiform profile numbers on land is observed rather later than the convective rain, namely, between 19 and 24 LT. During this period, the number of profiles in the ocean

**Figure 8.** Diurnal variation of profile number for stratiform (left), deep (middle), and shallow convective rains (right).

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**Figure 8** shows the diurnal cycle of the number of radar reflectivity profiles for each rain type. From the early morning until morning, that is, 01-06 local time (LT), the number of profiles on land is very small, especially for deep and shallow convective rains. Simultaneously, many stratiform profiles are observed in the coastal region of Sumatra and surrounding seas. In addition, the amount of stratiform rain is also widely observed around the coastal region of Papua.

**Figure 8.** Diurnal variation of profile number for stratiform (left), deep (middle), and shallow convective rains (right).

**Figure 7.** Same as **Figure 5**, but for stratiform rain.

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**4.3. Diurnal variation of radar reflectivity gradient**

is on the diurnal variation of precipitation vertical profile.

The diurnal cycle of precipitation is a prominent mode over the IMC. There have been many studies on the diurnal variation of surface rainfall over the IMC [3, 5, 7, 11], and the focus here

**Figure 8** shows the diurnal cycle of the number of radar reflectivity profiles for each rain type. From the early morning until morning, that is, 01-06 local time (LT), the number of profiles on land is very small, especially for deep and shallow convective rains. Simultaneously, many stratiform profiles are observed in the coastal region of Sumatra and surrounding seas. In addition, the amount of stratiform rain is also widely observed around the coastal region of Papua. The large number of shallow convective profiles is also widely observed in the Indian Ocean, especially in the southern region between 01 and 09 LT. From 13 LT, the number of convective profiles on land increases and reaches a peak between 16 and 18 LT. For shallow convective rain, the largest number of profiles on land is observed between 13 and 15 LT. On the other hand, the peak of stratiform profile numbers on land is observed rather later than the convective rain, namely, between 19 and 24 LT. During this period, the number of profiles in the ocean

**Figure 9.** Same as **Figure 8** but for rain top height.

is suppressed.Based on **Figure 8**, when the convective rain profile reaches a peak in the afternoon over land, the stratiform profile number is at a minimum during this period. Furthermore, when the stratiform profile reaches a peak in the ocean during the early morning, the convective rain profile is at a minimum over land. Migration of the evening convection over land and early morning convection over ocean has been described previously in some papers [5].

Mature convection over land is normally a result of surface solar heating and a gravity wave forced by convection. Land-based convection is associated with high rain top height due to a strong updraft. This can be seen in **Figure 9**, in which between 13 and 21 LT, the rain top over

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**Figure 10.** Same as **Figure 8** but for the ratio of DI to DD.

**Figure 10.** Same as **Figure 8** but for the ratio of DI to DD.

is suppressed.Based on **Figure 8**, when the convective rain profile reaches a peak in the afternoon over land, the stratiform profile number is at a minimum during this period. Furthermore, when the stratiform profile reaches a peak in the ocean during the early morning, the convective rain profile is at a minimum over land. Migration of the evening convection over land and early

morning convection over ocean has been described previously in some papers [5].

**Figure 9.** Same as **Figure 8** but for rain top height.

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Mature convection over land is normally a result of surface solar heating and a gravity wave forced by convection. Land-based convection is associated with high rain top height due to a strong updraft. This can be seen in **Figure 9**, in which between 13 and 21 LT, the rain top over land is higher than during other time periods. On the other hand, over the ocean, the diurnal variation in rain top height is not obvious.

the rain top height decreases, and the reflectivity gradient increases (**Figure 11c** and **d**), which

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The vertical structure of radar reflectivity has many applications, but it has not been comprehensively analyzed on the IMC, which is a region with complex precipitation formation due to the interaction of local circulation dominantly affected by the topography and some global circulations. In this chapter, we present the statistical analysis of seasonal and diurnal variations of such a profile. The gradient is calculated using a linear regression of radar reflectivity as a function of height, in which the positive gradient is denoted as DI and the negative gradient is the DD of radar reflectivity toward the surface. In general, the pattern of reflectivity gradient in this work is similar to that previously found on a global scale, in which the dominant DI is observed in the oceans and DD is observed predominantly on land. However, the diurnal and spatial variations in the gradient shows interesting feature. For convective rainfall, the ratio of DI to DD increases during the wet season such as DJF, whereas during the drier season (MAM and JJA), the number of DD pattern increases, especially over land as the rain top height increases due to the prevailing land-based convection. The stratiform rain does not show a significant seasonal variation, which is consistent with some previous studies on the seasonal variation in raindrops over the IMC. The vertical structure of radar reflectivity shows significant diurnal variations, and the pattern is similar to the land-ocean convection migration, which has been previously reported in some studies. The smallest DI ratio, especially over land, is observed during intense solar radiation. This indicates the reduction of raindrop concentration due to evaporation, especially small-sized raindrops, which are seen from a deficit of such raindrops during this period. The results in this chapter will be useful for the quantitative estimation of rainfall based on weather radar, particularly over the IMC.

This work was supported by the 2017 International Joint Collaboration and Scientific Publication grant from the Ministry of Research, Technology and Higher Education of the Republic of Indonesia (Contract no. 02/UN.16.1.17/PP.KLN/LPPM/2017). The authors thank to Japan Aerospace Exploration Agency and Goddard Space Flight Center for providing the

indicates raindrop growth.

**Acknowledgements**

data particularly 2A25 V7 data.

There is no conflict of interest.

**Conflict of interest**

**5. Conclusions**

The diurnal cycle of the DI to DD ratio is similar to the rain top height pattern. The ratio of DI to DD over land is relatively small when the rain top height is high, namely, between 13 and 18 LT. For the entire dataset (without diurnal classification), ratio of DI to DD for 01–06 LT, 07–12 LT, 13–18 LT, and 19–24 LT are 1.30, 1.29, 1.25, and 1.25, respectively. Deep convective has a slightly dominant DI, with a ratio of DI to DD being 1.34, 1.31, 1.18, and 1.28, respectively. Moreover, the shallow convective rain has a larger ratio, in which the ratio for each hour is 4.99, 4.72, 4.22, and 4.77, respectively. On the other hand, the ratios for stratiform rain are 0.62, 0.60, 0.56, and 0.59, respectively (**Figure 10**).

The summary of the aforementioned discussion is given in **Figure 11**. Two major peaks are observed, in which one is over land and the other is over ocean. Furthermore, the rain top height increases with increasing time, particularly between 12 and 24 LT. During this period, the reflectivity gradient is less positive or more negative, which is an indication of the reduction of raindrops toward the surface due to evaporation, particularly small-sized raindrops. Disdrometer observations in Sumatera have shown that the raindrop spectra from noon to evening contains less small-sized drops (<2 mm) than at other hours [37]. Between 00 and 12 LT,

**Figure 11.** Diurnal variation of number of radar reflectivity profile (a), the ratio of DI to DD (b), mean VPRG (c), and mean RTH (d). Symbols of S, D and SH indicate stratiform, deep and shallow convective, respectively. Left *y* axis is for shallow convective rain.

the rain top height decreases, and the reflectivity gradient increases (**Figure 11c** and **d**), which indicates raindrop growth.
