On the Vertical Distribution of Mean Vertical Velocities in the Convective Regions during the Wet and Dry Spells of the Monsoon over Gadanki

K. N. Uma Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum, India

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K. Kishore Kumar Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum, India

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Siddarth Shankar Das Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum, India

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T. N. Rao National Atmospheric Research Laboratory, Tirupati, India

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T. M. Satyanarayana National Atmospheric Research Laboratory, Tirupati, India

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Abstract

The Indian Mesosphere–Stratosphere–Troposphere (MST) radar observations of vertical distribution of mean vertical velocities w in convective regions during the wet and dry spells of the Indian summer monsoon over a tropical station at Gadanki, India (13.5°N, 79.2°E) are discussed. The composite w profile during the wet spell consistently shows a single peak at ~13 km whereas during the dry spell it shows two peaks, one at 5 km and another at 11–13 km. The characteristics of this altitudinal distribution in w are discussed in terms of background wind and thermal structure during both spells of the monsoon. Background w obtained from NCEP–NCAR reanalysis shows subsidence throughout the depth of the troposphere during the dry spell of the monsoon over Gadanki. Analysis of background wind and thermal structure clearly reveal that wind shear and temperature inversion in the midtroposphere are different in the dry spell compared to that of the wet spell, which may be the possible reason for the observed double-peak w structure during the dry spell of the monsoon. The present analysis for the first time brought out the distinct vertical distribution in w and the background meteorological conditions during the wet and dry spells of the monsoon over Gadanki, which may have implications in understanding the monsoon convective systems during the wet and dry spells of the Indian summer monsoon.

Corresponding author address: Dr. K. N. Uma, Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum 695022, India. E-mail: urmi_nmrf@yahoo.co.in

Abstract

The Indian Mesosphere–Stratosphere–Troposphere (MST) radar observations of vertical distribution of mean vertical velocities w in convective regions during the wet and dry spells of the Indian summer monsoon over a tropical station at Gadanki, India (13.5°N, 79.2°E) are discussed. The composite w profile during the wet spell consistently shows a single peak at ~13 km whereas during the dry spell it shows two peaks, one at 5 km and another at 11–13 km. The characteristics of this altitudinal distribution in w are discussed in terms of background wind and thermal structure during both spells of the monsoon. Background w obtained from NCEP–NCAR reanalysis shows subsidence throughout the depth of the troposphere during the dry spell of the monsoon over Gadanki. Analysis of background wind and thermal structure clearly reveal that wind shear and temperature inversion in the midtroposphere are different in the dry spell compared to that of the wet spell, which may be the possible reason for the observed double-peak w structure during the dry spell of the monsoon. The present analysis for the first time brought out the distinct vertical distribution in w and the background meteorological conditions during the wet and dry spells of the monsoon over Gadanki, which may have implications in understanding the monsoon convective systems during the wet and dry spells of the Indian summer monsoon.

Corresponding author address: Dr. K. N. Uma, Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum 695022, India. E-mail: urmi_nmrf@yahoo.co.in

1. Introduction

Tropical deep convection is a very important component to understand the global circulation. Most of the deep convection occurs in association with mesoscale convective systems (MCSs), which couples the atmosphere through mass, heat, and momentum transport (Houze 1989, and references therein). To understand the microphysical characteristics of convection and its interaction with large-scale environment, the knowledge of vertical air motion w is very important. Furthermore, better characterization of w will be useful for parameterization and simulation of the convective storms in numerical weather prediction models. Moreover, the knowledge of w in MCS is essential to understand the vertical distribution of latent heat. Extensive studies have then been carried out all over the globe to understand the characteristics of w in convection and most of them were based on several field campaigns, including the Global Atmospheric Research Program (GARP) Atlantic Tropical Experiment (GATE), the Winter Monsoon Experiment (MONEX), the Australian Monsoon Experiment (AMEX), the Equatorial Mesoscale Experiment (EMEX), the Convection and Precipitation/Electrification Experiment (CaPE), the Down Under Doppler and Electricity Experiment (DUNDEE), and Tropical Rainfall Measuring Mission Large-scale Biosphere–Atmosphere experiment (TRMM LBA). A variety of instruments were employed in these campaigns to retrieve vertical air motion profiles (LeMone and Zipser 1980; Zipser and Lemone 1980; Houze 1989; Lucas et al. 1994; Yuter and Houze 1995; Cifelli and Rutledge 1994, hereafter CR94; Cifelli and Rutledge 1998, hereafter CR98; Cifelli et al. 2002). With the advent of wind profilers it became possible to measure w with high vertical and temporal resolutions. Extensive studies were then carried out on w during MCS using wind profilers (Balsley et al. 1988; CR94; CR98; May and Rajopadhyaya 1999, hereafter MR99; Kumar et al. 2005; Uma and Rao 2009; Rao et al. 2009). The above campaigns carried out in the past to divulge the MCS, emphasize the importance of such observations.

Uma and Rao (2009) have characterized draft cores in terms of their size, shape, vertical extent, and magnitude in different stages of convection such as shallow, deep, and decay using the Indian Mesosphere–Stratosphere–Troposphere (MST) radar collected in several monsoon seasons over a tropical station Gadanki, India. There are two distinctive features of the Indian summer monsoon (June–September; viz., active and break spells). The general characteristics observed during the active spell are the intensification of low-level (850 hPa) westerlies, popularly known as the low-level jet (LLJ), which extends from Arabian Sea to southern peninsular India and the occurrence of a large amount of rainfall in the monsoon zone (north and central India). During the break spell, the monsoon trough shifts northward close to foothills of the Himalayas and produces rain in those areas, but the rainfall ceases over the north and central parts of India. The LLJ moves south from the southern peninsular and settles between Sri Lanka and the equator, but the wind direction remains the same, westerly (Joseph and Sijikumar 2004).While our understanding of circulation, rainfall patterns, and convection have improved considerably, the characteristics of vertical velocity draft cores are poorly documented over the monsoon regions.

Detailed studies on convective systems during the active and the break phases of the monsoon have been carried out over Darwin, Australia (CR94; CR98; Cifelli et al. 2002) but only limited studies have been carried out over the Asian summer monsoon(Webster et al. 2002; Bhat et al. 2002; Rao et al. 2009). A recent study by Rao et al. (2009) found that the characteristics of vertical velocity cores during convection exhibit significant differences during the wet and dry spells of the monsoon. The distribution of w during deep convection is found to be large in magnitude in the lower troposphere and upper troposphere for the dry spell, while it is large only in the upper troposphere and lower stratosphere for the wet spell. One interesting aspect that they reported is that the composite w profile for the dry spell shows double-peak altitudinal structure with peaks at 5 and 11–13 km, and during the wet spell, composite w shows a single peak at around 13–14 km. The causative mechanism for such a distinct distribution in w during these spells is still unknown. Rao et al. (2009) have also reported that because of the absence of information on the spatial distribution of convection (which is possible with scanning radar) and vertical temperature soundings, it was not possible for them to attribute any physical or dynamical mechanism(s) to the observed difference in the w profiles. They in fact utilized National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis data to relate the vertical structure of w with buoyancy. The knowledge of the profiles of w and the mechanism involved in the above processes are very important as they play a crucial role in organization of the convective system.

The focal objective of the present study is to expose the observed differences in the composite w profiles during the dry and wet spells of the Indian summer monsoon and to discuss the differences in the background meteorological conditions during these spells over Gadanki, which might be the possible reasons behind the observed differences in the composite w profiles. Background thermodynamical and wind conditions were obtained using GPS sonde, MST radar, and NCEP–NCAR reanalysis observations. In Section 2, a brief description of the Indian MST radar and the experimental specifications employed in the present study are provided. Results obtained from the present study along with statistics and background meteorological conditions are provided in Section 3. The plausible mechanisms for the differences in the vertical air motion distribution and the background meteorological conditions during the wet and dry spell of the monsoon are also discussed in Section 3. Finally, the conclusions are provided in Section 4.

2. Data and system description

For the present study, MST radar measurements made during the passage of several convective systems over Gadanki during the Indian summer monsoon are used. The Indian MST radar is a high-powered, monostatic, pulsed coherent Doppler radar, which operates at 53 MHz with a peak power aperture product of 3 × 1010 W m−2. A complete description of the Indian MST radar can be found in Rao et al. (1995). The Indian MST radar observations of convective systems used for the present study span from 1999 to 2006. The MST radar measurements are augmented by surface rainfall measurements made by an optical rain gauge (ORG) at Gadanki and (1° × 1°) resolution daily rainfall maps over India, generated by Rajeevan et al. (2006) in order to classify the wet and dry spells of the monsoon. The high-resolution gridded dataset is generated from 1803 (where data are available for more than 90% of the time during 1951–2003) stations scattered all over India. The rainfall at any grid point is the sum of weighted rainfall measured at nearby stations. The weight depends on the station distance from the grid point; however, the interpolation is restricted to the radius of influence. ORG provides high-resolution (1 min) rainfall rate data with good accuracy (95% on accumulation data). The continuous measurements of ORG are available from 1999. Further details on this classification can be found in Rao et al. (2009).

Using the above data, Rao et al. (2009) have shown the spatial distribution of rainfall fraction (%) in break periods defined by De et al. (1998), Gadgil and Joseph (2003), Rajeevan et al. (2006), and Krishnan et al. (2000). The rainfall fraction Rb in break period is defined as 100 times the ratio between the rainfall occurred during the dry periods and the total rainfall during the period considered in the respective studies cited above. It is found that Rb is very small in the entire central and north India in addition to the west coast of south India, but it increases over southeast peninsular India during the all-India break period of the monsoon (defined by the India Meteorological Department when the entire central and northern parts of India do not get rain). This clearly indicates that an alternate definition is required to define the active (wet) and break (dry) periods on a regional scale. The rainfall data collected with ORG at Gadanki during each southwest monsoon period from 1999 to 2006 have been used to identify the epochs as followed by Rao et al. (2009). The period is termed as active (or wet) if the rain is seen nearly continuously on all days during the period. Any convective event that occurred during that period is considered to be associated with the wet spell. If convection occurs only on a particular day in a period of mostly dry weather (no rainfall), then the convection event is treated as associated with the break (dry) spell. Out of 37 convective events presented in the study (Rao et al. 2009), only 19 convective events are considered further for the present study, which fall into the deep stage of the convective system as explained in Section 3. These 19 convective events are tabulated in Table 1. NCEP–NCAR reanalysis data and GPS sonde observations have also been utilized to understand the background wind and thermodynamical conditions associated with the wet and dry spells of the monsoon, respectively.

Table 1.

Days of deep convective events during the wet and dry spells of the monsoon.

Table 1.

3. Results and discussion

a. Wet spell of the monsoon (18 September 2001)

Figure 1 shows MST radar observations of a mesoscale convective system, which passed over the radar site on 18 September 2001. The height–time sections of the signal-to-noise ratio (SNR), vertical velocity w, and mean w during the deep stages of convection are shown in Figs. 1a,b,c, respectively. The spurious echoes associated with low SNR have been removed and the data gaps that resulted from this were filled by employing spline interpolation. However, there were very few spurious points in the data. This analysis assures the quality of the data used in the present study. The height–time section of SNR in Fig. 1a shows that the enhanced SNR is reaching as high as 17–18 km and subsequently coming down. Enhanced SNR observed near 17–18 km marks the tropopause. The enhanced echoes at the tropopause are due to the sharp gradients in the refractive index, which is a function of temperature and humidity gradients in the neutral atmosphere. Figure 1b depicts the vertical velocity cores embedded in the deep convective systems followed by the decaying stage of convection. The deep convective core is identified by its characteristic deep and intense up- and/or downdrafts with clear acceleration of w above the freezing level (=4.85 km; Rao et al. 2009). The convective core is defined as the region in which contiguous measurements of vertical velocity should exceed a threshold value. In our analysis, the threshold is chosen as 1 m s−1 (≤−1 m s−1) for updraft (downdraft) core (LeMone and Zipser 1980; Houze 1993; Atlas et al. 1999), which is close to the threshold value (1.5 m s−1) used by MR99. The sensitivity of w threshold (1 m s−1) on draft statistics is examined by varying the threshold from 1 to 1.5 to 2 m s−1 (Uma and Rao 2009). These analyses revealed that the draft core statistics for deep convection are not affected by the changes in the threshold of w. The cores are then considered if they exist for 10 min in time or 500 m in height. Furthermore, if the data in the isolated small pockets of stronger velocities exists for less than 500 m or 10 min, then they were not considered as cores and neglected for further investigation. The methodology and detailed analysis are given in Uma and Rao (2009).

Fig. 1.
Fig. 1.

The time–height section of (a) SNR, (b) vertical air velocity, and (c) mean w derived from MST radar measurements during the deep stage of convection during the passage of an MCS on 18 Sep 2001 for the wet spell of the monsoon. The mean profile is obtained by averaging the vertical velocities during 1520 to 1535 LT. The closed contours indicate the downdraft for (<−1 m s−1).

Citation: Monthly Weather Review 140, 2; 10.1175/MWR-D-11-00044.1

In the present case, large w of the order of ~15 m s −1 is observed during the passage of vertical velocity cores over the radar site. The vertical extent of the core is found to be ~11 km extending from 6 to 17 km. Figure 1c shows the composite w during the deep stage of convection. The mean profile of w calculated during the deep stage of the convection (1520 to 1535 LT in the present case) is referred to as composite profile. It is clear from the figure that the composite w is downward below 5 km and upward above this height and reaches a maximum of about 4.5 m s−1 around 13 km. The composite w exhibits a single peak in the midtroposphere as evident from the figure. These types of profiles were observed in almost all the wet spell events. Similar composite w is also observed over tropical West Africa and midlatitude MCS (Chong et al. 1983). Balsley et al. (1988) have observed a single upper-tropospheric peak during convection over western pacific (Pohnpei Island). The mechanism responsible for the single-peak altitudinal structure during the wet spell of the monsoon will be explained after discussing the characteristics of composite w during the dry spell of the monsoon.

b. Dry spell of the monsoon (31 July 2001)

Figures 2a,b show the height–time section of SNR and w during the passage of the mesoscale convective system over the radar site on 31 July 2001, which occurred during the dry spell of the monsoon. This event depicts all the three types of convection: with shallow (1815 LT) followed by deep (1830–1915 LT) and then decaying (1915–1955 LT) stages of convection. Generally, during shallow convection the cloud tops stay below the freezing level and, hence, warm rain dominates. The vertical extent of the shallow core is less and it is also short lived. The deep stage shows a distinct acceleration above the mean freezing level with strong updrafts with a magnitude of 6 m s−1. Downdrafts are seen above the updrafts (1825–1845 LT). This feature of downdrafts lying above the updrafts was also reported using wind profiler observations (Balsley et al. 1988; CR94) and dual-Doppler radar analysis (Smull and Houze, 1987). The vertical extent of the deep vertical velocity core observed in the present case is about 9 km with a core base at 4 km. It is found in an earlier study that the updraft cores in the dry spell have their base at lower altitudes as compared to the wet spell (Rao et al. 2009). Figure 2c shows the composite w during the deep stage of convection as similar to case 1. The composite w for the dry spell shows descending motion below 3 km and ascending motion above it. Furthermore, the composite w for the dry spell is stronger than their counterpart in the wet spell in the height region 3–7 km and weaker in the height region of 8–18 km. One interesting feature that forms the basis of the present study is that composite w profile for the dry spell shows two peaks, one at 5 and another at 10–11 km, in contrast to a single-peak w distribution during the wet spell. The w composite in the dry spell with a double peak is somewhat similar to the composite vertical air velocity profiles seen at other geographical locations (CR94; CR98; Gamache and Houze 1985). CR94 and CR98 observed this type of vertical distribution in composite vertical air motion profiles at Darwin, during the break regime of the Australian monsoon. The double-peaked updraft structure persisted for several hours and is seen in all break regime MCSs observed by the profiler (CR94).They attributed the low-level peak in the break regime to the convection forming along the leading edge of the convective line. The composite thermal buoyancy profiles also showed large differences from active to break spells in the lower troposphere, consistent with the peaks in the draft profiles (CR98). The upper-level peak in the break phase, not seen clearly in CR98 but is apparent in the composite w in CR94, was attributed to the deep and mature convection formed behind the leading edge of the squall line (CR94). Gamache and Houze (1985) noticed the double-peaked structure in composite w profile retrieved from data in tropical oceanic squall lines during the GATE campaign in the eastern Atlantic Ocean region. In a seminal work, Houze (1989) compared the mean w profiles in the convective regions obtained from various geographical locations and showed that these profiles vary significantly from one location to another. The author also showed the importance of vertical distribution and shape of vertical velocities for evaluating the large-scale heating by MCS. In the present study, we obtained the vertical distribution of mean vertical velocities in the convective regions of several MCS and found the differences in their distributions during two distinct background conditions (viz., the wet and dry spells of the Indian summer monsoon over Gadanki).

Fig. 2.
Fig. 2.

As in Fig. 1, but for the MCS observed on 31 Jul 2001 during the dry spell of the monsoon. The mean profile is obtained by averaging the vertical velocities during 1840–1915 LT.

Citation: Monthly Weather Review 140, 2; 10.1175/MWR-D-11-00044.1

Before carrying out further analysis, it is necessary to find out whether this single- and double-peak distribution of w is a persistent feature or not during all the events of deep convection in both the spells. To verify this, we have analyzed 9 (10) deep events in the wet (dry) spell of the monsoon. The composite w profiles for each deep convective event are constructed and are shown in Figs. 3a,b for the wet and dry spells, respectively. The mean profile is also plotted along with individual profiles and it is represented by a solid line in the figure. One interesting feature in Fig. 3a is that the composite w profiles for many of the events are strikingly similar, except for one event in the wet spell (case 1 in the present study shown in Fig. 1c, where weak downward motion is observed up to 6 km). In the wet spell, all composite w profiles show weaker velocities up to an altitude of 7 km and strong ascending motion above it. All the composite w profiles show peak values in the upper troposphere (at around 13 km). However, the magnitude of the peak varied considerably (more than 4 m s−1) from event to event, with the maximum variance occurring in the height region of 11–14 km. Also interestingly, many of the composite profiles for the dry spell not only show the double-peak distribution, but also the heights of the peaks in the distribution are strikingly similar. Furthermore, all the dry composites show downward motion below 3.5 km, which is expected as evaporation dominates at the lower level during this spell. Another conspicuous feature from Fig. 3 is that the composites are weaker in the dry spell above 15 km, whereas the wet composites show considerable values of w up to an altitude of 20 km. It is also clearly evident from the mean profile that during the wet spell the peak occurs at around ~13 km and during the dry spell two peaks occur at ~5 and ~11 km. Thus, it is very clear from the above analysis that there is clear difference in the composite w between both spells of the monsoon and the doubly peaked nature of the average of the profiles is due to a persistent double-peaked profile as against a bimodal distribution of profiles. Now we will explore the possible reasons that might be responsible for this distribution.

Fig. 3.
Fig. 3.

Vertical variation of composite w (dashed line) and its average (solid line) along with standard deviation (horizontal line) for deep cores in the (a) wet and (b) dry spells. Each profile is a composite of one convective event.

Citation: Monthly Weather Review 140, 2; 10.1175/MWR-D-11-00044.1

c. Background meteorological conditions

1) Background wind during the wet and dry spells of the monsoon

We analyzed the characteristics of background vertical wind prevailing during both these spells of the monsoon. Figures 4a,b show the vertical velocity derived from NCEP–NCAR reanalysis from 1999 to 2006 during the wet and dry spells of the monsoon. It is averaged over 10°–15°N latitude and shown over a longitude of 75°–85°E to represent the background vertical wind conditions over Gadanki. The data have been taken for the days whenever convection occurred during the wet and the dry spells given in Table 1 using the classification scheme described in Section 2. Figure 4a reveals that during the wet spell of the monsoon over Gadanki and nearby longitudes, there is a weak descending motion in the lower troposphere whereas in the upper troposphere, there is an ascending motion. On the other hand, during the dry spell of the monsoon, there is subsidence throughout the depth of the troposphere. Using the Meteosat water vapor channel and NCEP data, Rao et al. (2004) investigated humidity variations (from 600 to 200 hPa) during both phases of the monsoon. They showed strong low-level convergence and upper-level divergence (above the tropopause) and moistening of the upper troposphere in the wet phase of the monsoon. On the other hand, they observed a weak low- and high- level convergence of winds, midlevel divergence, and dry upper troposphere in the dry phase. The weakening of low-level convergence reduces the moisture supply, thereby inhibiting the convective core from developing during the dry phase.

Fig. 4.
Fig. 4.

Mean vertical wind averaged over 10°–15°N latitude for (left) the wet and (right) the dry spell of the monsoon around Gadanki longitudes for the convective events tabulated in Table 1.

Citation: Monthly Weather Review 140, 2; 10.1175/MWR-D-11-00044.1

Furthermore, to examine whether is there any distinct differences in the background wind shear, we have estimated the mean height profiles of wind shear from MST radar observations during the dry spell of the monsoon corresponding to four cases discussed in the present study (as we do not have horizontal wind measurements during all the cases presented here) and it is shown in Figs. 5a,d. One should note that the assumption of spatial homogeneity within the radar beams is not valid during convection and, therefore, the horizontal wind estimation during convection will have large errors. Hence, it is not feasible to calculate the horizontal wind and, hence, the shear during the passage of the convective systems. However, the horizontal winds estimated prior and after the passage of the system are considered for the present analysis. It can be seen from the figure that there is a strong shear in the midtroposphere where the dip in mean w is occurred, except on 18 May 2006 where the peak in the shear is located somewhat above the dip in mean w. This shows that wind shear may play a role in splitting the updrafts and, hence, can contribute to the bimodal distribution during the dry spell of the monsoon. Since we do not have MST radar observations of horizontal winds during all the case studies considered here, we estimated zonal and meridional winds from NCEP–NCAR for all the days given in Table 1 and a composite profile is given in Figs. 6a,b, respectively. One can observe that during the wet spell of the monsoon, the zonal wind reversal occurs at around 4 km whereas during the dry spell, it occurs at a higher altitude around 8 km. The meridional wind also reverses at around 8 km, but for both spells. However, the zonal and meridional wind reversals during the dry spell at 8 km would produce a strong directional wind shear that generally can split the updrafts in the convective systems. High wind shear acts to tear a storm apart, because it will separate the low-level convergence from the top of the thunderstorm, hence, cutting the convective storm from the source (Houze and Hobbs 1982, Houze 2004; Newton and Newton 1959). The splitting of the updraft due to the strong wind shear in the midtroposphere might be a possible reason for the observed double-peak structure in the mean vertical velocities during the dry spell. This inference is further substantiated by the height–time sections of vertical velocities shown Figs. 1b and 2b. In Fig. 1b, a coherent vertical velocity core can be observed whereas in Fig. 2b a distorted vertical velocity core can be observed. On the other hand, the shear found in the lower troposphere during the wet spell does not affect the draft cores because the base of the convective cores is always found at ~8 km, which is situated well above the shear zone (Rao et al. 2009).

Fig. 5.
Fig. 5.

Height profile of mean w and wind shear for four cases during the dry spell of the monsoon on (a) 18 May 2006, (b) 12 Jun 2002, (c) 31 Jul 2001, and (d) 31 Aug 2005.

Citation: Monthly Weather Review 140, 2; 10.1175/MWR-D-11-00044.1

Fig. 6.
Fig. 6.

Mean (a) zonal and (b) meridional wind profiles from NCEP–NCAR reanalysis for the wet and dry spells of the monsoon for the convective events given in Table 1.

Citation: Monthly Weather Review 140, 2; 10.1175/MWR-D-11-00044.1

In addition to the above analysis, we have estimated the wind shear using the GPS sonde launched at Gadanki for 2006 because regular launches started in 2006. For this analysis, we have separated the days of the wet and dry spells of the monsoon in 2006 as described in Section 2, irrespective of whether convection occurred or not, in order to explain the prevailing background wind during both these spells over Gadanki; these days are given in Table 2. There were about 9 (35) days in the wet (dry) spells. The number of days in the dry spell is more due to the prolonged dry spell that prevailed during 2006. The wind shear is estimated for each day and then averaged over each spell. Figures 7a,b show the wind shear for all the days (given by dotted lines) along with mean profile (thick line with standard deviation) for both spells of the monsoon. It is clearly evident from the figure that the shears are stronger in the height region of 10–13 km for most of these days during the dry spell of the monsoon. The mean profile is also reflecting the same with strong wind shear observed in the height region of 10–12 km during the dry spell, which is not observed during the wet spell. However, the strong shear is observed in the height region of 14–15 km in the wet spell for most of the days and one can clearly observe that the updraft is losing its strength considerably at that height region. Held et al. (1993) found that by applying a vertical wind shear in the model destroyed the localization of convection. This clearly exhibits that wind shear may be one of the primary candidates in determining the double-peaked vertical distribution of mean w during the dry spell of the monsoon. On the other hand, during the wet spell, such strong shears are not observed in the midtroposphere, which may be the plausible reason for updrafts not being split and, hence, for the observed single peak in the vertical distribution of mean w.

Table 2.

Days of the wet and dry spells of the monsoon during 2006.

Table 2.
Fig. 7.
Fig. 7.

(a) Square of vertical shear of horizontal wind along with standard deviation during the wet spell. (b) As in (a), but for the dry spell of the monsoon for the dates given in Table 2.

Citation: Monthly Weather Review 140, 2; 10.1175/MWR-D-11-00044.1

2) Background temperature during the wet and dry spells of the monsoon

It is elucidated from the above analysis that background winds may play a crucial role in creating differences in the distribution of mean w during both spells of the monsoon. Earlier studies by Firestone and Albrecht (1986) and Kloesel and Albrecht (1989) have found that 60% of nonconvective soundings are associated with the trade cumulus inversion in the equatorial central Pacific Ocean during the first GARP experiment. Their analyses significantly suggested a statistical linkage between the deep convection and the thermodynamical structure. Now in order to find out whether any temperature inversion is observed in the height region of w minimum (between the two peaks) found during the dry spell of the monsoon, we have examined the background thermal structure during both spells of the monsoon. We have analyzed the temperature for the month of June–September during 2006 for the days given in Table 2 during both spells of the monsoon from the GPS sonde launched at Gadanki (shown in Figs. 8a,b). However, there is no temperature information available during the case studies considered here. Figures 8a,b shows the height profile of temperature during both the spells along with the standard deviation. The figure shows that during the wet spell of the monsoon there is an inversion observed at around 2.5 km and thereafter no inversion is observed throughout the troposphere. On the other hand during the dry spell of the monsoon, a strong inversion is observed at around 7.9 km in the midtroposphere. Generally, if an inversion is observed in the lower levels, there will be an accumulation of water vapor and subsequently there will be enhancement in the convective available potential energy, which will aid the air parcel to overshoot this inversion layer and to develop into a deep convection. This inversion might have been caused by poleward flow and conservation of potential vorticity. This is also clearly evident in Fig. 3 where the composite w for the wet spell is stronger than their counterpart in the dry spell in the height region of 8–18 km. But during the dry spell, when the convective core starts moving upward, it encounters an inversion at the midtroposphere and the inversion tends to trap the vertical development of convection. This aspect is clearly evident from the Fig. 3 where the minimum in the mean w is also observed in that height region. At that height region since the humidity is also less, there will be fewer chances for the convective core to gain energy and develop into deep convection. Uma et al. (2011) have found that the midtroposphere is relatively dry with humidity values decreasing to 30%–40% during the dry spell, whereas during the wet spell the atmosphere is moist throughout the troposphere with humidity values of about 80%. Bhat et al. (2002) also observed large differences in the humidity during the wet and the dry spells of the monsoon. The observed relative humidity was high in the midtroposphere during the wet phase, but the entire troposphere was relatively dry during the dry spell of the monsoon. The height of inversion, which acts as a lid to convective updrafts, also agrees very well with the altitude where the minimum in mean w is observed during the dry spell. In addition to this, to substantiate the occurrence of midtropospheric inversion during the dry spell, we estimated the temperature gradient in the height region of 6–8 km during both the spells and it is depicted in Fig. 8c. In the present analysis, if the temperature gradient is above zero (positive), we consider the presence of inversion. Figure 8c shows the cumulative counts of temperature gradient of the height region 6–8 km for the dry and the wet spells. This figure clearly shows that the occurrence of inversion during the dry spell is very high as compared to that of the wet spell. It is also noticed that the temperature gradient above +2 K km−1 is only observed in the dry spell. In addition to it, we have also plotted equivalent potential temperature in Fig. 8d for both these spells and the midtropospheric inversion during the dry spell is evident from the figure. From the vertical profiles of equivalent potential temperature during the wet and the dry spells, it is also evident that the background atmosphere is more unstable during the wet spell compared to the dry spell. This clearly indicates that midtropospheric inversion may play a role in vertical distribution of w. From these analyses, it is clear that there is a midtropospheric shear and also inversion during the dry spell of the monsoon. In general, if shear is strong in a weakly unstable environment, it will suppress convective growth through the shearing of the updrafts. To explore this fact, we have estimated convective available potential energy (CAPE) during the wet and the dry spells of the monsoon and it is shown in Fig. 9.

Fig. 8.
Fig. 8.

(a) Mean temperature profile during the (a) wet and (b) dry spell of the monsoon. (c) Cumulative counts of temperature gradient. (d) Equivalent potential temperature for the year 2006 (for the days given in Table 2) from GPS sondes launched from Gadanki during the wet (square) and dry (circle) spells of the monsoon.

Citation: Monthly Weather Review 140, 2; 10.1175/MWR-D-11-00044.1

Fig. 9.
Fig. 9.

CAPE during the wet and the dry spell of the monsoon with its mean (square), median (circle), and standard deviation during 2006 for the days given in Table 2.

Citation: Monthly Weather Review 140, 2; 10.1175/MWR-D-11-00044.1

CAPE is cumulative buoyant energy and it is calculated by integrating vertically from the level of free convection (LFC; the level at which the parcel temperature exceeds the ambient temperature and parcels are unstable relative to their environment) to the equilibrium level or level of neutral buoyancy (LNB; the level at which the air temperature exceeds the parcel temperature and parcels are stable relative to their environment) (Moncrieff and Miller 1976; Emanuel 1994):
eq1
where Tvp and Tve are the temperatures of the parcel and the environment, respectively; and g is the acceleration due to gravity; and LFC and LNB are level of free convection and level of neutral buoyancy, respectively. Figure 9 shows the mean and median along with standard deviation of CAPE during the wet and dry spells of the monsoon. The mean value during the wet spell is about 2300 J kg−1, whereas during the dry spell it is about 500 J kg−1. This clearly shows that the buoyancy of the parcel is more during the wet spell compared to that of the dry spell of the monsoon. Hence, the lifted parcel that has large buoyancy during the wet spell of the monsoon, the midtropospheric shear, and inversion will not be important. On the other hand, during the dry spell, the lifted parcel has less buoyancy and hence shear and inversion will affect the convective core when it reaches the midtroposphere.

The above analysis clearly indicates that there exist differences in the background wind and thermodynamical conditions during the dry spell compared to that of the wet spell of the monsoon. Hence, the present study documents these differences in view of the vertical velocity obtained during both spells of the monsoon over Gadanki. Present analysis indicates that the midtropospheric inversion during the dry spell alone may be sufficient to account for observed double-peak structure in the vertical velocities. However, relatively large wind shears and low CAPE during the dry spell further supports the observed vertical velocity distribution. Now, the effect of this vertical distribution of mean vertical velocities on the large-scale heating and thus on the circulation over the tropics will be our focus in the near future.

4. Summary and conclusions

Indian MST radar observations, GPS sonde, and NCEP–NCAR reanalysis data have been utilized to study the potential mechanisms behind the vertical distribution of mean w during the wet and dry spells of the Indian summer monsoon over Gadanki. A total of 19 vertical velocity cores were analyzed during the passage of the convective system using MST radar to get the composite w in both the spells of the monsoon. Background wind and thermal structure is obtained from MST radar, GPS sonde, and NCEP–NCAR reanalysis. The composite w for the wet spell shows weak w up to an altitude of 5 km, then increases and reaches its maximum at ~13 km. On the other hand the composite w during the dry spell shows a descending motion below 3 km and then ascending motion above it. The composite w profile for the wet spell shows a single peak at ~13 km, while in the dry spell it shows two peaks one at 5 km and another at 11–13 km. The composite w profiles of individual deep draft cores are strikingly similar with double-peak (single peak) distribution in the dry (wet) spell. The background w using NCEP–NCAR reanalysis showed that there is subsidence throughout the depth of troposphere. The present study also revealed that the background wind shear and thermal structure in the midtroposphere are different in the dry spell as compared to that of the wet spell of the monsoon. CAPE is also smaller during the dry spell of the monsoon compared to that of the wet spell. Thus, the present observations for the first time revealed the differences in the background wind and thermodynamical conditions during the wet and the dry spell of the monsoon, which might be responsible for distinct vertical distribution of w in MCS observed during the wet and dry spelld of the monsoon over Gadanki, which will have important implications in large-scale heating of the atmosphere that in turn effects the circulation.

Acknowledgments

The Gadanki VHF radar belongs to National Atmospheric Research Laboratory (NARL), under the Department of Space, government of India. The authors would like to thank the NARL director and the technical staff for their support for conducting the radar experiments and GPS sonde launches.

REFERENCES

  • Atlas, D. C., W. Ulbrich, F. D. Marks, and C. Williams, 1999: Systematic variation of drop size and radar-rainfall relations. J. Geophys. Res., 104, 61556170.

    • Search Google Scholar
    • Export Citation
  • Balsley, B. B., W. L. Ecklund, D. A. Carter, A. C. Riddle, and K. S. Gage, 1988: Average vertical motions in the tropical atmosphere observed by a radar wind profiler on Pohnpei (7°N latitude, 157°E longitude). J. Atmos. Sci., 45, 396405.

    • Search Google Scholar
    • Export Citation
  • Bhat, G. S., A. Chakraborty, R. S. Nanjundiah, and J. Srinivasan, 2002: Vertical thermal structure of the atmosphere during active and weak phases of convection over the North Bay of Bengal: Observation and model results. Curr. Sci., 83, 296302.

    • Search Google Scholar
    • Export Citation
  • Chong, M., J. Testud, and F. Roux, 1983: Three-dimensional wind analysis from dual-Doppler radar data. Part II: Minimizing the error due to temporal variation. J. Climate Appl. Meteor., 22, 12161226.

    • Search Google Scholar
    • Export Citation
  • Cifelli, R., and S. A. Rutledge, 1994: Vertical motion structure in maritime continent mesoscale, convective systems: Results from 50-MHz profiler. J. Atmos. Sci., 51, 26312652.

    • Search Google Scholar
    • Export Citation
  • Cifelli, R., and S. A. Rutledge, 1998: Vertical motion, diabatic heating, and rainfall characteristics in north Australia convective systems. Quart. J. Roy. Meteor. Soc., 124, 11331162.

    • Search Google Scholar
    • Export Citation
  • Cifelli, R., W. A. Peterson, L. D. Carey, S. A. Rutledge, and M. A. F. da Silva Dias, 2002: Radar observations of the kinematic, microphysical, and precipitation characteristics of two MCS’s in TRMM LBA. J. Geophys. Res., 107, 8077, doi:10.1029/2000JD000264.

    • Search Google Scholar
    • Export Citation
  • De, U. S., R. R. Lele, and J. C. Natu, 1998: Breaks in southwest monsoon. Rep. 1998/3. India Meteorology Department, New Delhi, India, 32 pp.

    • Search Google Scholar
    • Export Citation
  • Emanuel, K. A., 1994: Atmospheric Convection. Oxford University Press, 580 pp.

  • Firestone, J. K., and B. A. Albrecht, 1986: The structure of the atmospheric boundary layer in the central equatorial Pacific during January and February of FGGE. Mon. Wea. Rev., 114, 22192230.

    • Search Google Scholar
    • Export Citation
  • Gadgil, S., and P. V. Joseph, 2003: On breaks of the Indian monsoon. Proc. Indian Acad. Sci. (Earth Planet. Sci.), 112, 529558.

  • Gamache, J. F., and R. A. Houze, 1985: Further analysis of the composite wind and thermodynamic structure of the 12 September GATE squall line. Mon. Wea. Rev., 113, 12411259.

    • Search Google Scholar
    • Export Citation
  • Held, I. M., R. S. Hemler, and V. Ramaswamy, 1993: Radiative-convective equilibrium with explicit two-dimensional moist convection. J. Atmos. Sci., 50, 39093927.

    • Search Google Scholar
    • Export Citation
  • Houze, R. A., Jr., 1989: Observed structure of mesoscale convective systems and implications on large-scale heating. Quart. J. Roy. Meteor. Soc., 115, 425461.

    • Search Google Scholar
    • Export Citation
  • Houze, R. A., Jr., 1993: Cloud Dynamics. Academic Press, 573 pp.

  • Houze, R. A., Jr., 2004: Mesoscale convective systems. Rev. Geophys., 42, RG4003, 10.1029/2004RG000150.

  • Houze, R. A., Jr., and P.V. Hobbs, 1982: Organization and structure of precipitating cloud systems. Advances in Geophysics, Vol. 24, Academic Press, 225–315.

    • Search Google Scholar
    • Export Citation
  • Joseph, P. V., and S. Sijikumar, 2004: Intraseasonal variability of the low-level jet stream of the Asian summer monsoon. J. Climate, 17, 14491458.

    • Search Google Scholar
    • Export Citation
  • Kloesel, K. A., and B. A. Albrecht, 1989: Low-level inversions over the tropical Pacific thermodynamic structure of the boundary layer and the above-inversion moisture structure. Mon. Wea. Rev., 117, 87101.

    • Search Google Scholar
    • Export Citation
  • Krishnan, R., C. Zhang, and M. Sugi, 2000: Dynamics of breaks in the Indian summer monsoon. J. Atmos. Sci., 57, 13541372.

  • Kumar, K. K., A. R. Jain, and D. N. Rao, 2005: VHF/UHF radar observations of tropical mesoscale convective systems over southern India. Ann. Geophys., 23, 16731683.

    • Search Google Scholar
    • Export Citation
  • LeMone, M. A., and E. J. Zipser, 1980: Cumulonimbus vertical velocity events in GATE. Part I: Diameter, intensity, and mass flux. J. Atmos. Sci., 37, 24442457.

    • Search Google Scholar
    • Export Citation
  • Lucas, C., E. J. Zipser, and M. A. Lemone, 1994: Vertical velocity in oceanic convection off tropical Australia. J. Atmos. Sci., 51, 31833193.

    • Search Google Scholar
    • Export Citation
  • May, P. T., and D. K. Rajopadhyaya, 1999: Vertical velocity characteristics of deep convection over Darwin, Australia. Mon. Wea. Rev., 127, 10561070.

    • Search Google Scholar
    • Export Citation
  • Moncrieff, M. W., and M. J. Miller, 1976: The dynamics and simulation of tropical cumulonimbus and squall line. Quart. J. Roy. Meteor. Soc., 102, 373394.

    • Search Google Scholar
    • Export Citation
  • Newton, C. W., and H. Newton, 1959: Dynamical interactions between large convective clouds and the environment with vertical shear. J. Meteor., 16, 483496.

    • Search Google Scholar
    • Export Citation
  • Rajeevan, M., J. Bhate, J. D. Kale, and B. Lal, 2006: High-resolution daily gridded rainfall data for the Indian region: Analysis of break and active monsoon spells. Curr. Sci., 91, 296306.

    • Search Google Scholar
    • Export Citation
  • Rao, K. G., M. Desbois, R. Roca, and K. Nakamura, 2004: Upper tropospheric drying and the “transition to break” in the Indian summer monsoon during 1999. Geophys. Res. Lett., 31, L03206, doi:10.1029/2003GL018269.

    • Search Google Scholar
    • Export Citation
  • Rao, P. B., A. R. Jain, P. Kishore, P. Balamuralidhar, S. H. Damle, and G. Viswanathan, 1995: Indian MST radar 1. System description and sample vector wind measurements using ST mode. Radio Sci., 30, 11251138.

    • Search Google Scholar
    • Export Citation
  • Rao, T. N., K. N. Uma, T. M. Satyanarayana, and D. N. Rao, 2009: Differences in draft core statistics from the wet to dry spell over Gadanki, India (13.5°N, 79.2°E). Mon. Wea. Rev., 137, 42934306.

    • Search Google Scholar
    • Export Citation
  • Smull, B. F., and R. A. Houze Jr., 1987: Dual-Doppler radar analysis of a midlatitude squall line with a trailing region of stratiform rain. J. Atmos. Sci., 44, 21282148.

    • Search Google Scholar
    • Export Citation
  • Uma, K. N., and T. N. Rao, 2009: Characteristics of vertical velocity cores in different convective systems observed over Gadanki, India. Mon. Wea. Rev., 137, 954975.

    • Search Google Scholar
    • Export Citation
  • Uma, K. N., K. K. Kumar, and T. N. Rao, 2011: VHF radar observed characteristics of convectively generated gravity waves during wet and dry spells of Indian summer monsoon. J. Atmos. Sol. Terr. Phys., 73, 815824.

    • Search Google Scholar
    • Export Citation
  • Webster, P. J., C. Clark, G. Cherikova, J. Fasullo, W. Han, J. Loschnigg, and K. Sahami, 2002: The monsoon as a self-regulating coupled ocean-atmosphere system. Meteorology at the Millennium, R. B. Pearce, Ed., International Geophysical Series, Vol. 83, Academic Press, 198–219.

    • Search Google Scholar
    • Export Citation
  • Yuter, S. E., and R. A. Houze Jr., 1995: Three-dimensional kinematic and microphysical evolution of Florida cumulonimbus. Part I: Spatial distribution of updrafts, downdrafts, and precipitation. Mon. Wea. Rev., 123, 19211940.

    • Search Google Scholar
    • Export Citation
  • Zipser, E. J., and M. A. Lemone, 1980: Cumulonimbus vertical velocity events in GATE. Part II: Synthesis and model structure. J. Atmos. Sci., 37, 24582469.

    • Search Google Scholar
    • Export Citation
Save
  • Atlas, D. C., W. Ulbrich, F. D. Marks, and C. Williams, 1999: Systematic variation of drop size and radar-rainfall relations. J. Geophys. Res., 104, 61556170.

    • Search Google Scholar
    • Export Citation
  • Balsley, B. B., W. L. Ecklund, D. A. Carter, A. C. Riddle, and K. S. Gage, 1988: Average vertical motions in the tropical atmosphere observed by a radar wind profiler on Pohnpei (7°N latitude, 157°E longitude). J. Atmos. Sci., 45, 396405.

    • Search Google Scholar
    • Export Citation
  • Bhat, G. S., A. Chakraborty, R. S. Nanjundiah, and J. Srinivasan, 2002: Vertical thermal structure of the atmosphere during active and weak phases of convection over the North Bay of Bengal: Observation and model results. Curr. Sci., 83, 296302.

    • Search Google Scholar
    • Export Citation
  • Chong, M., J. Testud, and F. Roux, 1983: Three-dimensional wind analysis from dual-Doppler radar data. Part II: Minimizing the error due to temporal variation. J. Climate Appl. Meteor., 22, 12161226.

    • Search Google Scholar
    • Export Citation
  • Cifelli, R., and S. A. Rutledge, 1994: Vertical motion structure in maritime continent mesoscale, convective systems: Results from 50-MHz profiler. J. Atmos. Sci., 51, 26312652.

    • Search Google Scholar
    • Export Citation
  • Cifelli, R., and S. A. Rutledge, 1998: Vertical motion, diabatic heating, and rainfall characteristics in north Australia convective systems. Quart. J. Roy. Meteor. Soc., 124, 11331162.

    • Search Google Scholar
    • Export Citation
  • Cifelli, R., W. A. Peterson, L. D. Carey, S. A. Rutledge, and M. A. F. da Silva Dias, 2002: Radar observations of the kinematic, microphysical, and precipitation characteristics of two MCS’s in TRMM LBA. J. Geophys. Res., 107, 8077, doi:10.1029/2000JD000264.

    • Search Google Scholar
    • Export Citation
  • De, U. S., R. R. Lele, and J. C. Natu, 1998: Breaks in southwest monsoon. Rep. 1998/3. India Meteorology Department, New Delhi, India, 32 pp.

    • Search Google Scholar
    • Export Citation
  • Emanuel, K. A., 1994: Atmospheric Convection. Oxford University Press, 580 pp.

  • Firestone, J. K., and B. A. Albrecht, 1986: The structure of the atmospheric boundary layer in the central equatorial Pacific during January and February of FGGE. Mon. Wea. Rev., 114, 22192230.

    • Search Google Scholar
    • Export Citation
  • Gadgil, S., and P. V. Joseph, 2003: On breaks of the Indian monsoon. Proc. Indian Acad. Sci. (Earth Planet. Sci.), 112, 529558.

  • Gamache, J. F., and R. A. Houze, 1985: Further analysis of the composite wind and thermodynamic structure of the 12 September GATE squall line. Mon. Wea. Rev., 113, 12411259.

    • Search Google Scholar
    • Export Citation
  • Held, I. M., R. S. Hemler, and V. Ramaswamy, 1993: Radiative-convective equilibrium with explicit two-dimensional moist convection. J. Atmos. Sci., 50, 39093927.

    • Search Google Scholar
    • Export Citation
  • Houze, R. A., Jr., 1989: Observed structure of mesoscale convective systems and implications on large-scale heating. Quart. J. Roy. Meteor. Soc., 115, 425461.

    • Search Google Scholar
    • Export Citation
  • Houze, R. A., Jr., 1993: Cloud Dynamics. Academic Press, 573 pp.

  • Houze, R. A., Jr., 2004: Mesoscale convective systems. Rev. Geophys., 42, RG4003, 10.1029/2004RG000150.

  • Houze, R. A., Jr., and P.V. Hobbs, 1982: Organization and structure of precipitating cloud systems. Advances in Geophysics, Vol. 24, Academic Press, 225–315.

    • Search Google Scholar
    • Export Citation
  • Joseph, P. V., and S. Sijikumar, 2004: Intraseasonal variability of the low-level jet stream of the Asian summer monsoon. J. Climate, 17, 14491458.

    • Search Google Scholar
    • Export Citation
  • Kloesel, K. A., and B. A. Albrecht, 1989: Low-level inversions over the tropical Pacific thermodynamic structure of the boundary layer and the above-inversion moisture structure. Mon. Wea. Rev., 117, 87101.

    • Search Google Scholar
    • Export Citation
  • Krishnan, R., C. Zhang, and M. Sugi, 2000: Dynamics of breaks in the Indian summer monsoon. J. Atmos. Sci., 57, 13541372.

  • Kumar, K. K., A. R. Jain, and D. N. Rao, 2005: VHF/UHF radar observations of tropical mesoscale convective systems over southern India. Ann. Geophys., 23, 16731683.

    • Search Google Scholar
    • Export Citation
  • LeMone, M. A., and E. J. Zipser, 1980: Cumulonimbus vertical velocity events in GATE. Part I: Diameter, intensity, and mass flux. J. Atmos. Sci., 37, 24442457.

    • Search Google Scholar
    • Export Citation
  • Lucas, C., E. J. Zipser, and M. A. Lemone, 1994: Vertical velocity in oceanic convection off tropical Australia. J. Atmos. Sci., 51, 31833193.

    • Search Google Scholar
    • Export Citation
  • May, P. T., and D. K. Rajopadhyaya, 1999: Vertical velocity characteristics of deep convection over Darwin, Australia. Mon. Wea. Rev., 127, 10561070.

    • Search Google Scholar
    • Export Citation
  • Moncrieff, M. W., and M. J. Miller, 1976: The dynamics and simulation of tropical cumulonimbus and squall line. Quart. J. Roy. Meteor. Soc., 102, 373394.

    • Search Google Scholar
    • Export Citation
  • Newton, C. W., and H. Newton, 1959: Dynamical interactions between large convective clouds and the environment with vertical shear. J. Meteor., 16, 483496.

    • Search Google Scholar
    • Export Citation
  • Rajeevan, M., J. Bhate, J. D. Kale, and B. Lal, 2006: High-resolution daily gridded rainfall data for the Indian region: Analysis of break and active monsoon spells. Curr. Sci., 91, 296306.

    • Search Google Scholar
    • Export Citation
  • Rao, K. G., M. Desbois, R. Roca, and K. Nakamura, 2004: Upper tropospheric drying and the “transition to break” in the Indian summer monsoon during 1999. Geophys. Res. Lett., 31, L03206, doi:10.1029/2003GL018269.

    • Search Google Scholar
    • Export Citation
  • Rao, P. B., A. R. Jain, P. Kishore, P. Balamuralidhar, S. H. Damle, and G. Viswanathan, 1995: Indian MST radar 1. System description and sample vector wind measurements using ST mode. Radio Sci., 30, 11251138.

    • Search Google Scholar
    • Export Citation
  • Rao, T. N., K. N. Uma, T. M. Satyanarayana, and D. N. Rao, 2009: Differences in draft core statistics from the wet to dry spell over Gadanki, India (13.5°N, 79.2°E). Mon. Wea. Rev., 137, 42934306.

    • Search Google Scholar
    • Export Citation
  • Smull, B. F., and R. A. Houze Jr., 1987: Dual-Doppler radar analysis of a midlatitude squall line with a trailing region of stratiform rain. J. Atmos. Sci., 44, 21282148.

    • Search Google Scholar
    • Export Citation
  • Uma, K. N., and T. N. Rao, 2009: Characteristics of vertical velocity cores in different convective systems observed over Gadanki, India. Mon. Wea. Rev., 137, 954975.

    • Search Google Scholar
    • Export Citation
  • Uma, K. N., K. K. Kumar, and T. N. Rao, 2011: VHF radar observed characteristics of convectively generated gravity waves during wet and dry spells of Indian summer monsoon. J. Atmos. Sol. Terr. Phys., 73, 815824.

    • Search Google Scholar
    • Export Citation
  • Webster, P. J., C. Clark, G. Cherikova, J. Fasullo, W. Han, J. Loschnigg, and K. Sahami, 2002: The monsoon as a self-regulating coupled ocean-atmosphere system. Meteorology at the Millennium, R. B. Pearce, Ed., International Geophysical Series, Vol. 83, Academic Press, 198–219.

    • Search Google Scholar
    • Export Citation
  • Yuter, S. E., and R. A. Houze Jr., 1995: Three-dimensional kinematic and microphysical evolution of Florida cumulonimbus. Part I: Spatial distribution of updrafts, downdrafts, and precipitation. Mon. Wea. Rev., 123, 19211940.

    • Search Google Scholar
    • Export Citation
  • Zipser, E. J., and M. A. Lemone, 1980: Cumulonimbus vertical velocity events in GATE. Part II: Synthesis and model structure. J. Atmos. Sci., 37, 24582469.

    • Search Google Scholar
    • Export Citation
  • Fig. 1.

    The time–height section of (a) SNR, (b) vertical air velocity, and (c) mean w derived from MST radar measurements during the deep stage of convection during the passage of an MCS on 18 Sep 2001 for the wet spell of the monsoon. The mean profile is obtained by averaging the vertical velocities during 1520 to 1535 LT. The closed contours indicate the downdraft for (<−1 m s−1).

  • Fig. 2.

    As in Fig. 1, but for the MCS observed on 31 Jul 2001 during the dry spell of the monsoon. The mean profile is obtained by averaging the vertical velocities during 1840–1915 LT.

  • Fig. 3.

    Vertical variation of composite w (dashed line) and its average (solid line) along with standard deviation (horizontal line) for deep cores in the (a) wet and (b) dry spells. Each profile is a composite of one convective event.

  • Fig. 4.

    Mean vertical wind averaged over 10°–15°N latitude for (left) the wet and (right) the dry spell of the monsoon around Gadanki longitudes for the convective events tabulated in Table 1.

  • Fig. 5.

    Height profile of mean w and wind shear for four cases during the dry spell of the monsoon on (a) 18 May 2006, (b) 12 Jun 2002, (c) 31 Jul 2001, and (d) 31 Aug 2005.

  • Fig. 6.

    Mean (a) zonal and (b) meridional wind profiles from NCEP–NCAR reanalysis for the wet and dry spells of the monsoon for the convective events given in Table 1.

  • Fig. 7.

    (a) Square of vertical shear of horizontal wind along with standard deviation during the wet spell. (b) As in (a), but for the dry spell of the monsoon for the dates given in Table 2.

  • Fig. 8.

    (a) Mean temperature profile during the (a) wet and (b) dry spell of the monsoon. (c) Cumulative counts of temperature gradient. (d) Equivalent potential temperature for the year 2006 (for the days given in Table 2) from GPS sondes launched from Gadanki during the wet (square) and dry (circle) spells of the monsoon.

  • Fig. 9.

    CAPE during the wet and the dry spell of the monsoon with its mean (square), median (circle), and standard deviation during 2006 for the days given in Table 2.

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