T. Narayana Rao

Tropical precipitating systems observed with Indian MST radar

Three campaigns are conducted with the Indian mesosphere-stratosphere-troposphere (MST) radar, located at Gadanki (13.5°N, 79.2°E), India, to study the precipitating systems in the tropics. This study mainly deals with (1) classification of precipitating clouds and the spectral characteristics of echoes associated with these cloud systems and (2) characteristics of the radar bright band. The radar gets echoes scattered both from refractive index fluctuations and precipitation particles in moderate to heavy precipitation conditions. These echoes are separated in the spectral domain to determine the vertical air motion and the Doppler velocity of hydrometeors simultaneously. The tropical precipitating systems are classified as stratiform and convective using the reflectivity and vertical velocity distribution. The echo power, spectral width, and vertical velocities of the ambient air and hydrometeors in both the cloud systems have been compared. Aspect sensitivity of the echoes from the hydrometeors and refractive index fluctuations in both stratiform and convective atmosphere is studied. A transition stage, where the stratiform precipitation is associated with the convection, is also reported. Backscattered power from precipitation particles is used to estimate the reflectivity factor (dBZ), and these values along with spectral width and vertical velocity values are used to identify the bright band structure. The reflectivity at the bright band, up to 42 dBZ, is found to be 10–12 dB more than the average value of reflectivity below the bright band. Discussion on the factors contributing to this enhancement is also included. A clear layered structure around the 0°C isotherm in the reflectivity profile of the precipitation echo confirms the presence of the bright band. The thickness of the bright band is estimated and is correlated with the peak reflectivity at the bright band. Comparison of the average terminal velocity of hydrometeors with their average Doppler velocities below the bright band shows the presence of gentle updrafts of a few cm s−1 in stratiform precipitation. These studies are made for the first time with the Indian MST radar and also demonstrate the capability of a VHF radar in studying precipitating systems in addition to the turbulence to which these radars are highly sensitive.

Characteristics of Vertical Velocity Cores in Different Convective Systems Observed over Gadanki, India

Abstract The Indian mesosphere–stratosphere–troposphere (MST) radar measurements during the passage of 60 convective systems are used to study the vertical air velocity (w) characteristics of tropical convection. The up- and downdraft cores and various stages/types of convection (shallow, deep, and decaying) are discerned from radar time–intensity maps of w. The characteristics of cores (speed, size, orientation, vertical extent, gravity wave activity, etc.) at different stages of convection are discussed with the help of three case studies. The cores stratified based on the type of convection are mostly erect in nature in all types of convective systems, except for deep updraft cores. A considerable percentage (35%) of deep updraft cores show inclined structure with elevation angles as low as 0°–20°. The variation of the horizontal wind field with height and the internal dynamics of mesoscale convective systems (MCSs) are thought to be responsible for this geometry. Further, the vertical extent of draft ...

Studies on refractivity structure constant, eddy dissipation rate, and momentum flux at a tropical latitude

VHF and UHF Doppler radars provide a unique database to estimate the refractivity structure constant Cn2, eddy dissipation rate ∈, and vertical flux of horizontal momentum. Using the data collected from the Indian MST radar, these parameters are studied at a tropical latitude. The refractivity turbulence structure constant is estimated from the backscattered power of the received echoes. Cn2 (radar) and Cn2 (model), derived from radiosonde observations, are compared, and a fairly good agreement is seen. Diurnal and seasonal variations of Cn2 are also presented. The eddy dissipation rate is estimated from the radar echoes employing the power and spectral width methods. A fairly good agreement is seen between the two methods. Values of e are found to vary from 10−6 to 10−3 m2 s−3 in a height range of 4–19 km. Cn2 and e are observed to be minimum during a moderate jet stream wind of 50–60 m s−1. Vertical flux of horizonal momentum is computed using the symmetrical two-beam method. Significant fluxes of westward and northward momentum are observed, and the values lie in the range of −1 to +1 m2 s−2. The implied accelarations are also estimated. The results presented are largely consistent with the results available in the literature.

Climatology of UTLS ozone and the ratio of ozone and potential vorticity over northern Europe

Annual and interannual variations of ozone in the upper troposphere and lower stratosphere (UTLS) region have been studied using ozonesonde data collected between 1994 and 2001 at several northern European stations. The climatology of ozone exhibits a prominent annual cycle in the UTLS region. The observed change in the phase of the annual cycle from late spring-early summer at 500 hPa to spring at 200 hPa and to winter-early spring at 100 hPa shows the switching of the ozone control from photochemical to dynamical. Traces of interannual variation in the lower stratosphere are seen not only in the upper troposphere but also in the middle troposphere (not necessarily always) indicating the dynamical influence on tropospheric ozone budget. Further, the correlation between ozone mixing ratio and potential vorticity (PV) is studied at three northern high-latitude stations. As expected, a good correlation is found in the lower stratosphere, while the correlation is fair in the middle troposphere, except during summer over the European Arctic. This weak correlation at high latitudes indicates the dominance of photochemistry over dynamics in the presence of prolonged hours of solar illumination. The correlation coefficients derived at high latitudes are smaller than those reported at midlatitudes. This could be due to the greater number of tropopause folds at midlatitudes than at high latitudes and this eventually leads to the conclusion that the downward cross-tropopause flux is greater at midlatitudes than at high latitudes. Absence of a significant north-south gradient in the ozone/PV ratio in the lower stratosphere suggests that a single ozone/PV ratio (however, the ratio varies with month) can be used to convert global PV fluxes to ozone fluxes. A few cases of tropopause folds (only one case study is reported in the present study) are selected and studied in detail with the help of a very high frequency radar and meteorological analysis. The ratio between ozone and PV for these case studies agrees reasonably well with the climatological ratios.

Statistical Characteristics of Raindrop Size Distribution in Southwest Monsoon Season

Abstract Raindrop size distribution (DSD) parameters are retrieved from dual-frequency (UHF and VHF) wind profiler measurements made at Gadanki, India, in a summer monsoon season. The convoluted UHF spectra are first corrected for vertical air motion and spectral broadening (using VHF measurements) and later are used for deriving DSD parameters. Two distinctly different case studies, a mesoscale convective system and a pure stratiform precipitation system, have been considered for a detailed study. DSD parameters obtained in these case studies reveal systematic variations of DSD from case to case and also from one rain regime to another within the same precipitating system. A statistical study has been carried out using the profiler data collected during the passage of 16 rain events. The retrieved DSD profiles are divided into separate rain regimes (stratiform and convection), based on reflectivity, to examine salient microphysical characteristics and the vertical variability of DSD in different precipit...

Coordinated MST radar and lidar observations for the study of mesospheric structures over a tropical station

Abstract VHF radar observations at a frequency of 53 MHz at Gadanki (13.5°N,79.2°E), India, during the period from September 1995 to August 1999 are used to study the tropical mesospheric structures. MST radar echoes have shown an enhancement in echo power of about 6– 8 dB above the average noise level. These echoes are intermittent in time and can be observed at an altitude between 70 and 76 km . The seasonal variation of these echoes shows the maximum percentage occurrence during summer, closely followed by equinoxes and minimum during winter. This seasonal variation is also accompanied by shifting of height to lower heights during winter. Coordinated MST radar and Nd:YAG lidar observations were conducted to study the mesospheric structures during March 1998–June 1999. In the Lidar temperature profiles, inversion of about 20– 30 K is observed at the heights of the MST radar-enhanced echoes. Enhanced radar reflectivity is observed on both the preceding and succeeding days of lidar observations of strong temperature inversion. Percentage occurrence of MST radar echoes and temperature inversions show one-to-one correspondence for most of the cases. The effect of temperature inversion on the radar reflectivity is also studied. The presence of temperature inversion and enhanced radar reflectivity are discussed in the light of gravity-wave breaking processes at that height region.

Differences in draft core statistics from the wet to dry spell over Gadanki, India (13.5°N, 79.2°E).

Abstract The Indian mesosphere–stratosphere–troposphere (MST) radar observations during the passage of 37 convective systems are utilized to investigate the characteristics of vertical air velocity w in different convection categories (shallow, deep, and decaying) and also the differences in draft core statistics from the wet to dry spell. The radar and optical rain gauge measurements show pronounced differences in core statistics (in terms of their vertical structure, draft strength, size, number, and the elevation angle) and surface rainfall characteristics from the dry to wet spell. The shallow convective cores are preponderant in the dry spell. Composite w profiles, retrieved from all deep cases and also from individual convection cases, depict an upper-tropospheric peak in the wet spell and a bimodal distribution (peaks at 5 and 11–13 km) in the dry spell, illustrating that they are characteristic features of wet and dry spells. The average vertical extents of the cores are nearly equal (about 8 km) ...

Morphology of the vertical structure of precipitation over India and adjoining oceans based on long-term measurements of TRMM PR

The spatial and seasonal variability of the vertical structure of precipitation has been studied using 15 years of Tropical Rainfall Measuring Missions Precipitation Radar (TRMM PR) version 7 data over India and adjoining oceans. Special emphasis has been put on six different climatic rain regimes and on different types of precipitation including the virga rain. The distribution of reflectivity factor (Z) above the freezing level height is broader in northwest India (NWI) and narrower over the Arabian Sea and west coast of India (ASWC) than in other selected regions, due to dominance of deep and shallow convective rain, respectively, in those regions. The height variation of contours in normalized distributions for Z indicates that evaporation of raindrops (low-level hydrometeor growth) could be significant in NWI (ASWC and Bay of Bengal). All the above features show clear seasonal variation and are observed predominantly during the southwest monsoon. The occurrence of virga rain clearly shows land-ocean contrast (less over the oceans) and seasonal variation (preponderant during premonsoon). Among different rain categories, the stratiform (convective) rain had highest (lowest) fraction of virga rain of >15–30% (<10%) over land regions. 1. The storm height (SH) vertical distributions show a peak in the vicinity of bright band (BB) in all regions, except for those regions and seasons, where convective precipitation is dominant. The well-defined BB feature and SH exhibit significant seasonal and regional variations, which are linked to variations in the occurrence of stratiform rain and height of BB. The spatial and seasonal variations of mean SH and the occurrence of deep and overshooting convective rain show good correspondence with the spatial variation of convective available potential energy.

VHF radar observations of tropical easterly jet stream over gadanki

The three dimensional wind velocities evaluated using the data obtained from the Indian Mesosphere-Stratosphere-Troposphere (MST) Radar are used to study the tropospheric and lower stratospheric dynamics. Temporal variation of the zonal wind velocities during September 1995 – September 1996 are used to identify and study the tropical easterly jet stream characteristics. The jet is active over the observation site during the months of June, July and August with maximum strength in July. The zonal winds are westerlies upto about 6 km and easterlies start above that height and the maximum winds during jet stream conditions occurred at around 16 km. A reversal in vertical velocities, i.e., descent to ascent, below the jet maximum is observed. Gravity waves generated by the jet induced wind shears are observed with 8 hours periodicity. The wind shears, refractivity turbulence structure constant, Cn2 and the eddy dissipation rate, e were calculated around the jet stream and the results are presented.

Identification and Validation of Homogeneous Rainfall Zones in India Using Correlation Analysis

AbstractDaily rainfall data obtained from 1025 rain gauges spread across the country over 51 years (1951–2001) are subjected to correlation analysis to identify homogeneous rainfall zones over India. In contrast to earlier studies, which were based on seasonal/annual rainfall, the present study identifies homogeneous rainfall regions with the help of seasonal [southwest monsoon (SWM) and northeast monsoon (NEM)] and annual rainfall. India is divided into 26 (20) homogeneous rainfall zones using annual and SWM (NEM) rainfall. The delineated homogeneous regions are compared and contrasted with those defined by earlier studies, employing a variety of schemes. The interseries correlations of rainfall within each zone are found to be better when the zones are identified by the present study than by other studies. The tests that are performed to evaluate coherency of zones reveal that the zones are homogeneous not only at different temporal scales (interannual and intraseasonal) but also in terms of rain amount...