M. Roja Raman

Intriguing Aspects of the Monsoon Low-Level Jet over Peninsular India Revealed by High-Resolution GPS Radiosonde Observations

AbstractThe strong cross-equatorial flow in the lower troposphere, widely known as the monsoon low-level jet (MLLJ), plays an important role in the Indian summer monsoon (ISM) rainfall during June–September. Using high-resolution GPS radiosonde observations over Gadanki (13.5°N, 79.2°E), some new aspects of MLLJ have been reported. In the present study it is found that, on average, the MLLJ exists at 710 hPa over southeastern peninsular India, rather than at 850 hPa as reported by earlier studies. It is observed that the ECMWF Re-Analysis (ERA)-Interim data provide better results on the spatial, temporal, and vertical variation of MLLJ. Further, the characteristics of the MLLJ during the active and break spells of ISM are also investigated; higher MLLJ core height and intensity are found during active phases of the Indian monsoon. This study emphasizes the use of high-resolution measurements for studying monsoon dynamics in detail.

Onset of Indian summer monsoon over Gadanki (13.5°N, 79.2°E): Study using lower atmospheric wind profiler

[1] The onset of Indian summer monsoon (ISM) over Gadanki (13.5°N; 79.2°E) is identified using variations in signal-to-noise ratio (SNR), wind speed, wind direction and vertical velocity at 1.5 km (∼850 hPa) using lower atmospheric wind profiler (LAWP). Strengthening of the low level wind speed attaining 8 ms -1 with directional change from south-easterlies to south-westerlies defines the beginning of the monsoon. Enhancement in SNR few days before with noticeable magnitude of 5-10 dB at the time of onset combined with clear-air vertical velocity reversal from downward to upward few days before onset and persisting during monsoon activity supplements the wind speed criteria in identifying onset. Hydrometeor velocity shows large downward values exceeding more than 1 ms -1 indicating occurrence of rainfall. It is proposed that UHF radar at a location can be used to identify the onset of ISM based on wind speed without considering rainfall separately.

Diurnal variation of ducts observed over a tropical station, Gadanki, using high‐resolution GPS radiosonde observations

A comprehensive study on diurnal variation of ducting using high vertical and temporal resolution radiosonde measurements over the Indian tropical region, Gadanki (13.5oN, 79.2oE) was presented. The diurnal variation of ducts was examined statistically based on both the refractivity and modified refractivity using 3 hourly radiosonde soundings obtained during October 2010 to March 2014 as a part of tropical tropopause dynamics (TTD) campaigns conducted under CAWSES India Phase II program. Strong diurnal variation in the altitude of occurrence of the duct has been found and is at maximum altitude (~2.5 km) during 11-17 LT. Interestingly, it is found that the occurrence of duct altitude closely follows the boundary layer altitude. Diurnal variation of duct altitude is maximum during post-monsoon followed by winter, monsoon and minimum in pre-monsoon. However, duct strength is maximum during winter followed by pre-monsoon, post-monsoon and minimum in monsoon. Duct thickness is found to be varying between 0.4 km and 1 km diurnally with the highest thickness during the winter season. Strong diurnal and seasonal variation in the percentage occurrence of the ducts was found with the highest percentage of ducts observed during winter (77%) followed by post-monsoon (51%), pre-monsoon (44%) and monsoon (10%). All the characteristics of ducts during all the seasons are maximum at 14 LT due to the high solar irradiance over Gadanki from 11-17 LT. The minimum frequency being trapped has been investigated and found that wave trapping occurs for the radars with frequencies 56-438 MHz over this station.

On the Detection of Onset and Activity of the Indian Summer Monsoon Using GPS RO Refractivity Profiles

AbstractGlobal positioning system (GPS) radio occultation (RO) data available during 2001–10 have been used to examine the variations in the refractivity during the onset of Indian summer monsoon (ISM) over the east Arabian Sea (5°–15°N, 65°–75°E). An enhancement of 5–10 N-units in the refractivity is observed around 4.8 km (~600 hPa) a few days (9.23 ± 3.6 days) before onset of the monsoon over Kerala, India. This is attributed to moisture buildup over the Arabian Sea during the monsoon onset phase. A sudden increase (1.5–2 K) in mean upper-tropospheric temperature at the time of onset and during the active phase of the monsoon is attributed to convective activity and the release of latent heat. On the day of monsoon onset over Kerala, an appreciable dip in the refractivity is observed that persisted for 1–3 days followed by an enhancement in refractivity with the active phase of the monsoon. An arbitrary value of 128 N-units difference between 4.8 km (~600 hPa) and 16 km (~100 hPa) coupled with a dip in...

Trends in Monthly Tropopause Characteristics Observed over Taipei, Taiwan*

AbstractThis study presents monthly trends in the cold-point tropopause (CPT), calculated using three decades of radiosonde data from 1981 to 2010 over Taipei, Taiwan (25°01′N, 121°27′E). Multivariate regression analysis has been used to suppress the effect of natural variations, such as quasi-biennial oscillation (QBO), ENSO, solar cycle, and volcanic eruptions. From the continuous time series, statistically insignificant heating and a decrease in the height of CPT are observed. However, the trends estimated using individual monthly time series revealed new features with statistically significant increasing trends in CPT temperature at a rate of approximately 0.03°C yr−1 and statistically significant decreasing trends in CPT height at a rate of approximately 4.7 m yr−1 during summer months. An enhanced heating rate in the upper troposphere along with a suppressed cooling rate in the lower stratosphere observed over Taipei might have caused the tropopause heating trend during summer. The possible relation...