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

2011 ◽  
Vol 68 (7) ◽  
pp. 1413-1423 ◽  
Author(s):  
M. Roja Raman ◽  
M. Venkat Ratnam ◽  
M. Rajeevan ◽  
V. V. M. Jagannadha Rao ◽  
S. Vijaya Bhaskara Rao
Keyword(s):  
High Resolution ◽  
Indian Monsoon ◽  
Lower Troposphere ◽  
Vertical Variation ◽  
Peninsular India ◽  
Low Level ◽  
Low Level Jet ◽  
Core Height ◽  
Gps Radiosonde ◽  
Monsoon Dynamics

Abstract The 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.

Response of the low‐level jet to precession and its implications for proxies of the Indian monsoon

2022 ◽  
Author(s):  
Chetankumar Jalihal ◽  
Jayaraman Srinivasan ◽  
Arindam Chakraborty
Keyword(s):  
Indian Monsoon ◽  
Low Level ◽  
Low Level Jet

scholarly journals Low-level stratiform clouds and dynamical features observed within the southern West African monsoon

2019 ◽  
Vol 19 (13) ◽  
pp. 8979-8997 ◽  
Author(s):  
Cheikh Dione ◽  
Fabienne Lohou ◽  
Marie Lothon ◽  
Bianca Adler ◽  
Karmen Babić ◽  
...  
Keyword(s):  
West Africa ◽  
Boreal Summer ◽  
Time Range ◽  
Atmospheric Conditions ◽  
Low Level ◽  
Stratus Clouds ◽  
Stratiform Clouds ◽  
Monsoon Flow ◽  
Low Level Jet ◽  
Core Height

Abstract. During the boreal summer, the monsoon season that takes place in West Africa is accompanied by low stratus clouds over land that stretch from the Guinean coast several hundred kilometers inland. Numerical climate and weather models need finer description and knowledge of cloud macrophysical characteristics and of the dynamical and thermodynamical structures occupying the lowest troposphere, in order to be properly evaluated in this region. The Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa (DACCIWA) field experiment, which took place in summer 2016, addresses this knowledge gap. Low-level atmospheric dynamics and stratiform low-level cloud macrophysical properties are analyzed using in situ and remote sensing measurements continuously collected from 20 June to 30 July at Savè, Benin, roughly 180 km from the coast. The macrophysical characteristics of the stratus clouds are deduced from a ceilometer, an infrared cloud camera, and cloud radar. Onset times, evolution, dissipation times, base heights, and thickness are evaluated. The data from an ultra-high-frequency (UHF) wind profiler, a microwave radiometer, and an energy balance station are used to quantify the occurrence and characteristics of the monsoon flow, the nocturnal low-level jet, and the cold air mass inflow propagating northward from the coast of the Gulf of Guinea. The results show that these dynamical structures are very regularly observed during the entire 41 d documented period. Monsoon flow is observed every day during our study period. The so-called “maritime inflow” and the nocturnal low-level jet are also systematic features in this area. According to synoptic atmospheric conditions, the maritime inflow reaches Savè around 18:00–19:00 UTC on average. This timing is correlated with the strength of the monsoon flow. This time of arrival is close to the time range of the nocturnal low-level jet settlement. As a result, these phenomena are difficult to distinguish at the Savè site. The low-level jet occurs every night, except during rain events, and is associated 65 % of the time with low stratus clouds. Stratus clouds form between 22:00 and 06:00 UTC at an elevation close to the nocturnal low-level jet core height. The cloud base height, 310±30 m above ground level (a.g.l.), is rather stationary during the night and remains below the jet core height. The cloud top height, at 640±100 m a.g.l., is typically found above the jet core. The nocturnal low-level jet, low-level stratiform clouds, monsoon flow, and maritime inflow reveal significant day-to-day and intra-seasonal variability during the summer given the importance of the different monsoon phases and synoptic atmospheric conditions. Distributions of strength, depth, onset time, breakup time, etc. are quantified here. These results contribute to satisfy the main goals of DACCIWA and allow a conceptual model of the dynamical structures in the lowest troposphere over the southern part of West Africa.

Remote influences on the Indian monsoon Low-Level Jet intraseasonal variations

2020 ◽  
Author(s):  
Takeshi Izumo ◽  
Maratt Satheesan Swathi ◽  
Matthieu Lengaigne ◽  
Jérôme Vialard ◽  
Dr Ramesh Kumar
Keyword(s):  
Indian Ocean ◽  
Indian Summer Monsoon ◽  
Arabian Sea ◽  
Large Scale ◽  
Indian Monsoon ◽  
Westerly Wind ◽  
Low Level ◽  
Intraseasonal Variations ◽  
The Indian Ocean ◽  
Low Level Jet

<p>A strong Low-Level Jet (LLJ), also known as the Findlater jet, develops over the Arabian Sea during the Indian summer monsoon. This jet is an essential source of moisture for monsoonal rainfall over the densely-populated Indian subcontinent and is a key contributor to the Indian Ocean oceanic productivity by sustaining the western Arabian Sea upwelling systems. The LLJ intensity fluctuates intraseasonally within the ~20- to 90-day band, in relation with the northward-propagating active and break phases of the Indian summer monsoon. Our observational analyses reveal that these large-scale regional convective perturbations  only explain about half of the intraseasonal LLJ variance, the other half being unrelated to large-scale convective perturbations over the Indian Ocean. We show that convective fluctuations in two regions outside the Indian Ocean can remotely force a LLJ intensification, four days later. Enhanced atmosphericdeep convection over the northwestern tropical Pacific yields westerly wind anomalies that propagate westward to the Arabian Sea as baroclinic atmospheric Rossby Waves. Suppressed convection over the eastern Pacific / North American monsoon region yields westerly wind anomalies that propagate eastward to the Indian Ocean as dry baroclinic equatorial Kelvin waves. Those largely independent remote influences jointly explain ~40% of the intraseasonal LLJ variance that is not related to convective perturbations over the Indian Ocean (i.e. ~20% of the total), with the northwestern Pacific contributing twice as much as the eastern Pacific. Taking into account these two remote influences should thus enhance the ability to predict the LLJ.</p><p> </p><p>Related reference: Swathi M.S, Takeshi Izumo, Matthieu Lengaigne, Jérôme Vialard and M.R. Ramesh Kumar:Remote influences on the Indian monsoon Low-Level Jet intraseasonal variations, accepted in Climate Dynamics.</p>

scholarly journals Modelling Hydrometeorological Extremes Associated to the Moisture Transport Driven by the Great Plains Low-Level Jet

2021 ◽  
Author(s):  
Luis Gimeno-Sotelo ◽  
Patricia de Zea Bermudez ◽  
Iago Algarra ◽  
Luis Gimeno
Keyword(s):  
Great Plains ◽  
North Atlantic ◽  
Source Region ◽  
Lower Troposphere ◽  
Dynamical Instability ◽  
Low Level ◽  
Extremal Dependence ◽  
The North ◽  
Low Level Jet ◽  
The North Atlantic

Abstract The Great Plains Low-Level Jet system consists of very strong winds in the lower troposphere that transport a huge amount of moisture from the Gulf of Mexico to the American Great Plains. This paper aims to study the extremes of the Transported Moisture (TM) from the GPLLJ source region to the jet domain; and, for low and high TM, to analyze the extremal dependence between the upper tail of the precipitation in the GPLLJ sink region and the lower tail of the tropospheric stability in that region (omega). The declustered extremes of TM were analyzed using Peaks Over Threshold (POT). A non-stationary Exponential model was fitted to the cluster maxima. Estimated return levels show that the extremes of TM are expected to decrease in the future. This is meteorologically congruent with the known displacement of the western edge of the North Atlantic Subtropical High, which controls atmospheric circulation in the North Atlantic, and to a higher scale with the change of phase from negative to positive of the Atlantic Multidecadal Oscillation. Bilogistic and Logistic models were fitted to the extremes of (-omega, precipitation) for low and high TM, respectively. The extremal dependence between "-omega" and precipitation proves to be stronger in the case of high TM. This confirms that dynamical instability represented by “-omega” is the most important parameter for achieving high values of precipitation once there is a mechanism that allows the continuous supply of large amounts of moisture, such as the derived from a low-level jet system.

scholarly journals Mixed layer depths via Doppler lidar during low-level jet events

2018 ◽  
Vol 176 ◽  
pp. 06017
Author(s):  
Brian Carroll ◽  
Belay Demoz ◽  
Timothy Bonin ◽  
Ruben Delgado
Keyword(s):  
Fuzzy Logic ◽  
Wind Speed ◽  
Mixed Layer ◽  
Wind Shear ◽  
Lower Troposphere ◽  
Doppler Lidar ◽  
High Wind ◽  
Fuzzy Logic Algorithm ◽  
Low Level ◽  
Low Level Jet

A low-level jet (LLJ) is a prominent wind speed peak in the lower troposphere. Nocturnal LLJs have been shown to transport and mix atmospheric constituents from the residual layer down to the surface, breaching quiescent nocturnal conditions due to high wind shear. A new fuzzy logic algorithm combining turbulence and aerosol information from Doppler lidar scans can resolve the strength and depth of this mixing below the jet. Conclusions will be drawn about LLJ relations to turbulence and mixing.

scholarly journals The life cycle of nocturnal low-level clouds over southern West Africa analysed using high-resolution simulations

2016 ◽  
Author(s):  
Bianca Adler ◽  
Norbert Kalthoff ◽  
Leonhard Gantner
Keyword(s):  
Life Cycle ◽  
High Resolution ◽  
West Africa ◽  
Single Case ◽  
Monsoon Season ◽  
Cloud Formation ◽  
Field Campaign ◽  
Low Level ◽  
Low Level Jet ◽  
Spatio Temporal

Abstract. We performed a high-resolution numerical simulation to study the life cycle of extensive low-level clouds which frequently form over southern West Africa during the monsoon season. This study was made in preparation for a field campaign in 2016 within the Dynamics-aerosol-chemistry-cloud interactions in West Africa (DACCIWA) project and focuses on an area around the city of Save in southern Benin. Nocturnal low-level clouds evolve a few hundred metres above the ground around the same level as a distinct low-level jet. Several processes are found to determine the spatio-temporal evolution of these clouds including (i) significant cooling of the nocturnal atmosphere due to horizontal advection with the south-westerly monsoon flow during the first half of the night, (ii) vertical cold air advection due to gravity waves leading to clouds in the wave crests and (iii) enhanced convergence and upward motion upstream of existing clouds that trigger new clouds. The latter is caused by an upward shift of the low-level jet in cloudy areas leading to horizontal convergence in the lower part and to horizontal divergence in the upper part of the cloud layer. Although this single case study hardly allows for a generalisation of the processes found, the results added to the optimisation of the measurements strategy for the field campaign and the observations will be used to the test the hypotheses for cloud formation resulting from this study.

Diurnal variations of the low-level jet over peninsular India during the onset of Asian summer monsoon

2014 ◽  
Vol 120 (1-2) ◽  
pp. 287-298 ◽  
Author(s):  
Sandhya K. Nair ◽  
Thara V. Prabha ◽  
N. Purushothaman ◽  
S. Sijikumar ◽  
S. Muralidharan ◽  
...  
Keyword(s):  
Summer Monsoon ◽  
Asian Summer Monsoon ◽  
Diurnal Variations ◽  
Peninsular India ◽  
Low Level ◽  
Asian Summer ◽  
Low Level Jet

scholarly journals Response of the low-level jet to precession and its implications for proxies of the Indian monsoon

2021 ◽  
Author(s):  
Chetankumar Jalihal ◽  
Jayaraman Srinivasan ◽  
Arindam Chakraborty
Keyword(s):  
Indian Monsoon ◽  
Low Level ◽  
Low Level Jet
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