The Orinoco Low-Level Jet and its association with the hydroclimatology of northern South America

2021 ◽  
Keyword(s):  
Wind Speed ◽  
South America ◽  
Moisture Flux ◽  
Spatial And Temporal Variability ◽  
Frequency Of Occurrence ◽  
Flux Divergence ◽  
Vapor Flux ◽  
Low Level ◽  
Low Level Jet ◽  
Northern South America

Abstract We investigated the relationship between the frequency of occurrence of the Orinoco Low-Level Jet (OLLJ) and hydroclimatic variables over northern South America. We use data from the ERA5 atmospheric reanalysis to characterize the spatial and temporal variability of the OLLJ in light of the LLJ-classification criteria available in the literature. An index for the frequency of occurrence of an LLJ was used, based on the hourly maxima of wind speed. The linkages among the OLLJ, water vapor flux, and precipitation were analyzed using a composite analysis. Our results show that during December–January–February (DJF), the OLLJ exhibits its maximum wind speed, with values around 8–10 m/s. During DJF, the analysis shows how the OLLJ transports atmospheric moisture from the Tropical North Atlantic Ocean. During this season, the predominant pathway of the OLLJ is associated with an area of moisture flux divergence located over northeastern South America. During JJA, an area of moisture flux convergence associated with the northernmost location of the ITCZ inhibits the entrance of moisture from northerlies. We also show that the occurrence of the OLLJ is associated with the so-called cross-equatorial flow. During DJF, the period of strongest activity of the OLLJ is associated with the northerly cross-equatorial flow and dry season, whereas during JJA the southerly cross-equatorial flow from the Amazon river basin predominates and contributes to the rainy season over the Orinoco region.

Strong activity of the Orinoco Low-Level Jet and its association with moisture transport in northern South America

2021 ◽  
Author(s):  
Alejandro Builes-Jaramillo ◽  
Johanna Yepes ◽  
Salas Hernán D.
Keyword(s):  
Time Series ◽  
Standard Deviation ◽  
Wind Speed ◽  
South America ◽  
Water Transport ◽  
Composite Analysis ◽  
Strong Activity ◽  
Easterly Wind ◽  
Low Level Jet ◽  
Northern South America

<p>We classified events of extreme Orinoco Low-Level Jet (OLLJ) activity using the ERA5 time series of daily winds at 925 hPa averaged over the 6°S–8°N/67°W–69°W area for the period 1981-2019. This area exhibits an overall mean of 3.7 m/s easterly wind speed and an overall standard deviation of 3.5 m/s. Then, during December-January-February (June-July-August), the season of strong (weak) OLLJ activity, we defined the events below (above) one standard deviation from the overall mean. Hence, days with easterly wind speeds higher than 7.2 m/s are considered events with strong activity during DJF. In contrast, days with westerly wind speed higher than 0.2 m/s are the events with weak activity during JJA. A composite analysis of precipitation from CHIRPS dataset during the days classified as strong or weak OLLJ activity showed that during the most active period (DJF), daily precipitation values are close to 0 mm/day; except for increased precipitation in the border between Colombia, Ecuador, Peru, and Brazil. In contrast, precipitation composites during the period of non-activity of the OLLJ (JJA), showed that precipitation increases in the range 5–10 mm/day along the OLLJ corridor. A detailed analysis of the precipitation time series used for composite analysis indicates that the probability of precipitation during DJF (JJA) is less (more) than 20% (80%) over Venezuela and the Guianas. In terms of advective water transport (qV) during the most active events of the OLLJ water is transported from the Tropical Atlantic towards northern South America through the OLLJ corridor, whilst during the less active events water transport along the OLLJ corridor comes from the north Amazon basin towards northern South America. In conclusion, during DJF the OLLJ is associated with the northerly cross-equatorial flow and dry season, whereas during JJA the southerly cross-equatorial flow from the Amazon river basin predominates, which contributes to the rainy season over the Orinoco region.</p>

scholarly journals Precipitation over Northern South America and Its Seasonal Variability as Simulated by the CMIP5 Models

2015 ◽  
Vol 2015 ◽  
pp. 1-22 ◽  
Author(s):  
Juan P. Sierra ◽  
Paola A. Arias ◽  
Sara C. Vieira
Keyword(s):  
South America ◽  
Cmip5 Models ◽  
Boreal Summer ◽  
Boreal Winter ◽  
Precipitation Pattern ◽  
Socioeconomic Impacts ◽  
Low Level ◽  
Low Level Jets ◽  
Low Level Jet ◽  
Northern South America

Northern South America is identified as one of the most vulnerable regions to be affected by climate change. Furthermore, recent extreme wet seasons over the region have induced socioeconomic impacts of wide proportions. Hence, the evaluation of rainfall simulations at seasonal and interannual time scales by the CMIP5 models is urgently required. Here, we evaluated the ability of seven CMIP5 models (selected based on literature review) to represent the seasonal mean precipitation and its interannual variability over northern South America. Our results suggest that it is easier for models to reproduce rainfall distribution during boreal summer and fall over both oceans and land. This is probably due to the fact that during these seasons, incoming radiation and ocean-atmosphere feedbacks over Atlantic and Pacific oceans locate the ITCZ on the Northern Hemisphere, as suggested by previous studies. Models exhibit the worse simulations during boreal winter and spring, when these processes have opposite effects locating the ITCZ. Our results suggest that the models with a better representation of the oceanic ITCZ and the local low-level jets over northern South America, such as the Choco low-level jet, are able to realistically simulate the main features of seasonal precipitation pattern over northern South America.

scholarly journals Impact of enriched analyses on regional numerical forecasts over southeastern South America during SALLJEX

2014 ◽  
Vol 29 (3) ◽  
pp. 315-330
Author(s):  
Yanina García Skabar ◽  
Matilde Nicolini
Keyword(s):  
Data Assimilation ◽  
South America ◽  
Atmospheric Modeling ◽  
South American ◽  
The South ◽  
Low Level ◽  
Positive Effects ◽  
Accumulated Rainfall ◽  
Low Level Jet ◽  
The Impact

During the warm season 2002-2003, the South American Low-Level Jet Experiment (SALLJEX) was carried out in southeastern South America. Taking advantage of the unique database collected in the region, a set of analyses is generated for the SALLJEX period assimilating all available data. The spatial and temporal resolution of this new set of analyses is higher than that of analyses available up to present for southeastern South America. The aim of this paper is to determine the impact of assimilating data into initial fields on mesoscale forecasts in the region, using the Brazilian Regional Atmospheric Modeling System (BRAMS) with particular emphasis on the South American Low-Level Jet (SALLJ) structure and on rainfall forecasts. For most variables, using analyses with data assimilated as initial fields has positive effects on short term forecast. Such effect is greater in wind variables, but not significant in forecasts longer than 24 hours. In particular, data assimilation does not improve forecasts of 24-hour accumulated rainfall, but it has slight positive effects on accumulated rainfall between 6 and 12 forecast hours. As the main focus is on the representation of the SALLJ, the effect of data assimilation in its forecast was explored. Results show that SALLJ is fairly predictable however assimilating additional observation data has small impact on the forecast of SALLJ timing and intensity. The strength of the SALLJ is underestimated independently of data assimilation. However, Root mean square error (RMSE) and BIAS values reveal the positive effect of data assimilation up to 18-hours forecasts with a greater impact near higher topography.

scholarly journals Impacts of Modifications to a Local Planetary Boundary Layer Scheme on Forecasts of the Great Plains Low-Level Jet Environment

2018 ◽  
Vol 33 (5) ◽  
pp. 1109-1120 ◽  
Author(s):  
David E. Jahn ◽  
William A. Gallus
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
Planetary Boundary Layer ◽  
Great Plains ◽  
Potential Temperature ◽  
Specific Humidity ◽  
Low Level ◽  
Stable Conditions ◽  
Pbl Scheme ◽  
Low Level Jet

Abstract The Great Plains low-level jet (LLJ) is influential in the initiation and evolution of nocturnal convection through the northward advection of heat and moisture, as well as convergence in the region of the LLJ nose. However, accurate numerical model forecasts of LLJs remain a challenge, related to the performance of the planetary boundary layer (PBL) scheme in the stable boundary layer. Evaluated here using a series of LLJ cases from the Plains Elevated Convection at Night (PECAN) program are modifications to a commonly used local PBL scheme, Mellor–Yamada–Nakanishi–Niino (MYNN), available in the Weather Research and Forecasting (WRF) Model. WRF forecast mean absolute error (MAE) and bias are calculated relative to PECAN rawinsonde observations. The first MYNN modification invokes a new set of constants for the scheme closure equations that, in the vicinity of the LLJ, decreases forecast MAEs of wind speed, potential temperature, and specific humidity more than 19%. For comparison, the Yonsei University (YSU) scheme results in wind speed MAEs 22% lower but specific humidity MAEs 17% greater than in the original MYNN scheme. The second MYNN modification, which incorporates the effects of potential kinetic energy and uses a nonzero mixing length in stable conditions as dependent on bulk shear, reduces wind speed MAEs 66% for levels below the LLJ, but increases MAEs at higher levels. Finally, Rapid Refresh analyses, which are often used for forecast verification, are evaluated here and found to exhibit a relatively large average wind speed bias of 3 m s−1 in the region below the LLJ, but with relatively small potential temperature and specific humidity biases.

Spatial and temporal variability of the Iberian Peninsula coastal low-level jet

2017 ◽  
Vol 38 (4) ◽  
pp. 1605-1622 ◽  
Author(s):  
Nadia Rijo ◽  
Alvaro Semedo ◽  
Pedro M. A. Miranda ◽  
Daniela Lima ◽  
Rita M. Cardoso ◽  
...  
Keyword(s):  
Iberian Peninsula ◽  
Temporal Variability ◽  
Spatial And Temporal Variability ◽  
Low Level ◽  
Low Level Jet

scholarly journals Hydrodynamics of the Caribbean Low-Level Jet and Its Relationship to Precipitation

2010 ◽  
Vol 23 (6) ◽  
pp. 1477-1494 ◽  
Author(s):  
Kerry H. Cook ◽  
Edward K. Vizy
Keyword(s):  
United States ◽  
South America ◽  
The United States ◽  
Caribbean Basin ◽  
Low Level ◽  
The North ◽  
South Central ◽  
Central United States ◽  
Low Level Jet ◽  
The Caribbean

Abstract The easterly Caribbean low-level jet (CLLJ) is a prominent climate feature over the Intra-America Seas, and it is associated with much of the water vapor transport from the tropical Atlantic into the Caribbean Basin. In this study, the North American Regional Reanalysis (NARR) is analyzed to improve the understanding of the dynamics of the CLLJ and its relationship to regional rainfall variations. Horizontal momentum balances are examined to understand how jet variations on both diurnal and seasonal time scales are controlled. The jet is geostrophic to the first order. Its previously documented semidiurnal cycle (with minima at about 0400 and 1600 LT) is caused by semidiurnal cycling of the meridional geopotential height gradient in association with changes in the westward extension of the North Atlantic subtropical high (NASH). A diurnal cycle is superimposed, associated with a meridional land–sea breeze (solenoidal circulation) onto the north coast of South America, so that the weakest jet velocities occur at 1600 LT. The CLLJ is present throughout the year, and it is known to vary in strength semiannually. Peak magnitudes in July are related to the seasonal cycle of the NASH, and a second maximum in February is caused by heating over northern South America. From May through September, zonal geopotential gradients associated with summer heating over Central America and Mexico induce meridional flow. The CLLJ splits into two branches, including a southerly branch that connects with the Great Plains low-level jet (GPLLJ) bringing moisture into the central United States. During the rest of the year, the flow remains essentially zonal across the Caribbean Basin and into the Pacific. A strong (weak) CLLJ is associated with reduced (enhanced) rainfall over the Caribbean Sea throughout the year in the NARR. The relationship with precipitation over land depends on the season. Despite the fact that the southerly branch of the CLLJ feeds into the meridional GPLLJ in May through September, variations in the CLLJ strength during these months do not impact U.S. precipitation, because the CLLJ strength is varying in response to regional-scale forcing and not to changes in the large-scale circulation. During the cool season, there are statistically significant correlations between the CLLJ index and rainfall over the United States. When the CLLJ is strong, there is anomalous northward moisture transport across the Gulf of Mexico into the central United States and pronounced rainfall increases over Louisiana and Texas. A weak jet is associated with anomalous westerly flow across the southern Caribbean region and significantly reduced rainfall over the south-central United States. No connection between the intensity of the CLLJ and drought over the central United States is found. There are only three drought summers in the NARR period (1980, 1988, and 2006), and the CLLJ was extremely weak in 1988 but not in 1980 or 2006.

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.

Sign in / Sign up

Export Citation Format

Share Document