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.

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.

Understanding the influence of ENSO on the Great Plains low-level jet in CMIP5 models

2017 ◽  
Vol 51 (4) ◽  
pp. 1537-1558 ◽  
Author(s):  
James F. Danco ◽  
Elinor R. Martin
Keyword(s):  
Great Plains ◽  
Cmip5 Models ◽  
Low Level ◽  
Low Level Jet

scholarly journals Later Wet Seasons with More Intense Rainfall over Africa under Future Climate Change

2018 ◽  
Vol 31 (23) ◽  
pp. 9719-9738 ◽  
Author(s):  
Caroline M. Dunning ◽  
Emily Black ◽  
Richard P. Allan
Keyword(s):  
Southern Africa ◽  
Crop Yields ◽  
Central Africa ◽  
Cmip5 Models ◽  
Future Climate Change ◽  
Boreal Summer ◽  
Wet Season ◽  
Socioeconomic Impacts ◽  
Continental Scale ◽  
Rainfall Frequency

Changes in the seasonality of precipitation over Africa have high potential for detrimental socioeconomic impacts due to high societal dependence upon seasonal rainfall. Here, for the first time we conduct a continental-scale analysis of changes in wet season characteristics under the RCP4.5 and RCP8.5 climate projection scenarios across an ensemble of CMIP5 models using an objective methodology to determine the onset and cessation of the wet season. A delay in the wet season over West Africa and the Sahel of over 5–10 days on average, and later onset of the wet season over southern Africa, is identified and associated with increasing strength of the Saharan heat low in late boreal summer and a northward shift in the position of the tropical rain belt over August–December. Over the Horn of Africa rainfall during the “short rains” season is projected to increase by over 100 mm on average by the end of the twenty-first century under the RCP8.5 scenario. Average rainfall per rainy day is projected to increase, while the number of rainy days in the wet season declines in regions of stable or declining rainfall (western and southern Africa) and remains constant in central Africa, where rainfall is projected to increase. Adaptation strategies should account for shorter wet seasons, increasing rainfall intensity, and decreasing rainfall frequency, which will have implications for crop yields and surface water supplies.

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 The Moisture Sources and Transport Processes for a Sudden Rainstorm Associated with Double Low-Level Jets in the Northeast Sichuan Basin of China

Atmosphere ◽  
2019 ◽  
Vol 10 (3) ◽  
pp. 160 ◽  
Author(s):  
Fangli Zhang ◽  
Guoping Li ◽  
Jun Yue
Keyword(s):  
South China Sea ◽  
South China ◽  
Moisture Transport ◽  
Sichuan Basin ◽  
Arabian Sea ◽  
Low Level ◽  
China Sea ◽  
Northeast Sichuan Basin ◽  
Low Level Jets ◽  
Low Level Jet

A sudden rainstorm that occurred in the northeast Sichuan Basin of China in early May 2017 was associated with a southwest low-level jet (SWLJ) and a mountainous low-level jet (MLLJ). This study investigates the impact of the double low-level jets (LLJs) on rainfall diurnal variation by using the data from ERA5 reanalysis, and explores the characteristics of water vapor transport, including the main paths and sources of moisture, by using the HYSPLIT-driven data of the ERA—interim, GDAS (Global Data Assimilation System), and NCEP/NCAR reanalysis data. The analysis shows that the sudden rainstorm in the mountain terrain was located at the left side of the large-scale SWLJ at 700 hPa, and at the exit region of the meso-scale MLLJ at 850 hPa. The double LLJs provide favorable moisture conditions, and the enhancement (weakening) of the LLJs is ahead of the start (end) of the rainstorm. The capacity of the LLJ at 850 hPa with respect to moisture convergence is superior to that at 700 hPa, especially when the MLLJ and the southerly LLJ at 850 hPa appear at the same time. The HYSPLIT backward trajectory model based on Lagrangian methods has favorable applicability in the event of sudden rainstorms in mountainous terrain, and there is no special path of moisture transport in this precipitation event. The main moisture sources of this process are the East China Sea–South China Sea, the Arabian Sea–Indian Peninsula, the Bay of Bengal, and the Middle East, accounting for 38%, 34%, 17% and 11% of the total moisture transport, respectively. Among them, the moisture transport in the Bay of Bengal and the South China Sea–East China Sea is mainly located in the lower troposphere, which is below 900 hPa, while the moisture transport in the Arabian Sea–Indian Peninsula and the Middle East is mainly in the middle and upper layers of the troposphere. The moisture changes of the transport trajectories are affected by the topography, especially the high mountains around the Sichuan Basin.

Observed relationship between the Turkana low-level jet and boreal summer convection

2019 ◽  
Vol 53 (7-8) ◽  
pp. 4037-4058 ◽  
Author(s):  
Edward K. Vizy ◽  
Kerry H. Cook
Keyword(s):  
Boreal Summer ◽  
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.

scholarly journals Wind profiler observations of a monsoon low-level jet over a tropical Indian station

2007 ◽  
Vol 25 (10) ◽  
pp. 2125-2137 ◽  
Author(s):  
M. C. R. Kalapureddy ◽  
D. N. Rao ◽  
A. R. Jain ◽  
Y. Ohno
Keyword(s):  
Boundary Layer ◽  
Mutual Influence ◽  
Boreal Summer ◽  
Wind Profiler ◽  
Strong Wind ◽  
Height Range ◽  
Diurnal Variability ◽  
Low Level ◽  
Low Level Jet ◽  
Wind Shears

Abstract. Three-year high-resolution wind observations of the wind profiler have been utilized to characterize the diurnal and seasonal features of the monsoon Low-Level Jet (LLJ) over a tropical station, Gadanki (13.5° N, 79.2° E), with a focus on the diurnal variability of low-level winds. The Boreal summer monsoon winds show a conspicuously strong westerly LLJ with average wind speed exceeding 20 m s−1. The L-band wind profiler measurements have shown an advantage of better height and time resolutions over the conventional radiosonde method for diurnal wind measurements. An interesting diurnal oscillation of LLJ core has been observed. It is varying in the height range of 1.8±0.6 km with the maximum and minimum intensity noticed during the early morning and afternoon hours, respectively. The jet core (wind maxima) height is observed to coincide with the inversion height. Strong wind shears are normally located beneath the LLJ core. The sole wind profiler observations are capable of identifying the monsoon phases, such as onset, break and active spells, etc. The mutual influence between the LLJ and the boundary layer has been discussed. One notices that the observed LLJ diurnal structures depend on the local convective activity, wind shears and turbulence activity associated with boundary layer winds. The day-to-day change in the LLJ structure depends on the latitudinal position of the LLJ core.

scholarly journals Summertime Low-Level Jets over the High-Latitude Arctic Ocean

2008 ◽  
Vol 47 (6) ◽  
pp. 1770-1784 ◽  
Author(s):  
Douglas O. ReVelle ◽  
E. Douglas Nilsson
Keyword(s):  
Boundary Layer ◽  
Eddy Viscosity ◽  
Low Level ◽  
Rayleigh Friction ◽  
Low Level Jets ◽  
Low Level Jet ◽  
Horizontal Momentum ◽  
Ocean Observations ◽  
Measured Wind Speed ◽  
Good Agreement

Abstract The application of a simple analytic boundary layer model developed by Thorpe and Guymer did not produce good agreement with observational data for oceanic low-level jet observations even though this model has worked well for the predictions of low-level jets over continental surfaces. This failure to properly predict the boundary layer wind maxima was very puzzling because more detailed numerical boundary layer models have properly predicted these low-level oceanic wind maxima. To understand the reasons for its failure to explain the ocean observations, the authors modified the frictional terms in the horizontal linear momentum equations of Thorpe and Guymer, using a standard eddy viscosity closure technique instead of the Rayleigh friction parameterization originally used. This improvement in the modeling of the dissipation terms, which has resulted in the use of an enhanced Rayleigh friction parameterization in the horizontal momentum equations, modified the boundary layer winds such that the continental predictions remained nearly identical to those predicted previously using the Thorpe and Guymer model while the oceanic predictions have now become more representative of the measured wind speed from recent Arctic expeditions.

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