Evolution of Downslope Flow under Strong Opposing Trade Winds and Frequent Trade-Wind Rainshowers over the Island of Hawaii

Abstract. La Réunion is a volcanic island in a tropical zone, which soil undergoes intense erosion. The possible contribution of rainfall to erosion is analyzed and quantified using one year of UHF radar profiler data located at sea level. Measurements of reflectivity, vertical and horizontal wind allow, with suitable assumptions, to determine raindrop vertical and horizontal energy fluxes, which are both essential parameters for erosion. After calibration of radar rain rates, one-year statistics between May 2009 to April 2010 allow to identify differences in rain vertical profiles depending on the season. During the cool dry season, the mean rain rate is less than 2.5 mm h−1 as high as 1.25 km and it decreases at higher altitudes due to the trade winds inversion. During the warm moist season, the mean rain rate is nearly uniform from ground up to 4 km, around 5 mm h−1. The dynamical and microphysical properties of rainfall events are investigated on three cases that are representative of meteorological events in La Réunion: summer deep convection, a cold front and a winter depression embedded in trade winds. For intense rainfall events, the rain rate deduced from the gamma function is in agreement with the rain rate deduced from the mere Marshall Palmer exponential relationship. For less intense events, the gamma function is necessary to represent rain distribution. The deep-convection event is associated to strong reflectivity reaching as high as 10 km, and strong negative vertical velocity. Wind shear is responsible for a deficiency of radar rain detection at the lower levels. During a cold front event, strong reflectivities reach the trade wind inversion (around 4 km high). The trade wind depression generates moderate rain only as high as 2 km. For all the altitudes, the horizontal kinetic energy fluxes are one order of magnitude stronger that than the vertical kinetic energy fluxes. A simple relationship between the reflectivity factor and vertical kinetic energy fluxes is found for each case study.

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Mesoscale spatio-temporal variability of airborne lidar-derived aerosol properties in the Barbados region during EUREC<sup>4</sup>A

10.5194/acp-2021-706 ◽

2021 ◽

Author(s):

Patrick Chazette ◽

Alexandre Baron ◽

Cyrille Flamant

Keyword(s):

Temporal Variability ◽

Cloud Cover ◽

Airborne Lidar ◽

Trade Wind ◽

Tropical Atlantic ◽

Trade Winds ◽

Airborne Measurements ◽

Fractional Cloud ◽

Spatio Temporal ◽

Humidity Field

Abstract. From 23 January to 13 February 2020, twenty ATR-42 scientific flights were conducted in the framework of the EUREC4A field campaign over the tropical Atlantic, off the coast of Barbados (−58°30' W 13°30' N). By means of a side-pointing lidar, these flights allowed to retrieve the optical properties of the aerosols found in the sub-cloud layer and below the trade winds inversion. Two distinct periods with significant aerosol contents were identified in relationship with the so-called trade wind and tropical regimes, respectively. A very strong spatial heterogeneity of the aerosol field has been highlighted at the airborne measurements scale of a few tens of kilometres. This heterogeneity, difficult to capture using spaceborne instruments, can be related to the highly variable relative humidity field and the fractional cloud cover encountered during all the flights.

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Understanding the Anthropogenically Forced Change of Equatorial Pacific Trade Winds in Coupled Climate Models*

Journal of Climate ◽

10.1175/jcli-d-14-00115.1 ◽

2014 ◽

Vol 27 (22) ◽

pp. 8510-8526 ◽

Cited By ~ 10

Author(s):

Baoqiang Xiang ◽

Bin Wang ◽

Juan Li ◽

Ming Zhao ◽

June-Yi Lee

Keyword(s):

Surface Temperature ◽

Sea Surface Temperature ◽

Climate Models ◽

Walker Circulation ◽

Tropical Pacific ◽

Equatorial Pacific ◽

Sea Surface ◽

Trade Wind ◽

Trade Winds ◽

The Walker Circulation

Abstract Understanding the change of equatorial Pacific trade winds is pivotal for understanding the global mean temperature change and the El Niño–Southern Oscillation (ENSO) property change. The weakening of the Walker circulation due to anthropogenic greenhouse gas (GHG) forcing was suggested as one of the most robust phenomena in current climate models by examining zonal sea level pressure gradient over the tropical Pacific. This study explores another component of the Walker circulation change focusing on equatorial Pacific trade wind change. Model sensitivity experiments demonstrate that the direct/fast response due to GHG forcing is to increase the trade winds, especially over the equatorial central-western Pacific (ECWP) (5°S–5°N, 140°E–150°W), while the indirect/slow response associated with sea surface temperature (SST) warming weakens the trade winds. Further, analysis of the results from 19 models in phase 5 of the Coupled Model Intercomparison Project (CMIP5) and the Parallel Ocean Program (POP)–Ocean Atmosphere Sea Ice Soil (OASIS)–ECHAM model (POEM) shows that the projected weakening of the trades is robust only in the equatorial eastern Pacific (EEP) ( 5°S–5°N, 150°–80°W), but highly uncertain over the ECWP with 9 out of 19 CMIP5 models producing intensified trades. The prominent and robust weakening of EEP trades is suggested to be mainly driven by a top-down mechanism: the mean vertical advection of more upper-tropospheric warming downward to generate a cyclonic circulation anomaly in the southeast tropical Pacific. In the ECWP, the large intermodel spread is primarily linked to model diversity in simulating the relative warming of the equatorial Pacific versus the tropical mean sea surface temperature. The possible root causes of the uncertainty for the trade wind change are also discussed.

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Processes Controlling Precipitation in Shallow, Orographic, Trade Wind Convection

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-14-0333.1 ◽

2015 ◽

Vol 72 (8) ◽

pp. 3051-3072 ◽

Cited By ~ 4

Author(s):

Campbell D. Watson ◽

Ronald B. Smith ◽

Alison D. Nugent

Keyword(s):

Air Entrainment ◽

Negative Influence ◽

Sea Salt ◽

Small Island ◽

Trade Wind ◽

Strong Wind ◽

Cumulus Clouds ◽

Trade Winds ◽

Dry Air ◽

Sea Salt Aerosol

Abstract A sharp reduction in precipitation was observed on the island of Dominica in the Caribbean during a 2011 field campaign when the trade winds weakened and convection transitioned from mechanically to thermally driven. The authors propose four hypotheses for this reduction, which relate to (i) the triggering mechanism, (ii) dry-air entrainment, (iii) giant sea-salt aerosol, and (iv) small-island-derived aerosol. The plausibility of the first three hypotheses is the focus of this study. Aircraft observations show the dynamics of the orographic cumulus clouds at flight level are surprisingly similar, irrespective of how they are triggered. However, the orographic cumulus clouds are consistently shallower when the trade winds are weak, which the authors attribute to a drier and shallower cloud layer compared to days with stronger trade winds. The strong negative influence of dry-air entrainment in a drier environment on cumulus depth and liquid water content is qualitatively demonstrated using an entraining plume model and the WRF Model. Although the models appear more sensitive than observations to entrainment and cloud size, the sensitivity tests have some resemblance to observations. The authors also find evidence of sea-salt aerosol entering the base of marine cumulus on strong wind days using an aircraft-mounted lidar and other instruments. Although each hypothesis is plausible, the complex interplay of these processes makes determining the controlling mechanisms difficult. Ultimately, the authors’ analysis rejects the hypothesis (i) triggering, while supporting (ii) entrainment and (iii) sea-salt aerosol.

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Impact of elevation and weather patterns on the isotopic composition of precipitation in a tropical montane rainforest

Hydrology and Earth System Sciences ◽

10.5194/hess-17-409-2013 ◽

2013 ◽

Vol 17 (1) ◽

pp. 409-419 ◽

Cited By ~ 65

Author(s):

D. Windhorst ◽

T. Waltz ◽

E. Timbe ◽

H.-G. Frede ◽

L. Breuer

Keyword(s):

Temporal Variability ◽

Cloud Forest ◽

Weather Conditions ◽

Trade Wind ◽

Spatial And Temporal Variability ◽

Austral Winter ◽

Deuterium Excess ◽

Altitude Effect ◽

Trade Winds ◽

Air Masses

Abstract. This study presents the spatial and temporal variability of δ18O and δ2H isotope signatures in precipitation of a south Ecuadorian montane cloud forest catchment (San Francisco catchment). From 2 September to 25 December 2010, event sampling of open rainfall was conducted along an altitudinal transect (1800 to 2800 m a.s.l.) to investigate possible effects of altitude and weather conditions on the isotope signature. The spatial variability is mainly affected by the altitude effect. The event based δ18O altitude effect for the study area averages −0.22‰ × 100 m−1 (δ2H: −1.12‰ × 100 m−1). The temporal variability is mostly controlled by prevailing air masses. Precipitation during the times of prevailing southeasterly trade winds is significantly enriched in heavy isotopes compared to precipitation during other weather conditions. In the study area, weather during austral winter is commonly controlled by southeasterly trade winds. Since the Amazon Basin contributes large amounts of recycled moisture to these air masses, trade wind-related precipitation is enriched in heavy isotopes. We used deuterium excess to further evaluate the contribution of recycled moisture to precipitation. Analogously to the δ18O and δ2H values, deuterium excess is significantly higher in trade wind-related precipitation. Consequently, it is assumed that evaporated moisture is responsible for high concentrations of heavy isotopes during austral winter.

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Loop Current Growth and Eddy Shedding Using Models and Observations: Numerical Process Experiments and Satellite Altimetry Data

Journal of Physical Oceanography ◽

10.1175/jpo-d-12-0139.1 ◽

2013 ◽

Vol 43 (3) ◽

pp. 669-689 ◽

Cited By ~ 23

Author(s):

Yu-Lin Chang ◽

L.-Y. Oey

Keyword(s):

Trade Wind ◽

Loop Current ◽

Reduced Gravity ◽

Trade Winds ◽

Numerical Process ◽

The Asymmetry ◽

Yucatan Channel ◽

Satellite Altimetry Data ◽

Eddy Shedding

Abstract Recent studies on Loop Current’s variability in the Gulf of Mexico suggest that the system may behave with some regularity forced by the biannually varying trade winds. The process is analyzed here using a reduced-gravity model and satellite data. The model shows that a biannual signal is produced by vorticity and transport fluctuations in the Yucatan Channel because of the piling up and retreat of warm water in the northwestern Caribbean Sea forced by the biannually varying trade wind. The Loop grows and expands with increased northward velocity and cyclonic vorticity of the Yucatan Current, and eddies are shed when these are near minima. Satellite sea surface height (SSH) data from 1993 to 2010 are analyzed. These show, consistent with the reduced-gravity experiments and previous studies, a (statistically) significant asymmetric biannual variation of the growth and wane of Loop Current: strong from summer to fall and weaker from winter to spring; the asymmetry being due to the asymmetry that also exists in the long-term observed wind. The biannual signal is contained in the two leading EOF modes, which together explain 47% of the total variance, and which additionally describe the eddy shedding and westward propagation from summer to fall. The EOFs also show connectivity between Loop Current and Caribbean Sea’s variability by mass and vorticity fluxes through the Yucatan Channel.

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Effects of Trade Wind Strength on Airflow and Cloudiness over O‘ahu

Monthly Weather Review ◽

10.1175/mwr-d-20-0399.1 ◽

2021 ◽

Author(s):

Feng Hsiao ◽

Yi-Leng Chen ◽

Hiep Van Nguyen ◽

David Eugene Hitzl ◽

Robert Ballard

Keyword(s):

Sea Breeze ◽

Windward Side ◽

Trade Wind ◽

Return Flow ◽

Western Coast ◽

Trade Winds ◽

Flow Deceleration ◽

Orographic Clouds ◽

Island Wake ◽

Orographic Cloud

AbstractSatellite observations and high-resolution modeling during July–August 2013 are used to study the effects of trade wind strength on island wake circulations and cloudiness over O‘ahu, Hawai‘i. O‘ahu is composed of two northwest–southeast orientated mountain ranges: the Wai‘anae Range (~1227 m) along the western leeside coast and the Ko‘olau Range (~944 m) along the eastern windward coast. At night, the flow deceleration of the incoming northeasterly trade winds on the eastern windward side is more significant when trades are stronger.In the afternoon hours, effective albedo and simulated cloud water are greater over the Ko‘olau Range when trades are stronger, and clouds are advected downstream by the trade winds aloft. Over the Wai‘anae Range, orographic clouds are more significant when trades are weaker due to less moisture removal by orographic precipitation over the Ko‘olau Range and the development of both upslope flow on the eastern slope and upslope/sea-breeze flow along the western coast, the latter of which brings in warm, moist air from the ocean. When trades are weaker, cloudiness off the western leeside coast is more extensive and originates from orographic cloud development over the Wai‘anae Range, which drifts downstream due to a combination of trade winds and the easterly return flow aloft. The latter is associated with the low-level sea-breeze/upslope flow.

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Cold tongue/Warm pool and ENSO dynamics in the Pliocene

Climate of the Past Discussions ◽

10.5194/cpd-7-997-2011 ◽

2011 ◽

Vol 7 (2) ◽

pp. 997-1027 ◽

Cited By ~ 4

Author(s):

A. S. von der Heydt ◽

A. Nnafie ◽

H. A. Dijkstra

Keyword(s):

El Niño ◽

El Nino ◽

Southern Oscillation ◽

Warm Pool ◽

Sensitivity Study ◽

Trade Wind ◽

Cold Tongue ◽

Trade Winds ◽

Enso Variability ◽

East West

Abstract. It has been suggested that a "permanent" El Niño climate state has existed in the warm Pliocene. One of the main pieces of evidence of such conditions is the small east-west sea surface temperature (SST) difference that is found in proxy temperature records of the equatorial Pacific. Using a coupled version of the Zebiak-Cane model of intermediate complexity for the tropical Pacific, we study the sensitivity of the time-mean Pacific background state and El Niño/Southern Oscillation (ENSO) variability to Pliocene climate changes. The parameters varied in this sensitivity study include changes in the trade wind strength due to a reduced equator-to-pole temperature gradient, higher global mean temperatures and an open Panama gateway. All these changes lead to a westward shift of the position of the cold tongue along the equator by up to 2000 km. This result is consistent with data from the PRISM3D Pliocene SST reconstruction. Our model further suggests that ENSO variability is present in the Pliocene climate with only slight changes as compared to today. A background climate that would resemble a "permanent" El Niño with weak to no east-west temperature difference along the equator is only found for very weak trade winds which seem unrealistic for the Pliocene climate.

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Impact of elevation and weather patterns on the isotopic composition of precipitation in a tropical montane rainforest

Hydrology and Earth System Sciences Discussions ◽

10.5194/hessd-9-8425-2012 ◽

2012 ◽

Vol 9 (7) ◽

pp. 8425-8453 ◽

Cited By ~ 2

Author(s):

D. Windhorst ◽

T. Waltz ◽

H.-G. Frede ◽

L. Breuer

Keyword(s):

Temporal Variability ◽

Cloud Forest ◽

Weather Conditions ◽

Trade Wind ◽

Spatial And Temporal Variability ◽

Austral Winter ◽

Deuterium Excess ◽

Altitude Effect ◽

Trade Winds ◽

Air Masses

Abstract. This study presents the spatial and temporal variability of δ18O and δ2H isotope signatures in precipitation of a south Ecuadorian montane cloud forest catchment (San Francisco Catchment). From 2 September to 25 December 2010, event sampling of open rainfall was conducted along an altitudinal transect (1800 m a.s.l. to 2800 m a.s.l.) to investigate possible effects of altitude and weather conditions on the isotope signature. The spatial variability is mainly affected by the altitude effect. The event based δ18O altitude effect for the study area averages −0.22‰ × 100 m−1 (δ2H: −1.12‰ × 100 m−1). The temporal variability is mostly controlled by prevailing air masses. Precipitation during the times of prevailing southeasterly trade winds is significantly enriched in heavy isotopes compared to precipitation during other weather conditions. In the study area, weather during austral winter is commonly controlled by southeasterly trade winds. Since the Amazon Basin contributes large amounts of recycled moisture to these air masses, trade wind-related precipitation is enriched in heavy isotopes. We used deuterium excess to further evaluate the contribution of recycled moisture to precipitation. Analogously to the δ18O and δ2H values, deuterium excess is significantly higher in trade wind related precipitation. Consequently, it is assumed that evaporated moisture is responsible for high concentrations of heavy isotopes during austral winter.

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Decoupling of the Agulhas Leakage from the Agulhas Current

Journal of Physical Oceanography ◽

10.1175/jpo-d-13-093.1 ◽

2014 ◽

Vol 44 (7) ◽

pp. 1776-1797 ◽

Cited By ~ 39

Author(s):

Benjamin R. Loveday ◽

Jonathan V. Durgadoo ◽

Chris J. C. Reason ◽

Arne Biastoch ◽

Pierrick Penven

Keyword(s):

Wind Stress ◽

Ocean Basin ◽

Global Ocean ◽

Trade Wind ◽

Current Transport ◽

Trade Winds ◽

Agulhas Current ◽

Basin Scale ◽

Stress Changes ◽

The Indian Ocean

Abstract The relationship between the Agulhas Current and the Agulhas leakage is not well understood. Here, this is investigated using two basin-scale and two global ocean models of incrementally increasing resolution. The response of the Agulhas Current is evaluated under a series of sensitivity experiments, in which idealized anomalies, designed to geometrically modulate zonal trade wind stress, are applied across the Indian Ocean Basin. The imposed wind stress changes exceed plus or minus two standard deviations from the annual-mean trade winds and, in the case of intensification, are partially representative of recently observed trends. The Agulhas leakage is quantified using complimentary techniques based on Lagrangian virtual floats and Eulerian passive tracer flux. As resolution increases, model behavior converges and the sensitivity of the leakage to Agulhas Current transport anomalies is reduced. In the two eddy-resolving configurations tested, the leakage is insensitive to changes in Agulhas Current transport at 32°S, though substantial eddy kinetic energy anomalies are evident. Consistent with observations, the position of the retroflection remains stable. The decoupling of Agulhas Current variability from the Agulhas leakage suggests that while correlations between the two may exist, they may not have a clear dynamical basis. It is suggested that present and future Agulhas leakage proxies should be considered in the context of potentially transient forcing regimes.

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