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

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.

Characteristics of Summer Trade Wind Rainfall over Oahu

2010 ◽  
Vol 25 (6) ◽  
pp. 1797-1815 ◽  
Author(s):  
Treena Marie Hartley ◽  
Yi-Leng Chen
Keyword(s):  
Land Surface ◽  
Early Morning ◽  
Windward Side ◽  
Trade Wind ◽  
Sea Breezes ◽  
Orographic Rainfall ◽  
Flow Deceleration ◽  
Study Characteristics ◽  
Cloud Development ◽  
Variable Wind

Abstract In this study, characteristics of summer trade wind rainfall over Oahu, Hawaii, are analyzed. In the early morning before sunrise, flow deceleration on the windward coastal area is the greatest when the island land surface is the coldest. Furthermore, relatively calm winds are found over central Oahu between the Ko’olau Mountains and the Waianae Mountains, with weak westerly katabatic winds on the windward side of the Waianae Mountains. Most windward stations have an early morning rainfall maximum with a secondary rainfall maximum in the early evening. Morning (afternoon) land (sea) breezes dominate under variable winds, and are more pronounced over leeward Oahu. Precipitation on the western leeside coast has a slight peak in the afternoon due to an increase of cloud development from the afternoon sea breezes. Daily orographic rainfall over Oahu is greater under the strong trade wind regime, and less under weak trade wind and variable-wind flow regimes. However, the maximum correlation between daily trade wind rainfall and trade wind speed is <0.3. Days with high rainfall generally occur under strong trades, but not all strong trade wind days produce significant rainfall. With its relatively low terrain height compared to lifted condensation level (LCL) and relatively small size, orographic lifting alone is inadequate to initiate precipitation through warm rain processes. The existence of trade wind cumuli upstream is necessary. In addition, a deeper moist layer and higher moisture content upstream are two conditions that are favorable for higher orographic precipitation over Oahu under undisturbed summertime trade wind weather.

scholarly journals The Convection, Aerosol, and Synoptic-Effects in the Tropics (CAST) Experiment: Building an Understanding of Multiscale Impacts on Caribbean Weather via Field Campaigns

2017 ◽  
Vol 98 (8) ◽  
pp. 1593-1600 ◽  
Author(s):  
N. Hosannah ◽  
J. González ◽  
R. Rodriguez-Solis ◽  
H. Parsiani ◽  
F. Moshary ◽  
...  
Keyword(s):  
Puerto Rico ◽  
Sea Breeze ◽  
Convective Precipitation ◽  
Trade Wind ◽  
Test Case ◽  
Western Coast ◽  
Short Article ◽  
Tropical Regions ◽  
Soil Moisture Sensors ◽  
The Tropics

Abstract Modulated by global-, continental-, regional-, and local-scale processes, convective precipitation in coastal tropical regions is paramount in maintaining the ecological balance and socioeconomic health within them. The western coast of the Caribbean island of Puerto Rico is ideal for observing local convective dynamics as interactions between complex processes involving orography, surface heating, land cover, and sea-breeze–trade wind convergence influence different rainfall climatologies across the island. A multiseason observational effort entitled the Convection, Aerosol, and Synoptic-Effects in the Tropics (CAST) experiment was undertaken using Puerto Rico as a test case, to improve the understanding of island-scale processes and their effects on precipitation. Puerto Rico has a wide network of observational instruments, including ground weather stations, soil moisture sensors, a Next Generation Weather Radar (NEXRAD), twice-daily radiosonde launches, and Aerosol Robotic Network (AERONET) sunphotometers. To achieve the goals of CAST, researchers from multiple institutions supplemented existing observational networks with additional radiosonde launches, three high-resolution radars, continuous ceilometer monitoring, and air sampling in western Puerto Rico to monitor convective precipitation events. Observations during three CAST measurement phases (22 June–10 July 2015, 6–22 February 2016, and 24 April–7 May 2016) captured the most extreme drought in recent history (summer 2015), in addition to anomalously wet early rainfall and dry-season (2016) phases. This short article presents an overview of CAST along with selected campaign data.

Numerical Simulations of the Island-Induced Circulations over the Island of Hawaii during HaRP

2005 ◽  
Vol 133 (12) ◽  
pp. 3693-3713 ◽  
Author(s):  
Yang Yang ◽  
Yi-Leng Chen ◽  
Francis M. Fujioka
Keyword(s):  
Land Surface ◽  
Diurnal Variations ◽  
Windward Side ◽  
Trade Wind ◽  
Wind Flow ◽  
State University ◽  
Thermal Contrast ◽  
Sea Breezes ◽  
Flow Deceleration ◽  
Trade Wind Inversion

Abstract The fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5)/land surface model (LSM) is used to simulate the diurnal island-scale circulations over the island of Hawaii during the Hawaiian Rainband Project (HaRP, 11 July–24 August 1990). The model is initialized with the NCEP–NCAR reanalysis data. The diurnal variations of the land–sea thermal contrast at the land surface and the planetary boundary layer at Hilo, Hawaii, are well simulated. The main discrepancy occurs on the leeside areas of mountains or ridges below the trade wind inversion (2 km), where the simulated afternoon land–sea thermal contrast at the surface is 1°–3°C lower than observed mainly due to the misrepresentation of lava rocks by the bare ground category in the U.S. Geological Survey (USGS) data and stronger than observed simulated sea breezes bringing in relatively cool maritime air. The flow deceleration and splitting of the incoming trade wind flow and the evolution of the diurnal circulation cells on the windward side, the thermally driven diurnal winds, and the wake circulations on the lee side are well simulated. The simulated diurnal variations in rainfall are also in good agreement with observations. However, the simulated winds in areas well exposed to the trade wind flow are weaker (1–3 m s−1) than observed mainly due to the underestimation of trade wind flow in the NCEP–NCAR reanalysis. The simulated rainfall over windward lowlands at night is underestimated and the maximum rainfall axis shifts farther toward the coast as compared with observations, due to an underestimation of orographic lifting aloft and a relatively large horizontal extent of the simulated katabatic flow because of the weaker- than-observed trade wind flow in the initial conditions. In the afternoon hours on the windward side, the strongest winds (anabatic/sea breeze and trade wind flow) are simulated in low levels over land in response to the surface heating, with a westerly wind deviation beneath the mean trade wind inversion (2 km) and sinking motion over the adjacent oceans. The simulated low-level flow deceleration of the incoming trade wind flow is most significant in the early morning as a combination of island blocking and nocturnal cooling over land. At that time, the simulated upward motion representing the rising branch of the thermally direct circulation extends more than 40 km offshore. Sensitivity tests show that with better surface conditions in the model coupled with the Oregon State University (OSU) LSM, the simulated thermal forcing over land is improved. The improvements in simulated ground temperature, land–sea thermal contrast at the land surface, and mixing ratio lead to better simulation of the strength of land/sea breezes over the island.

scholarly journals Cloud Trails Past the Lesser Antilles

2015 ◽  
Vol 143 (4) ◽  
pp. 995-1017 ◽  
Author(s):  
Daniel J. Kirshbaum ◽  
Jonathan G. Fairman
Keyword(s):  
Wind Speed ◽  
Thermal Anomaly ◽  
Surface Fluxes ◽  
Lesser Antilles ◽  
Trade Wind ◽  
Trade Winds ◽  
Cold Pools ◽  
Wake Turbulence ◽  
Island Wake ◽  
One Year

Abstract Observations and cloud-resolving simulations of elongated cloud plumes (or “cloud trails”) past the Lesser Antilles islands in the Caribbean Sea are presented. Analysis of one year of visible satellite images reveals that each island forms cloud trails on 30%–40% of days, typically in the afternoon in response to diurnal island heating. On around 10% of days the cloud bands are very well organized, with lengths of and durations of min. Radiosonde analysis suggests that the well-organized events are favored by moderate-to-strong trade winds (6–10 m s−1) and stronger trade inversions. The simulated cloud trails, which are consistent with observations in their morphology and diurnal cycle, are organized by quasi-linear bands of thermally forced convergence within the heated island wake. They are sensitive to overland surface fluxes, inversion strength and height, terrain height, and trade-wind speed. While surface fluxes control the strength of the wake thermal circulations, the inversion controls precipitation and the disruption of cloud trails by subcloud cold pools. The impacts of terrain height and wind speed are multifaceted, including control over (i) the mechanical flow regime, (ii) the intensity of wake turbulence, (iii) the cloud-trail length, (iv) the wake thermal anomaly, and (v) elevated-heating effects (which strengthen the thermal convergence). Dimensional analysis is used to develop empirical scalings for the wake thermal circulation, which describe the suite of numerical sensitivity tests reasonably well.

scholarly journals Cloud Radar Observations of Diurnal and Seasonal Cloudiness over Reunion Island

Atmosphere ◽  
2021 ◽  
Vol 12 (7) ◽  
pp. 868
Author(s):  
Jonathan Durand ◽  
Edouard Lees ◽  
Olivier Bousquet ◽  
Julien Delanoë ◽  
François Bonnardot
Keyword(s):  
Sea Breeze ◽  
Satellite System ◽  
Research Infrastructure ◽  
Weather Data ◽  
Reunion Island ◽  
Wet Season ◽  
Trade Winds ◽  
Cloud Radar ◽  
Réunion Island ◽  
Global Navigation Satellite

In November 2016, a 95 GHz cloud radar was permanently deployed in Reunion Island to investigate the vertical distribution of tropical clouds and monitor the temporal variability of cloudiness in the frame of the pan-European research infrastructure Aerosol, Clouds and Trace gases Research InfraStructure (ACTRIS). In the present study, reflectivity observations collected during the two first years of operation (2016–2018) of this vertically pointing cloud radar are relied upon to investigate the diurnal and seasonal cycle of cloudiness in the northern part of this island. During the wet season (December–March), cloudiness is particularly pronounced between 1–3 km above sea level (with a frequency of cloud occurrence of 45% between 12:00–19:00 LST) and 8–12 km (with a frequency of cloud occurrence of 15% between 14:00–19:00 LST). During the dry season (June–September), this bimodal vertical mode is no longer observed and the vertical cloud extension is essentially limited to a height of 3 km due to both the drop-in humidity resulting from the northward migration of the ITCZ and the capping effect of the trade winds inversion. The frequency of cloud occurrence is at its maximum between 13:00–18:00 LST, with a probability of 35% at 15 LST near an altitude of 2 km. The analysis of global navigation satellite system (GNSS)-derived weather data also shows that the diurnal cycle of low- (1–3 km) and mid-to-high level (5–10 km) clouds is strongly correlated with the diurnal evolution of tropospheric humidity, suggesting that additional moisture is advected towards the island by the sea breeze regime. The detailed analysis of cloudiness observations collected during the four seasons sampled in 2017 and 2018 also shows substantial differences between the two years, possibly associated with a strong positive Indian Ocean Southern Dipole (IOSD) event extending throughout the year 2017.

Quantifying snowfall from orographic cloud seeding

2020 ◽  
Author(s):  
Katja Friedrich ◽  
Kyoko Ikeda ◽  
Sarah Tessendorf ◽  
Jeffrey French ◽  
Robert Rauber ◽  
...  
Keyword(s):  
Water Management ◽  
Silver Iodide ◽  
Management Strategy ◽  
Radar Reflectivity ◽  
Cloud Seeding ◽  
Straight Line ◽  
Orographic Clouds ◽  
Increasing Demand ◽  
Orographic Cloud ◽  
Water Management Strategy

<p>Cloud seeding has been used as one water management strategy to overcome the increasing demand for water despite decades of inconclusive results on the efficacy of cloud seeding. In this study snowfall accumulation from glaciogenic cloud seeding is quantified based on snow gauge and radar observations from three days in January 2017, when orographic clouds in the absent of natural precipitation were seeded with silver iodide (AgI) in the Payette basin of Idaho during the Seeded and Natural Orographic Wintertime Clouds: The Idaho Experiment (SNOWIE). On each day, a seeding aircraft equipped with AgI flares flew back and forth on a straight-line flight track producing a zig-zag pattern representing two to eight lines of clouds visible through enhancements in radar reflectivity. As these seeding lines started to form precipitation, they passed over several snow gauges and through the radar observational domain. For the three cases presented here, precipitation gauges measured increases between 0.05-0.3 mm as precipitation generated by cloud seeding pass over the instruments. A variety of relationships between radar reflectivity factor and liquid equivalent snowfall rate were used to quantify snowfall within the radar observation domain. For the three cases, snowfall occurred within the radar observational domain between 25 -160 min producing a total amount of water generated by cloud seeding ranging from 123,220 to 339,540 m3 using the best-match Ze-S relationship. Uncertainties in radar reflectivity estimated snowfall are provided by considering not only the best-match Ze-S relationship but also an ensemble of Ze-S relationships based on the range of coefficients published from previous studies and then examining the percentile of snowfall estimates based on all of the Ze-S relationships within the ensemble. Considering the interquartile range and 5<sup>th</sup>/95<sup>th</sup> percentiles, uncertainties in total amount of water generated by cloud seeding can range between 20-45% compared to the best-math estimates. These results provide new insights towards understanding how cloud seeding impacts precipitation and its distribution across a region.</p>

Hypotheses for the Climax Wintertime Orographic Cloud Seeding Experiments

1986 ◽  
Vol 43 ◽  
pp. 105-108 ◽  
Author(s):  
Lewis O. Grant
Keyword(s):  
Hypothesis Testing ◽  
Remotely Sensed ◽  
Cloud Seeding ◽  
Statistical Experiments ◽  
Orographic Clouds ◽  
Orographic Cloud

Abstract The hypothesis used for the initial Climax wintertime cloud seeding experiment and for subsequent Climax replication-type experiments are described and briefly discussed. More recent physical studies of Colorado orographic clouds and seeding hypotheses are briefly summarized. These later tests and studies of orographic cloud seeding hypotheses emphasized direct and remotely sensed cloud and precipitation measurements utilizing instrumentation and modeling capabilities not available during the Climax statistical experiments. The conclusions suggested from the hypothesis testing, considering both the statistical experiments and the later physical studies, are summarized.

Idealized Simulations of the Intertropical Convergence Zone and Its Multilevel Flows

2010 ◽  
Vol 67 (12) ◽  
pp. 4028-4053 ◽  
Author(s):  
David S. Nolan ◽  
Scott W. Powell ◽  
Chidong Zhang ◽  
Brian E. Mapes
Keyword(s):  
Surface Temperature ◽  
Dynamic Instability ◽  
Vertical Motion ◽  
Sea Breeze ◽  
Return Flow ◽  
Eddy Fluxes ◽  
Convergence Zones ◽  
Upper Level ◽  
Static Energy ◽  
Regional Budgets

Abstract A mesoscale numerical model with an idealized tropical channel environment is used to study the dynamics of intertropical convergence zones (ITCZs) and the recently identified shallow return flow (SRF) and midlevel inflow (MLI). Four idealized sea surface temperature (SST) distributions are used: a meridionally symmetric SST profile with a sharply peaked SST maximum at the equator, a similar profile with the maximum SST shifted off the equator, a cosine-shaped SST profile with zero gradient at the equator, and an idealized SST profile modeled after the observed SST of the eastern Pacific. The simulations show that both the SRF and the MLI are robust features of the ITCZ. The prior result that the SRF is a sea-breeze-like response to surface temperature gradients is further supported, whereas the MLI is caused by the upper-level maxima in diabatic heating and vertical motion. Simulations with the SST maximum at the equator generate long-lasting, convectively coupled Kelvin waves. When the SST maximum is off the equator, the meridional circulations become highly asymmetric with strong cross-equatorial flow. Tropical cyclones are frequently generated by dynamic instability of the off-equatorial ITCZs. The contributions of the multilevel circulations to regional budgets of mass, water, and moist static energy (MSE) are computed. About 10% of the total water transported into the ITCZ region is transported out by the SRF. The water transport of the MLI is minimal, but its mass and MSE transports are significant, accounting for about ⅓ of the mass and MSE entering the ITCZ region. Eddy fluxes are found to be a large component of the net vertically integrated transport of MSE out of the ITCZ.

scholarly journals One-year analysis of rain and rain erosivity in a tropical volcanic island from UHF wind profiler measurements

2013 ◽  
Vol 6 (2) ◽  
pp. 3249-3277 ◽  
Author(s):  
A. Réchou ◽  
M. Plu ◽  
B. Campistron ◽  
R. Decoupes
Keyword(s):  
Kinetic Energy ◽  
Rain Rate ◽  
Cold Front ◽  
Deep Convection ◽  
Trade Wind ◽  
Energy Fluxes ◽  
Trade Winds ◽  
Rainfall Events ◽  
The Mean ◽  
One Year

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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