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This study applies a quantitative method to analyze variation in Pacific trade winds in 1950–2020. It's found that the southeast and northeast trade winds enter an acceleration period since the mid to late 1990s while the wind are enhanced in each month of the year. The enhancement of southeast trade wind is less intense than that of northeast trade wind. Further analysis shows that after the acceleration, the correlation between sea level pressure/sea surface temperature (SST) and the trade wind speed in the Pacific subtropical high area are obviously strengthened. The negative correlation between sea level pressure and SST in this area is also enhanced, which may be due to the enhancement of trade wind that have caused a decrease in SST. The reduction further enhanced and maintained a higher sea level pressure, thus forming a configuration which is conducive to the intensification of trade wind. Plain Language Summary: Under the background of global climate change in recent decades, the Pacific trade winds have strengthened since the mid to late 1990s. Further analysis shows that after the intensification, the correlation between sea level pressure/sea surface temperature (SST) and the trade wind speed in the Pacific subtropical high area are obviously strengthened. In the Pacific subtropical high area, the reverse changes of SST and sea level pressure are more obvious. This may be due to the fact that when the trade wind is strong, sea temperature decreases and the reduction maintains a higher sea level pressure. This reverse relationship helps to maintain the trade wind at a strong level. The discovery of this relationship is important for future research on global climate change related to trade winds. Key Points: The Pacific trade winds strengthened in the 1990s and the enhancement of southeast trade wind is less than that of northeast trade windAfter acceleration, correlation between sea level pressure/sea surface temperature (SST) and wind speed in Pacific subtropical high area enhanceThe acceleration of Pacific trade wind could possibly be related to the synergy of sea level pressure and SST [ABSTRACT FROM AUTHOR]

A growing body of literature investigates convective organization, but few studies to date have sought to investigate how wind shear plays a role in the spatial organization of shallow (trade‐wind) convection. The present study hence investigates the morphology of precipitating marine cumulus convection using large‐eddy‐simulation experiments with zonal forward and backward shear and without shear. One set of simulations includes evaporation of precipitation, promoting cold‐pool development, and another set inhibits the evaporation of precipitation and thus cold‐pool formation. Without (or with only weak) subcloud‐layer shear, conditions are unfavorable for convective deepening, as clouds remain stationary relative to their subcloud‐layer roots so that precipitative downdrafts interfere with emerging updrafts. Under subcloud‐layer forward shear (FS), where the wind strengthens with height (a condition that is commonly found in the trades), clouds move at greater speed than their roots and precipitation falls downwind away from emerging updrafts. FS in the subcloud layer appears to promote the development of stronger subcloud circulations, with greater divergence in the cold‐pool area downwind of the original cell and larger convergence and stronger uplift at the gust front boundary. As clouds shear forward, a larger fraction of precipitation falls outside of clouds, leading to more moistening within the cold pool (gust front). Plain Language Summary: The most common type of clouds in Earth's trade wind regions are precipitating cumulus clouds with tops up to 4 km height. The precipitation from such clouds is frequent and intense enough to cause so‐called cold pools: cold and dry air that spreads out laterally near the surface in a circular fashion triggering new clouds in arc‐like patterns. We used a high‐resolution atmospheric model to investigate how the morphology of such clouds and the associated cold pools is affected by vertical changes in the wind speed (shear). When the wind speed at the surface and at cloud base is the same, clouds remain above their "roots," and downward‐moving air associated with rain falls into those cloud roots, which hinders the consequent deepening of these clouds. When the wind speed increases from the surface to cloud base (which it often does), clouds move away from their roots, which separates the location of updrafts and downdrafts, allowing for the development of deeper clouds. This type of shear also promotes the formation of new clouds further away at the edge of cold pools. Key Points: Without cold pools, new convection is triggered upwind of existing cells, and with cold pools, new convection is triggered downwindIn the absence of (or with only weak) subcloud‐layer shear, rain falls into emerging updrafts, hindering the convective developmentUplift at the cold‐pool gust fronts is strongest under subcloud‐layer forward shear (increasing wind speed with height) [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract Motivated by an observed relationship between marine low cloud cover and surface wind speed, this study investigates how vertical wind shear affects trade‐wind cumulus convection, including shallow cumulus and congestus with tops below the freezing level. We ran large‐eddy simulations for an idealized case of trade‐wind convection using different vertical shears in the zonal wind. Backward shear, whereby surface easterlies become upper westerlies, is effective at limiting vertical cloud development, which leads to a moister, shallower, and cloudier trade‐wind layer. Without shear or with forward shear, shallow convection tends to deepen more, but clouds tops are still limited under forward shear. A number of mechanisms explain the observed behavior: First, shear leads to different surface wind speeds and, in turn, surface heat and moisture fluxes due to momentum transport, whereby the weakest surface wind speeds develop under backward shear. Second, a forward shear profile in the subcloud layer enhances moisture aggregation and leads to larger cloud clusters, but only on large domains that generally support cloud organization. Third, any absolute amount of shear across the cloud layer limits updraft speeds by enhancing the downward oriented pressure perturbation force. Backward shear—the most typical shear found in the winter trades—can thus be argued a key ingredient at setting the typical structure of the trade‐wind layer.

Ground surface visibility during Harmattan season over the years has impeded road and air transportations, causing numerous infectious diseases in West African sub region. Surface degraded visibility is one of the prominent climatic parameters that define Harmattan season in the region. The presence of dust in the atmosphere brought by the North east trade winds during the Harmattan season plays a vital role in absorbing and scattering solar radiation. The degraded visibility has various effects on low temperature and relative humidity experienced during the season. The study assessed the temporal and spatial variations of surface visibility in Northeastern Nigeria during the Harmattan season with the aim of ascertaining its variability, patterns and trends in the last three decades. Descriptive statistics of mean, standard deviation, coefficient of variation, and time series analysis with ArcGIS10.3 was used in assessing the temporal and spatial variations of surface visibility from 1984–2014 in six synoptic stations of Northeastern Nigeria. The findings show that surface visibility varied both temporally and spatially over the years. The pattern of visibility in the last three decades has shown rising trends with above mean variations dominating the entire distributions in all the six synoptic stations of the study area.

Motivated by an observed relationship between marine low cloud cover and surface wind speed, this study investigates how vertical wind shear affects trade‐wind cumulus convection, including shallow cumulus and congestus with tops below the freezing level. We ran large‐eddy simulations for an idealized case of trade‐wind convection using different vertical shears in the zonal wind. Backward shear, whereby surface easterlies become upper westerlies, is effective at limiting vertical cloud development, which leads to a moister, shallower, and cloudier trade‐wind layer. Without shear or with forward shear, shallow convection tends to deepen more, but clouds tops are still limited under forward shear. A number of mechanisms explain the observed behavior: First, shear leads to different surface wind speeds and, in turn, surface heat and moisture fluxes due to momentum transport, whereby the weakest surface wind speeds develop under backward shear. Second, a forward shear profile in the subcloud layer enhances moisture aggregation and leads to larger cloud clusters, but only on large domains that generally support cloud organization. Third, any absolute amount of shear across the cloud layer limits updraft speeds by enhancing the downward oriented pressure perturbation force. Backward shear—the most typical shear found in the winter trades—can thus be argued a key ingredient at setting the typical structure of the trade‐wind layer. Plain Language Summary: We used a high‐resolution weather model to investigate the influence of the shape of the wind profile (i.e., whether the wind blows faster, slower, or with the same velocity at greater altitudes compared to the surface) on shallow cumulus clouds typical of the North Atlantic trade‐wind region. In this region, easterly winds that decrease with height (and eventually turn westerly) are most common. Generally, the surface winds are also affected by how the wind blows further aloft, influencing what kind of clouds form. But even when we eliminate this effect in our study, we find that when the wind blows faster or slower at greater heights, clouds are not only tilted but also wider and both effects increase the overall cloud cover. Furthermore, if the wind speed changes with height, the updraft speed within clouds is diminished, which potentially decreases the height of clouds. However, if the wind speed increases with height (which only rarely occurs in the trades), clouds tend to cluster more, which "offsets" the weaker updrafts and thus still allows for deeper clouds. Key Points: Shear in the zonal wind influences cloud‐top heights via the effect of momentum transport on the surface wind and surface fluxesBackward shear (surface easterlies turn westerlies) lowers cloud tops and shallows and moistens the trade‐wind layerAny absolute amount of wind shear limits in‐cloud updraft speeds and enhances low‐level cloud fraction [ABSTRACT FROM AUTHOR]

Global positioning system radio occultation (GPS RO) refractivity data obtained from the first Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) for the years 2007 to 2012 were used to estimate an overall climatology for the height of the marine boundary layer (MBL) over the central North Pacific Ocean including the Hawaiian Island region (10°N–45°N; 125°W–175°W). The trade wind days are identified based on the six-year National Centers for Environmental Prediction (NCEP) global analysis for the same period. About 87% of the RO soundings in summer (June–July–August, JJA) and 47% in winter (December–January–February, DJF) are under trade wind conditions. The MBL height climatology under trade wind conditions is derived and compared to the overall climatology. In general, MBL heights are lowest adjacent to the southern coast of California and gradually increase to the south and west. During the summer (JJA) when the northeasterly trade winds are the dominant surface flow, the median MBL height decreases from 2.0 km over Kauai to 1.9 km over the Big Island with an approximate 2 km maximum that progresses from southwest to northeast throughout the year. If the surface flow is restricted to trade winds only, the maximum MBL heights are located over the same areas, but they increase to a median height of 1.8 km during DJF and 2.1 km during JJA. For the first time, the GPS RO technique allows the depiction of the spatial variations of the MBL height climatology over the central North Pacific. [ABSTRACT FROM AUTHOR]