An idealized convection‐permitting simulation is performed to investigate the physical mechanisms responsible for the nighttime offshore propagation of convection around tropical islands. An ...idealized island is placed in the middle of a long, nonrotating channel oceanic domain with constant sea surface temperature. A strong diurnal cycle typical of a tropical island is simulated, with a thermally forced sea breeze in daytime and the associated inland propagation of precipitation. Offshore propagation of a land breeze and its associated convection is simulated every night but with varying extent. Gravity waves of first and second baroclinic modes trigger convection far from the coast if the offshore conditions are favorable. This accelerates the propagation speed of the land breeze as it reduces the onshore wind associated with the lower branch of the overturning large‐scale circulation. Higher‐order modes may trigger convection or reinforce existing convection but less systematically. The distance of propagation is particularly sensitive to humidity and temperature at the top of the boundary layer, with occasional incursions of a dry anomaly at the top of the boundary layer near the island preventing convection from developing far from the island.
Plain Language Summary
Climate models have problems modeling the precipitation pattern, timing, and atmospheric circulation associated with tropical island regions. This impacts the prediction of planetary scale events such as the Madden‐Julian Oscillation. Part of this problem originates from the poor representation of islands and their topography, which affects the diurnal cycle of convection over land and the surrounding oceans. This diurnal cycle generates a sea breeze during daytime, which triggers convection over the island in the afternoon, early evening. At night, the breeze reverses and becomes a land breeze that propagates over the ocean. In order to understand what controls this offshore propagation, we study the movement of convection around an idealized tropical island. We show that a land breeze is systematically present even though its distance of propagation depends on the presence of offshore convection triggered by gravity waves earlier and on the humidity and temperature between 1 and 2 km of altitude close to coast. Properly modeling these individual elements may improve the precipitation pattern and atmospheric circulation around tropical islands and be beneficial for improving the representation of atmospheric phenomenons impacted by these regions.
Key Points
Systematic offshore propagation of convection is observed at night, with varying distance of propagation
Offshore convection triggered by gravity waves modulates the propagation speed of the land breeze
Large‐scale environment controls propagation with a dry and warm anomaly at the top of the boundary layer
This study summarizes an assessment of the wind resource at selected locations in Fiji for the potential of future utility-scale wind-power development. We use 2–8 years of near surface wind ...observations (2011–2018) from thirty automatic weather stations. The standard wind-industry software, WAsP is used to simulate the wind resource in terms of mean wind speed, dominant wind direction, power density and annual energy production (AEP) using a Vergnet 275-kW wind turbine. Our analysis identifies three sites: Rakiraki, Nabouwalu and Udu, which should be considered for a future comprehensive resource assessment for utility-scale wind-power development once further wind resource data is available. High-resolution resource maps for each wind resource parameter at a horizontal resolution of 50 m are produced for 6 km×6 km domains around these sites. Rakiraki, Nabouwalu and Udu have average wind speeds of 7.6 m/s, 7.1 m/s and 7.0 m/s, with an average power density of 401 W/m2, 512 W/m2 and 294 W/m2, and a potential average AEP of 0.91 GWh, 0.80 GWh and 0.72 GWh at 55 m AGL, respectively. The dominant wind direction is southeasterly. Modelling a 10 MW wind farm at each site yields a net AEP of 43 GWh, 42 GWh and 37 GWh for Rakiraki, Nabouwalu and Udu, respectively with capacity factors of 0.42–0.48 and wind farm efficiencies of 97–98%.
•Assessment of wind resource and energy potential for new onshore locations in Fiji.•Three potential sites of Rakiraki, Nabouwalu and Udu identified.•Sites have potential for future utility-scale wind-power development.•Each site can accommodate a 10 MW wind farm using Vergnet 275-kW wind turbines.•Modelling a 10 MW wind farm at each site yields an AEP of 43 GWh, 42 GWh and 37 GWh.
A set of idealized convection‐permitting simulations is performed to investigate the influence of topography on the physical mechanisms responsible for the nocturnal offshore propagation of ...convection around tropical islands. All simulations have an idealized island in the middle of a long channel oceanic domain, with constant sea surface temperature and without rotation. To diagnose the impact of topography, we compare a flat island simulation with two simulations with mountain ranges of different shapes. The topography over the island has a strong impact on the diurnal cycle of convection as clouds tend to remain all day over the highest topography. This weakens the diurnal cycle and the land breeze front and triggers a comparatively less frequent long‐distance offshore propagation of convection. As in the flat simulation, the distance of offshore propagation is particularly sensitive to humidity and temperature at the top of the boundary layer. A shallow circulation that is asymmetric with respect to the island influences the boundary layer top humidity and can favor propagation on one side of the island or the other. These results mimic cloud and precipitation patterns observed prior to the Madden‐Julian Oscillation propagation over the Maritime Continent. The shape of the topography does not seem to influence the offshore propagation of convection significantly except for mountain‐valley breezes that reinforce the land breeze and the establishment of the asymmetric shallow circulation.
Plain Language Summary
In Part I of this paper, we looked at the mechanisms controlling the offshore propagation of convection around an idealized flat tropical island. But tropical islands generally have high topography reaching thousands of meters over Papua New Guinea, Borneo, and Sumatra. The impact of topography is investigated using the same framework as before but with a mountain range on the island. We test two different shapes to investigate valley breeze effects and the dependence on the mountain shape: one is a simple ridge and the other has a peak and a pass. The presence of a mountain range strongly affects the offshore propagation of convection as clouds tend to stay above the highest topography at night, which weakens the land breeze. Gravity waves triggering convection far from the coast are present but only for days when clouds move away from the coast at night. The valley is found to reinforce the land breeze and, more surprisingly, allows the development of asymmetric offshore convection relative to the island. This asymmetry, also found with the flat island simulation, is responsible for the advection of a dry anomaly that forces convection to stay close to the coast more frequently.
Key Points
Clouds clinging to topography weaken nocturnal offshore propagation of convection
The same mechanisms trigger offshore convection with flat island and with topography
Valley breeze effect reinforces the propagation speed of the land breeze
Aimed at reducing deficiencies in representing the Madden‐Julian oscillation (MJO) in general circulation models (GCMs), a global model evaluation project on vertical structure and physical processes ...of the MJO was coordinated. In this paper, results from the climate simulation component of this project are reported. It is shown that the MJO remains a great challenge in these latest generation GCMs. The systematic eastward propagation of the MJO is only well simulated in about one fourth of the total participating models. The observed vertical westward tilt with altitude of the MJO is well simulated in good MJO models but not in the poor ones. Damped Kelvin wave responses to the east of convection in the lower troposphere could be responsible for the missing MJO preconditioning process in these poor MJO models. Several process‐oriented diagnostics were conducted to discriminate key processes for realistic MJO simulations. While large‐scale rainfall partition and low‐level mean zonal winds over the Indo‐Pacific in a model are not found to be closely associated with its MJO skill, two metrics, including the low‐level relative humidity difference between high‐ and low‐rain events and seasonal mean gross moist stability, exhibit statistically significant correlations with the MJO performance. It is further indicated that increased cloud‐radiative feedback tends to be associated with reduced amplitude of intraseasonal variability, which is incompatible with the radiative instability theory previously proposed for the MJO. Results in this study confirm that inclusion of air‐sea interaction can lead to significant improvement in simulating the MJO.
Key Points
Realistic vertical structure is closely related to good MJO simulations
Environment moisture and convection‐circulation feedback are critical for MJO
Air‐sea interaction can significantly improve MJO simulations in many GCMs
The double intertropical convergence zone (ITCZ) bias still affects all the models that participate to CMIP5 (Coupled Model Intercomparison Project, phase 5). As an ensemble, general circulation ...models have improved little between CMIP3 and CMIP5 as far as the double ITCZ is concerned. The present study proposes a new process-oriented metrics that provides a robust statistical relationship between atmospheric processes and the double ITCZ bias, additionally to the existing relationship between the sea surface temperature (SST) and the double ITCZ bias. The SST contribution is examined using the THR-MLT index (Bellucci et al. in J Clim 5:1127–1145,
2010
), which combines biases on the representation of local SSTs and the SST threshold leading to the onset of ascent in the double ITCZ region. As a metrics of a model’s bias in simulating the interaction between circulation and precipitation, we propose to use the Combined Precipitation Circulation Error (CPCE). It is computed as the quadratic error on the contribution of each vertical regime to the total precipitation over the tropical oceans. CPCE is a global measure of the circulation-precipitation coupling that characterizes the model physical parameterizations rather than the regional characteristics of the eastern Pacific. A linear regression analysis shows that most of the double ITCZ spread among CMIP5 coupled ocean–atmosphere models is attributed to SST biases, and that the precipitation large-scale dynamics relationship explains a significant fraction of the bias in these models, as well as in the atmosphere-only models.
This study presents an analysis of the proposed Weather Research and Forecasting–Wind Atlas Analysis and Application Program (WRF–WAsP) models coupling methodology and evaluates the improvement in ...the accuracy of the wind predictions for the small island developing state (SIDS) of Fiji in the tropical Southwest Pacific region. The results revealed that the proposed WRF–WAsP coupling methodology can be used as a wind resource assessment methodology provided very-high resolution dynamically downscaled wind resource data is available in the order of 10 years for utility-scale wind power applications. The analysis also revealed that the 1 km × 1 km WRF model data from the finest (high-resolution) domain (d03) is best for coupling WRF–WAsP for wind resource parameters evaluation. The coupling methodology of WRF–WAsP improved the accuracy of the wind prediction by 0.2–6% for the wind resource parameters evaluated for Fiji.
A linear model of intermediate complexity based on quasi‐equilibrium theory is used in a zonally symmetric aquaplanet configuration to investigate the stability of the mean summer monsoon flow in the ...south Asian region. In the control case, the mean state has one linearly unstable mode that corresponds in period and structure to the variability in the nonlinear model described by Bellon and Sobel (2008) and to the observed 30‐ to 60‐d mode of intraseasonal variability. This mode features propagation of the precipitation anomalies from the equator to the monsoon trough around 20°N. Both the period of this mode and its direction of propagation are determined by the direction and magnitude of the mean meridional baroclinic flow. The wind‐induced surface heat fluxes associated with the surface westerlies in the northern tropics are an important contributor to the instability of the mode. The mechanisms of propagation and reinitiation of convection are further described. (1) Meridional advection of vorticity by the mean flow is responsible for the creation of free‐tropospheric vorticity north of the convection center. This vorticity in turn creates boundary layer convergence via Coriolis acceleration and subsequent northward movement of the convection. (2) Meridional advection of zonal momentum creates equatorial free‐tropospheric easterlies on the equator. These easterlies in turn generate low‐level convergence via the β term of the Coriolis acceleration and cause the reemergence of convection at the equator.
Abstract
We investigate the steady dynamical response of the atmosphere on the equatorial β plane to a steady, localized, midtropospheric heating source at the equator. Expanding Gill’s seminal work, ...we vary the latitudinal and longitudinal scales of the diabatic heating pattern while keeping its total amount fixed. We focus on characteristics of the response that would be particularly important if the circulation interacted with the hydrologic and energy cycles: the overturning circulation and the low-level wind. In the limit of very small scale in either the longitudinal or latitudinal direction, the vertical energy transport balances the diabatic heating and this sets the intensity of the overturning circulation. In this limit, a fast low-level westerly jet is located around the center of diabatic heating. With increasing longitudinal or latitudinal scale of the diabatic heating, the intensity of the overturning circulation decreases and the low-level westerly jet decreases in maximum velocity and spatial extent relative to the spatial extent of this heating. The associated low-level eastward mass transport decreases only with increasing longitudinal scale. These results suggest that moisture-convergence feedbacks will favor small-scale equatorial convective disturbances while surface-heat-flux feedbacks would favor small-scale disturbances in mean westerlies and large-scale disturbances in mean easterlies. Part II investigates the case of off-equatorial heating.
Abstract
We investigate the steady dynamical response of the atmosphere on the equatorial β plane to a steady, localized, midtropospheric heating source. Following Part I, which investigates the case ...of an equatorial diabatic heating, we explore the sensitivity of the Gill circulation to the latitudinal location of the heating, together with the sensitivity to its horizontal scale. Again, we focus on characteristics of the response that would be particularly important if the circulation interacted with the hydrologic and energy cycles: overturning circulation and low-level wind. In the off-equatorial case, the intensity of the overturning circulation has the same limit as in the equatorial case for small horizontal extent of the diabatic heating, which is also the limit in the
f
-plane case. The decrease in this intensity with increasing horizontal scale of the diabatic heating is slightly faster in the off-equatorial case than in the equatorial case, which is due to the increase of rotational winds at the expense of divergent winds. The low-level westerly jet is more intense than in the equatorial case, with larger maximum wind and eastward mass transport that tend to infinity for small horizontal extent of the diabatic heating. In terms of spatial characteristics, this jet has a similar latitudinal extent as in the equatorial case but, unlike in the equatorial case, it extends farther equatorward than poleward of the diabatic-heating center. It also extends farther eastward than in the equatorial case.
PDF
