Abstract

The direct radiative effects of Saharan mineral dust (SMD) aerosols on the nonlinear evolution of the African easterly jet–African easterly wave (AEJ–AEW) system is examined using the Weather Research and Forecasting Model coupled to an online dust model. The SMD-modified AEW life cycles are characterized by four stages: enhanced linear growth, weakened nonlinear stabilization, larger peak amplitude, and smaller long-time amplitude. During the linear growth and nonlinear stabilization stages, the SMD increases the generation of eddy available potential energy (APE); this occurs where the maximum in the mean meridional SMD gradient is coincident with the critical surface. As the AEWs evolve beyond the nonlinear stabilization stage, the discrimination between SMD particle sizes due to sedimentation becomes more pronounced; the finer particles meridionally expand, while the coarser particles settle to the surface. The result is a reduction in the eddy APE at the base and the top of the plume.

The SMD enhances the Eliassen–Palm (EP) flux divergence and residual-mean meridional circulation, which generally oppose each other throughout the AEW life cycle. The SMD-modified residual-mean meridional circulation initially dominates to accelerate the flow but quickly surrenders to the EP flux divergence, which causes an SMD-enhanced deceleration of the AEJ during the linear growth and nonlinear stabilization stages. Throughout the AEW life cycle, the SMD-modified AEJ is elevated and the peak winds are larger than without SMD. During the first (second) half of the AEW life cycle, the SMD-modified wave fluxes shift the AEJ axis farther equatorward (poleward) of its original SMD-free position.

Abstract

The direct radiative effects of Saharan mineral dust aerosols on the linear dynamics of African easterly waves (AEWs) are examined analytically and numerically. The analytical analysis combines the thermodynamic equation with a dust continuity equation to form an expression for the dust-modified generation of eddy available potential energy . The dust-modified is a function of the transmissivity and spatial gradients of the dust, which are modulated by the Doppler-shifted frequency. The expression for predicts that for a fixed dust distribution, the wave response will be largest in regions where the dust gradients are maximized and the Doppler-shifted frequency vanishes. The numerical analysis uses the Weather Research and Forecasting (WRF) Model coupled to an online dust model to calculate the linear dynamics of AEWs. Zonally averaged basic states for wind, temperature, and dust are chosen consistent with summertime conditions over North Africa. For the fastest-growing AEW, the dust increases the growth rate from ~15% to 90% for aerosol optical depths ranging from τ = 1.0 to τ = 2.5. A local energetics analysis shows that for τ = 1.0, the dust increases the maximum barotropic and baroclinic energy conversions by ~50% and ~100%, respectively. The maxima in the generation and conversions of energy are collocated and occur where the meridional dust gradient is maximized near the critical surface—that is, where the Doppler-shifted frequency is small, in agreement with the prediction from the analytical analysis.

Abstract

Analytical and numerical analyses are used to examine how structural changes to the African easterly jet (AEJ) mediate the effects of Saharan mineral dust aerosols on the linear dynamics of African easterly waves (AEWs). An analytical expression for the generation of eddy available potential energy (APE) is derived that exposes how the AEJ and dust combine to affect the energetics of the AEWs. The expression is also used to interpret the numerical results, which are obtained by radiatively coupling a simplified version of the Weather Research and Forecasting Model to a conservation equation for dust. The WRF-Dust model is used to conduct linear simulations based on five observationally consistent zonal-mean AEJs: a reference AEJ and four other AEJs that are obtained by perturbing the maximum meridional and vertical shear. For a dust distribution consistent with summertime observations over North Africa, the numerical simulations show the following: (i) Irrespective of the AEJ structure or the zonal scale of the AEWs, the dust increases the growth rates of the AEWs. (ii) The growth rates of the AEWs are optimized when the ratio of baroclinic to barotropic energy conversions is largest. (iii) When the energy conversions are sufficiently large, the zonal scale of the fastest-growing AEW shortens. The numerical results confirm the analytical analysis, which shows that the dust effects, which are modulated by the Doppler-shifted frequency, are strongest north of the AEJ axis, a region where the dust augments the preexisting meridional temperature gradient.

Abstract

A spurious wavelike pattern in the monthly rain day statistics exists within the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis precipitation product. The anomaly, which is an artifact of the parameterization of moisture diffusion, occurs during the winter months in the Northern and Southern Hemisphere high latitudes. The anomaly is corrected by using monthly statistics from three different global precipitation products from 1) the University of Washington (UW), 2) the Global Precipitation Climate Project (GPCP), and 3) the Climatic Research Unit (CRU), resulting in three slightly different corrected precipitation products. The correction methodology, however, compromises spatial consistency (e.g., storm tracking) on a daily time scale. The effect that the precipitation correction has on the reanalysis-derived global land surface water budgets is investigated by forcing the Variable Infiltration Capacity (VIC) land surface model with all four datasets (i.e., the original reanalysis product and the three corrected datasets). The main components of the land surface water budget cycle are not affected substantially; however, the increased spatial variability in precipitation is reflected in the evaporation and runoff components but reduced in the case of soil moisture. Furthermore, the partitioning of precipitation into canopy evaporation and throughfall is sensitive to the rain day statistics of the correcting dataset, especially in the Tropics, and this has implications for the required accuracy of the correcting dataset. The output fields from these long-term land surface simulations provide a global, consistent dataset of water and energy states and fluxes that can be used for model intercomparisons, studies of annual and seasonal climate variability, and comparisons with current versions of numerical weather prediction models.

Abstract

Kilometer-scale climate model simulations are useful tools to investigate past and future changes in extreme precipitation, particularly in mountain regions, where convection is influenced by complex topography and land–atmosphere interactions. In this study, we evaluate simulations of a flood-producing mesoscale convective system (MCS) downstream of the Tibetan Plateau (TP) in the Sichuan basin from a kilometer-scale multimodel and multiphysics ensemble. The aim is to better understand the physical processes that need to be correctly simulated for successfully capturing downstream MCS formation. We assess how the ensemble members simulate these processes and how sensitive the simulations are to different model configurations. The preceding vortex evolution over the TP, its interaction with the jet stream, and water vapor advection into the basin are identified as key processes for the MCS formation. Most modeling systems struggle to capture the interaction between the vortex and jet stream, and perturbing the model physics has little impact, while constraining the large-scale flow by spectral nudging improves the simulation. This suggests that an accurate representation of the large-scale forcing is crucial to correctly simulate the MCS and associated precipitation. To verify whether the identified shortcomings systematically affect the MCS climatology in longer-term simulations, we evaluate a 1-yr WRF simulation and find that the seasonal cycle and spatial distribution of MCSs are reasonably well captured and not improved by spectral nudging. While the simulations of the MCS case highlight challenges in extreme precipitation forecasting, we conclude that these challenges do not systematically affect simulated climatological MCS characteristics.

Abstract

Combined impacts of the Pacific decadal oscillation (PDO) and two types of La Niña on climate anomalies in Europe are studied. Particularly, the conjunction of the negative PDO phase and two different types of La Niña events favors strong and significant North Atlantic Oscillation (NAO) pattern anomalies with opposite polarity. For the central Pacific (CP) La Niña, a clear positive NAO signal can be detected, which is accompanied by positive surface air temperature (SAT) anomaly and a dipolar structure of precipitation anomalies in Europe. In addition, a typical negative Pacific–North America (PNA) teleconnection pattern forms, including a high pressure anomaly over the southeastern United States, which may contribute to the development and maintenance of the NAO anomaly by strengthening the baroclinicity and the local eddy–mean flow interaction. However, for the eastern Pacific (EP) La Niña, a zonal wave train in the high latitudes can be observed, which is quite different from the typical PNA structure. Here, an anomalous anticyclone over southern Greenland supports a negative NAO pattern through the local eddy–mean flow interaction and the associated vorticity advection. Hence, reversed SAT and precipitation anomalies occur over Europe. Further analyses indicate that the wave trains emanating from the North Pacific and the synoptic eddy–mean flow interaction play essential roles in forming the anomalous NAO phases. The different wave trains for the CP and EP La Niña events may be attributed to the differences in the location and intensity of anomalous convection induced by different types of SST anomaly as well as by the corresponding background westerly wind anomalies in the upper troposphere.

Abstract

The hydrological impact of forest removal on the southeast Tibetan Plateau during the second half of the last century is investigated in this study using an atmospheric general circulation model. The effects of deforestation are investigated by examining the differences between the forest replacement and control experiments. Model results demonstrate that deforestation of the southeast Tibetan Plateau would influence the local and the remote climate as well. It would lead to decreased transpiration and increased summer precipitation in the deforested area and a wetter and warmer climate on the Tibetan Plateau in summer. This may produce more runoff into the rivers originating from the Tibetan Plateau and worsen flooding disasters in the downstream areas. The numerical experiments also show that deforestation would remotely impact Asian climate, and even global climate, although the statistical significance is small. A strong drought is found at middle and lower reaches of the Yellow River, where livelihoods and economics have suffered from recent droughts. Ecosystem research on the Tibetan Plateau is a relatively new topic and needs further interdisciplinary investigation.

Abstract

A theoretical framework is presented that exposes the radiative–dynamical relationships that govern the subcritical destabilization of African easterly waves (AEWs) by Saharan mineral dust (SMD) aerosols. The framework is built on coupled equations for quasigeostrophic potential vorticity (PV), temperature, and SMD mixing ratio. A perturbation analysis yields, for a subcritical, but otherwise arbitrary, zonal-mean background state, analytical expressions for the growth rate and frequency of the AEWs. The expressions are functions of the domain-averaged wave activity, which is generated by the direct radiative effects of the SMD. The wave activity is primarily modulated by the Doppler-shifted phase speed and the background gradients in PV and SMD.

Using an idealized version of the Weather Research and Forecasting (WRF) Model coupled to an interactive dust model, a linear analysis shows that, for a subcritical African easterly jet (AEJ) and a background SMD distribution that are consistent with observations, the SMD destabilizes the AEWs and slows their westward propagation, in agreement with the theoretical prediction. The SMD-induced growth rates are commensurate with, and can sometimes exceed, those obtained in previous dust-free studies in which the AEWs grow on AEJs that are supercritical with respect to the threshold for barotropic–baroclinic instability. The clarity of the theoretical framework can serve as a tool for understanding and predicting the effects of SMD aerosols on the linear instability of AEWs in subcritical, zonal-mean AEJs.

Abstract

A case study of a relatively dry front during TAMEX IOP-4 is presented. At 0000 UTC 27 May, the broad cloud band extended from the China plain and southern Japan to east of 150°E, along and north of the surface front. This front possessed appreciable baroclinity.

As the midtropospheric trough moved eastward, the surface front advanced southeastward and eventually separated from the quasi-stationary cloud band. The depth of the cold air decreased as it penetrated to the subtropics. In the vicinity of Taiwan, the frontal surface was shallow with a depth ≈ 1 km. The convection occurred in two different regimes; the large-scale cloud band associated with the eastward-moving trough, and cumuli along the surface front.

As the surface front approached Taiwan, the southwest flow started to increase, and a mesolow formed over the southeastern coast due to the interaction between the southwest flow and the mountainous topography. The air moved over the mountains and descended adiabatically. During the frontal passage, the mesolow deepened as the approaching front from the north also became a flow barrier. After the frontal passage, the mesolow was filled by the advancing cold air.

As the mesolow in southeastern Taiwan filled, a new mesolow formed over the southwestern plain. The surface front and isobars were distorted during the frontal passage as the shallow cold air moved around the topography. The relatively low pressure over the southwestern plain resulted hydrostatically from the cold air behind the front not reaching the southwestern plain, while surrounding areas experienced rising pressure with the arrival of the cold air.

Abstract

Infrared digital data from geostationary satellites were used to demonstrate the usefulness of remotely sensed surface temperature data to delineate microscale and mesoscale climates. Nocturnal winter data (December-February) from 1976–77 to 1978–79 over Florida revealed noticeable contrasts in surface temperature patterns. Colder areas were associated with low soil moisture content in the upper layers of excessively-drained and well-drained sandy soils, whereas warmer areas were associated with bodies of water, wetlands, or poorly drained soils. An unexpected surface temperature pattern for one night where the north-central Florida climatic zone was colder than the north Florida climatic zone was found to be caused by differences in antecedent frontal rainfall. Differences in surface radiant energy fluxes over these two areas at 0200 EST 20 December 1977, based on average satellite-sensed surface temperatures, were compared with differences in soil heat fluxes that were computed from 1.5 m climatological temperatures and soil thermal properties by use of a simplified surface energy balance equation. The difference in computed soil heat fluxes was in reasonable agreement with the difference in radiant energy fluxes from the surface of the two areas. It was therefore concluded that this method could be used to compute differences in thermal inertia of the surface layer of different areas based on satellite and climatological temperature data.