Chidong Zhang

African dust outbreaks: A satellite perspective of temporal and spatial variability over the tropical Atlantic Ocean

1000 km d −1 , reaching the Caribbean or South America in a week’s time. The advance of a dust front is associated with decreases in water vapor (up to −1.0 g kg −1 ) and increases in temperature (up to 1.0 K) and, behind the fronts, an anticyclonic circulation. We used Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) data to characterize dust altitude distributions. The vertical distribution of warm dry air is similar to that of dust observed in CALIPSO. The dust layer altitude decreases during transport across the Atlantic and is significantly lower in boreal winter than summer. The study highlights the temporal and spatial variability of African dust outbreaks, which are important to improving our understanding of climate impacts of African dust and Atlantic climate variability in general.

CALIPSO-Derived three-dimensional structure of aerosol over the atlantic basin and adjacent continents

AbstractAccurate modeling of the impact of aerosols on climate requires a detailed understanding of the vertical distribution of aerosols. The Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) provides continuous high-resolution vertical profiles of aerosol properties on a near-global scale. Here the CALIPSO Vertical Feature Mask is used to document the three-dimensional (3D) frequency-of-occurrence distribution of aerosols over a broad region of the Atlantic Ocean, Africa, Europe, and the Americas. The 3D distributions illustrate the seasonal cycle in the zonal and meridional variability of the vertical profiles of mineral dust, biomass-burning smoke, polluted dust (external mixture of dust and smoke), and polluted continental aerosol, and also of their emissions sources and transport pathways. Four aerosol domains stand out in the product: dust over North Africa and the Middle East and smoke over southern Africa and South America. The transport pathways of African dust and smo...

African aerosol and large-scale precipitation variability over West Africa

We investigated the large-scale connection between African aerosol and precipitation in the West African Monsoon (WAM) region using 8-year (2000–2007) monthly and daily Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol products (aerosol optical depth, fine mode fraction) and Tropical Rainfall Measuring Mission (TRMM) precipitation and rain type. These high-quality data further confirmed our previous results that the large-scale link between aerosol and precipitation in this region undergoes distinct seasonal and spatial variability. Previously detected suppression of precipitation during months of high aerosol concentration occurs in both convective and stratiform rain, but not systematically in shallow rain. This suggests the suppression of deep convection due to the aerosol. Based on the seasonal cycle of dust and smoke and their geographical distribution, our data suggest that both dust (coarse mode aerosol) and smoke (fine mode aerosol) contribute to the precipitation suppression. However, the dust effect is evident over the Gulf of Guinea while the smoke effect is evident over both land and ocean. A back trajectory analysis further demonstrates that the precipitation reduction is statistically linked to the upwind aerosol concentration. This study suggests that African aerosol outbreaks in the WAM region can influence precipitation in the local monsoon system which has direct societal impact on the local community. It calls for more systematic investigations to determine the modulating mechanisms using both observational and modeling approaches.

Aerosol-Induced Large-Scale Variability in Precipitation over the Tropical Atlantic

Abstract Multiyear satellite observations are used to document a relationship between the large-scale variability in precipitation over the tropical Atlantic and aerosol traced to African sources. During boreal winter and spring there is a significant reduction in precipitation south of the Atlantic marine intertropical convergence zone (ITCZ) during months when aerosol concentrations are anomalously high over a large domain of the tropical Atlantic Ocean. This reduction cannot be linearly attributed to known climate factors such as El Nino–Southern Oscillation, the North Atlantic Oscillation, and zonal and meridional modes of tropical Atlantic sea surface temperature or to meteorological factors such as water vapor. The fractional variance in precipitation related to aerosol is about 12% of the total interannual variance, which is of the same order of magnitude as that related to each of the known climate and weather factors. A backward trajectory analysis confirms the African origin of aerosols that dir...

Possible root causes of surface westerly biases over the equatorial atlantic in global climate models

AbstractMost global climate models (GCMs) suffer from biases of a reversed zonal gradient in sea surface temperature (SST) and weak surface easterlies (the westerly bias) in the equatorial Atlantic during boreal spring. These biases exist in atmospheric GCMs (AGCMs) and are amplified by air–sea interactions in atmospheric–oceanic GCMs. This problem has persisted despite considerable model improvements in other aspects. This study proposes a hypothesis that there are two possible root causes for the westerly bias. The first is insufficient lower-tropospheric diabatic heating over Amazonia. The second is erroneously weak zonal momentum flux (entrainment) across the top of the boundary layer. This hypothesis is based on a scale analysis of a simple model for a well-mixed equatorial boundary layer and diagnoses of simulations from eight AGCMs. Severe westerly biases in AGCMs tend to occur when the diabatic heating at low levels (850–700 hPa) over Amazonia is too weak. Deficient low-level diabatic heating weak...

The Role of Shallow Cloud Moistening in MJO and Non-MJO Convective Events over the ARM Manus Site

AbstractObservations from the Atmospheric Radiation Measurement Program (ARM) site at Manus Island in the western Pacific and (re)analysis products are used to investigate moistening by shallow cumulus clouds and by the circulation in large-scale convective events. Large-scale convective events are defined as rainfall anomalies larger than one standard deviation for a minimum of three consecutive days over a 10° × 10° domain centered at Manus. These events are categorized into two groups: Madden–Julian oscillation (MJO) events, with eastward propagation, and non-MJO events, without propagation. Shallow cumulus clouds are identified as continuous time–height echoes from 1-min cloud radar observations with their tops below the freezing level and their bases within the boundary layer. Daily moistening tendencies of shallow clouds, estimated from differences between their mean liquid water content and precipitation over their presumed life spans, and those of physical processes and advection from (re)analysis...

Sensitivity of the water cycle over the Indian Ocean and Maritime Continent to parameterized physics in a regional model

A regional model was used to simulate the water cycle over the Indian Ocean (IO) and Maritime Continent (MC). Sixteen 92 day simulations were performed using different combinations of eight cumulus parameterization schemes and three planetary boundary-layer (PBL) parameterization schemes. The strength of the water cycle in the IO and MC, measured by its domain mean precipitation and precipitable water, differs substantially among the simulations. The large spread of water cycle strength is mainly toward dry biases in comparison to global data assimilation products. The simulated water cycle, its spread, and biases differ between the IO and MC. Influences of PBL schemes can penetrate into the upper troposphere and those by cumulus schemes into the boundary layer. Dry biases in the simulations are produced mainly because of feedbacks among erroneously low diabatic heating peaks, shallow moisture convergence layers, dry lower troposphere, and weak surface evaporation. There is no single type of parameterization scheme that can be identified to be the main sources of the dry biases. It is the combination of errors from three types of parameterization schemes, namely, cumulus, PBL, and microphysics, that makes the simulated water cycle unrealistic. The lesson learned is that the tropical water cycle can be better simulated only by improving parameterization schemes of different processes all together as a package.

Role of water vapor and convection‐circulation decoupling in MJO simulations by a tropical channel model

A tropical channel model failed to simulate an observed MJO event. As a means to find reasons for the failure, model errors in water vapor (qv) and other fields were corrected to different extents by nudging towards a global reanalysis product. An MJO metric was developed to quantify the realism of a simulated MJO event in the amplitude and zonal propagation speed of its precipitation and zonal wind at 850 hPa (U850). Results show that correcting errors in qv alone is necessary and sufficient for a realistic simulation of the MJO event. Correcting errors in other fields is insufficient by itself and unnecessary with water vapor correction. For a realistic MJO simulation, error corrections are needed for qv perturbations of the planetary scales (zonal wave number 1–3) together with its zonal mean in both the lower and middle troposphere. Correcting errors in qv at upper levels, planetary-scale perturbations of any single zonal wave number, synoptic-scale perturbations, or temporal/zonal means alone is not sufficient for a realistic MJO simulation. Amplitudes of MJO precipitation and U850 are significantly correlated, but their propagation speeds are not. MJO signals in U850 are easier than those in precipitation to improve by correcting errors in qv. In the absence of MJO intrinsic dynamics, inserted MJO signals in qv in a simulation may create a phantom MJO event in precipitation but not in the circulation. This study confirms the existing knowledge on the importance of qv to MJO simulations, sheds new lights on the role of the qv spatial structure, and presents evidence for possible precipitation-circulation decoupling in MJO simulations.

Stratiform and Convective Precipitation Observed by Multiple Radars during the DYNAMO/AMIE Experiment

AbstractIn this study, methods of convective/stratiform precipitation classification and surface rain-rate estimation based on the Atmospheric Radiation Measurement Program (ARM) cloud radar measurements were developed and evaluated. Simultaneous and collocated observations of the Ka-band ARM zenith radar (KAZR), two scanning precipitation radars [NCAR S-band/Ka-band Dual Polarization, Dual Wavelength Doppler Radar (S-PolKa) and Texas A&M University Shared Mobile Atmospheric Research and Teaching Radar (SMART-R)], and surface precipitation during the Dynamics of the Madden–Julian Oscillation/ARM MJO Investigation Experiment (DYNAMO/AMIE) field campaign were used. The motivation of this study is to apply the unique long-term ARM cloud radar observations without accompanying precipitation radars to the study of cloud life cycle and precipitation features under different weather and climate regimes. The resulting convective/stratiform classification from KAZR was evaluated against precipitation radars. Preci...

Low-cloud characteristics over the tropical western Pacific from ARM observations and CAM5 simulations

This study evaluates the ability of the Community Atmospheric Model version 5 (CAM5) to reproduce low clouds observed by the Atmospheric Radiation Measurement (ARM) cloud radar at Manus Island of the tropical western Pacific during the Years of Tropical Convection. Here low clouds are defined as clouds with their tops below the freezing level and bases within the boundary layer. Low-cloud statistics in CAM5 simulations and ARM observations are compared in terms of their general occurrence, mean vertical profiles, fraction of precipitating versus nonprecipitating events, diurnal cycle, and monthly time series. Other types of clouds are included to put the comparison in a broader context. The comparison shows that the model overproduces total clouds and their precipitation fraction but underestimates low clouds in general. The model, however, produces excessive low clouds in a thin layer between 954 and 930 hPa, which coincides with excessive humidity near the top of the mixed layer. This suggests that the erroneously excessive low clouds stem from parameterization of both cloud and turbulence mixing. The model also fails to produce the observed diurnal cycle in low clouds, not exclusively due to the model coarse grid spacing that does not resolve Manus Island. This study demonstrates the utility of ARM long-term cloud observations in the tropical western Pacific in verifying low clouds simulated by global climate models, illustrates issues of using ARM observations in model validation, and provides an example of severe model biases in producing observed low clouds in the tropical western Pacific.