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Sun Wong and João Teixeira

Abstract

Changes in tropical convective events provide a test bed for understanding changes of extreme convection in a warming climate. Because convective cloud top in deep convection is associated with cold brightness temperatures (BTs) in infrared window channels, variability in global convective events can be studied by spaceborne measurements of BTs. The sensitivity of BTs, directly measured by an Atmospheric Infrared Sounder (AIRS) window channel, to natural changes (the seasonal cycle and El Niño–Southern Oscillation) in tropical sea surface temperature (SST) is examined. It is found that tropical average BTs (over the ocean) at the low percentiles of their probability distributions scale with tropical average SSTs (higher SST leading to colder BTs), with the lower percentiles being significantly more sensitive to changes in SST. The sensitivity is reduced for high percentiles of BT and is insignificant for the median BT, and has similar magnitudes for the two natural changes used in the study. The regions where the lower-percentile BTs are most sensitive to SST are near the edges of the convection active areas (intertropical convergence zone and South Pacific convergence zone), including areas with active tropical cyclone activity. Since cold BTs of lower percentiles represent stronger convective events, this study provides, for the first time, global observational evidence of higher sensitivity of changes in stronger convective activity to a changing SST. This result has important potential implications in answering the key climate question of how severe tropical convection will change in a warming world.

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Drew T. Shindell, Sun Wong, and David Rind

Abstract

To study the interannual variability of the Antarctic ozone hole, a physically realistic parameterization of the chemistry responsible for severe polar ozone loss has been included in the GISS GCM. The ensuing ozone hole agrees well with observations, as do modeled surface UV increases of up to 42%. The presence of the ozone hole causes a reduction in lower stratospheric solar heating and an increase in upper stratospheric descent and dynamical heating in the model, as expected. Both the degree of ozone depletion and the dynamical response exhibit large interannual variability, however. These variations are driven by differences in the midwinter buildup of tropospheric wave energy in the model, which affect the dynamics globally for several months according to the mechanism detailed herein. Starting by July, strong tropospheric wave activity leads to greater energy reaching the lower stratosphere, and therefore warmer temperatures, than in years with weak wave activity. The warmer temperatures persist throughout the austral spring, resulting in ozone losses that are only ∼80% of those seen in the years with weaker wave activity. Significant differences also occur in the zonal wind field, setting up conditions that ultimately affect the propagation of wave energy in the spring. Differences in the propagation of wave energy lead to an October increase in upper stratospheric dynamical heating that is more than three times larger in the years with weak wave activity than in years with strong wave activity. Modeled interannual variations in both upper stratospheric temperatures and ozone loss are of similar magnitude to observations, though the largest observed variations exceed those seen here, indicating that unforced variability likely plays a significant role in addition to periodic forcings such as the QBO. The results are in accord with observational studies showing a strong anticorrelation between the interannual variability of tropospheric wave forcing and of the Antarctic ozone hole, suggesting that midwinter tropospheric wave energy may be the best predictor of the severity of the ozone hole the following spring.

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Xianan Jiang, Terence L. Kubar, Sun Wong, William S. Olson, and Duane E. Waliser

Abstract

Owing to its profound influences on global energy balance, accurate representation of low cloud variability in climate models is an urgent need for future climate projection. In the present study, marine low cloud variability on intraseasonal time scales is characterized, with a particular focus over the Pacific basin during boreal summer and its association with the dominant mode of tropical intraseasonal variability (TISV) over the eastern Pacific (EPAC) intertropical convergence zone (ITCZ). Analyses indicate that, when anomalous TISV convection is enhanced over the elongated EPAC ITCZ, reduction of low cloud fraction (LCF) is evident over a vast area of the central North Pacific. Subsequently, when the enhanced TISV convection migrates to the northern part of the EPAC warm pool, a “comma shaped” pattern of reduced LCF prevails over the subtropical North Pacific, along with a pronounced reduction of LCF present over the southeast Pacific (SEPAC). Further analyses indicate that surface latent heat fluxes and boundary heights induced by anomalous low-level circulation through temperature advection and changes of total wind speed, as well as midlevel vertical velocity associated with the EPAC TISV, could be the most prominent factors in regulating the intraseasonal variability of LCF over the North Pacific. For the SEPAC, temperature anomalies at the top of the boundary inversion layer between 850 and 800 hPa play a critical role in the local LCF intraseasonal variations. Results presented in this study provide not only improved understanding of variability of marine low clouds and the underlying physics, but also a prominent benchmark in constraining and evaluating the representation of low clouds in climate models.

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Qing Yue, Eric J. Fetzer, Brian H. Kahn, Sun Wong, Gerald Manipon, Alexandre Guillaume, and Brian Wilson

Abstract

The precision, accuracy, and potential sampling biases of temperature T and water vapor q vertical profiles obtained by satellite infrared sounding instruments are highly cloud-state dependent and poorly quantified. The authors describe progress toward a comprehensive T and q climatology derived from the Atmospheric Infrared Sounder (AIRS) suite that is a function of cloud state based on collocated CloudSat observations. The AIRS sampling rates, biases, and center root-mean-square differences (CRMSD) are determined through comparisons of pixel-scale collocated ECMWF model analysis data. The results show that AIRS provides a realistic representation of most meteorological regimes in most geographical regions, including those dominated by high thin cirrus and shallow boundary layer clouds. The mean AIRS observational biases relative to the ECMWF analysis between the surface and 200 hPa are within ±1 K in T and from −1 to +0.5 g kg−1 in q. Biases because of cloud-state-dependent sampling dominate the total biases in the AIRS data and are largest in the presence of deep convective (DC) and nimbostratus (Ns) clouds. Systematic cold and dry biases are found throughout the free troposphere for DC and Ns. Somewhat larger biases are found over land and in the midlatitudes than over the oceans and in the tropics, respectively. Tropical and oceanic regions generally have a smaller CRMSD than the midlatitudes and over land, suggesting agreement of T and q variability between AIRS and ECMWF in these regions. The magnitude of CRMSD is also strongly dependent on cloud type.

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Sun Wong, Tristan S. L’Ecuyer, William S. Olson, Xianan Jiang, and Eric J. Fetzer

Abstract

The authors quantify systematic differences between modern observation- and reanalysis-based estimates of atmospheric heating rates and identify dominant variability modes over tropical oceans. Convergence of heat fluxes between the top of the atmosphere and the surface are calculated over the oceans using satellite-based radiative and sensible heat fluxes and latent heating from precipitation estimates. The convergence is then compared with column-integrated atmospheric heating based on Tropical Rainfall Measuring Mission data as well as the heating calculated using temperatures from the Atmospheric Infrared Sounder and wind fields from the Modern-Era Retrospective Analysis for Research and Applications (MERRA). Corresponding calculations using MERRA and the European Centre for Medium-Range Weather Forecasts Interim Re-Analysis heating rates and heat fluxes are also performed. The geographical patterns of atmospheric heating rates show heating regimes over the intertropical convergence zone and summertime monsoons and cooling regimes over subsidence areas in the subtropical oceans. Compared to observation-based datasets, the reanalyses have larger atmospheric heating rates in heating regimes and smaller cooling rates in cooling regimes. For the averaged heating rates over the oceans in 40°S–40°N, the observation-based datasets have net atmospheric cooling rates (from −15 to −22 W m−2) compared to the reanalyses net warming rates (5.0–5.2 W m−2). This discrepancy implies different pictures of atmospheric heat transport. Wavelet spectra of atmospheric heating rates show distinct maxima of variability in annual, semiannual, and/or intraseasonal time scales. In regimes where deep convection frequently occurs, variability is mainly driven by latent heating. In the subtropical subsidence areas, variability in radiative heating is comparable to that in latent heating.

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Sun Wong, Andrew E. Dessler, Natalie M. Mahowald, Ping Yang, and Qian Feng

Abstract

The role of Saharan dust and dry anomaly in maintaining the temperature inversion in the Saharan air layer (SAL) is investigated. The dust aerosol optical thickness (AOT) in the SAL is inferred from the measurements taken by Aqua Moderate Resolution Imaging Spectroradiometer (MODIS), and the corresponding temperature and specific humidity anomalies are identified using the National Centers for Environmental Prediction (NCEP) data in August–September over the North Atlantic tropical cyclone (TC) main development region (MDR; 10°–20°N, 40°–60°W). The authors also study the SAL simulated in the National Center of Atmospheric Research (NCAR) Community Atmosphere Model, version 3 (CAM3), coupled with dust radiative effect. It is found that higher AOT is associated with warmer and dryer anomalies below 700 hPa, which increases the atmospheric stability. The calculated instantaneous radiative heating anomalies from a radiative transfer model indicate that both the dust and low humidity are essential to maintaining the temperature structure in the SAL against thermal relaxation. At 850 hPa, heating anomalies caused by both the dust and dry anomalies (for AOT > 0.8) are 0.2–0.4 K day−1. The dust heats the atmosphere below 600 hPa, while the dry anomaly cools the atmosphere below 925 hPa, resulting in a peak of heating rate anomaly located at 700–850 hPa. In the eastern Atlantic, dust contributes about 50% of the heating rate anomaly. Westward of 40°W, when the dust content becomes small (AOT < 0.6), the heating rates are more sensitive to the water vapor profile used in the radiative transfer calculation. Retrieving or simulating correct water vapor profiles is essential to the assessment of the SAL heating budgets in regions where the dust content in the SAL is small.

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Sun Wong, Catherine M. Naud, Brian H. Kahn, Longtao Wu, and Eric J. Fetzer

Abstract

Precipitation (from TMPA) and cloud structures (from MODIS) in extratropical cyclones (ETCs) are modulated by phases of large-scale moisture flux convergence (from MERRA-2) in the sectors of ETCs, which are studied in a new coordinate system with directions of both surface warm fronts (WFs) and surface cold fronts (CFs) fixed. The phase of moisture flux convergence is described by moisture dynamical convergence Q cnvg and moisture advection Q advt. Precipitation and occurrence frequencies of deep convective clouds are sensitive to changes in Q cnvg, while moisture tendency is sensitive to changes in Q advt. Increasing Q cnvg and Q advt during the advance of the WF is associated with increasing occurrences of both deep convective and high-level stratiform clouds. A rapid decrease in Q advt with a relatively steady Q cnvg during the advance of the CF is associated with high-level cloud distribution weighting toward deep convective clouds. Behind the CF (cold sector or area with polar air intrusion), the moisture flux is divergent with abundant low- and midlevel clouds. From deepening to decaying stages, the pre-WF and WF sectors experience high-level clouds shifting to more convective and less stratiform because of decreasing Q advt with relatively steady Q cnvg, and the CF experiences shifting from high-level to midlevel clouds. Sectors of moisture flux divergence are less influenced by cyclone evolution. Surface evaporation is the largest in the cold sector and the CF during the deepening stage. Deepening cyclones are more efficient in poleward transport of water vapor.

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Qing Yue, Brian H. Kahn, Eric J. Fetzer, Mathias Schreier, Sun Wong, Xiuhong Chen, and Xianglei Huang

Abstract

The authors present a new method to derive both the broadband and spectral longwave observation-based cloud radiative kernels (CRKs) using cloud radiative forcing (CRF) and cloud fraction (CF) for different cloud types using multisensor A-Train observations and MERRA data collocated on the pixel scale. Both observation-based CRKs and model-based CRKs derived from the Fu–Liou radiative transfer model are shown. Good agreement between observation- and model-derived CRKs is found for optically thick clouds. For optically thin clouds, the observation-based CRKs show a larger radiative sensitivity at TOA to cloud-cover change than model-derived CRKs. Four types of possible uncertainties in the observed CRKs are investigated: 1) uncertainties in Moderate Resolution Imaging Spectroradiometer cloud properties, 2) the contributions of clear-sky changes to the CRF, 3) the assumptions regarding clear-sky thresholds in the observations, and 4) the assumption of a single-layer cloud. The observation-based CRKs show the TOA radiative sensitivity of cloud types to unit cloud fraction change as observed by the A-Train. Therefore, a combination of observation-based CRKs with cloud changes observed by these instruments over time will provide an estimate of the short-term cloud feedback by maintaining consistency between CRKs and cloud responses to climate variability.

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Sun Wong, Eric J. Fetzer, Baijun Tian, Bjorn Lambrigtsen, and Hengchun Ye

Abstract

The possibility of using remote sensing retrievals to estimate apparent water vapor sinks and heat sources is explored. The apparent water vapor sinks and heat sources are estimated from a combination of remote sensing, specific humidity, and temperature from the Atmospheric Infrared Sounder/Advanced Microwave Sounding Unit (AIRS) and wind fields from the National Aeronautics and Space Administration (NASA)’s Goddard Space Flight Center (GSFC)’s Modern Era Retrospective-Analysis for Research and Applications (MERRA). The intraseasonal oscillation (ISO) of the Indian summer monsoon is used as a test bed to evaluate the apparent water vapor sink and heat source. The ISO-related northward movement of the column-integrated apparent water vapor sink matches that of precipitation observed by the Tropical Rainfall Measuring Mission (TRMM) minus the MERRA surface evaporation, although the amplitude of the variation is underestimated by 50%. The diagnosed water vapor and heat budgets associated with convective events during various phases of the ISO agree with the moisture–convection feedback mechanism. The apparent heat source moves northward coherently with the apparent water vapor sink associated with the deep convective activity, which is consistent with the northward migration of the precipitation anomaly. The horizontal advection of water vapor and dynamical warming are strong north of the convective area, causing the northward movement of the convection by the destabilization of the atmosphere. The spatial distribution of the apparent heat source anomalies associated with different phases of the ISO is consistent with that of the diabatic heating anomalies from the trained heating (TRAIN Q1) dataset. Further diagnostics of the TRAIN Q1 heating anomalies indicate that the ISO in the apparent heat source is dominated by a variation in latent heating associated with the precipitation.

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Chen-Geng Ma, Edmund K. M. Chang, Sun Wong, Rui Zhang, Minghua Zhang, and Anthony Del Genio

Abstract

Previous studies have shown that variations in extratropical cyclone activity significantly affect the frequency of extreme precipitation events over the Ohio Valley and northwestern United States. In this study, we examine the similarities and differences between the dynamics governing these events in these two regions. In the Ohio Valley, extreme precipitation events are associated with midlatitude synoptic-scale convergence northeast of cyclones and a southwestward oriented ridge near the Atlantic coast that drives strong water vapor transport from the Gulf of Mexico into the Ohio Valley. In the northwestern United States, extreme precipitation events are associated with a cyclonic and anticyclonic circulation pair aligned northwest to southeast, which together drive a long and strong moisture transport corridor from the lower latitude of the central Pacific Ocean toward the northwestern United States. Moisture budget analysis shows that moisture convergence due to dynamical convergence dominates in the Ohio Valley, whereas moisture advection dominates over the Pacific Northwest. Differences between the cases in the same region are examined by an empirical orthogonal function (EOF) analysis conducted on the vertically integrated moisture flux. Different EOFs highlight shifts in spatial location, orientation, and intensity of the moisture flux but demonstrate consistent roles of dynamics in the two regions. Composites based on these EOFs highlight the range of likely synoptic scenarios that can give rise to precipitation extremes over these two regions.

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