Ren, D., Fu, R., Dickinson, R.E., Leslie, L.M. & Wang, X. Aviation Impacts on Fuel Efficiency of a Future More Viscous Atmosphere. Bulletin of the American Meteorological Society (2020).Abstract
Aircraft cruising near the tropopause currently benefit from the highest thermal efficiency and the least viscous (sticky) air, within the lowest 50 km of the Earth’s atmosphere. Both advantages wane in a warming climate, because atmospheric dynamic viscosity increases with temperature, in synergy with the simultaneous engine efficiency reduction. Here, skin friction drag, the dominant term for extra aviation fuel consumption in a future warming climate, is quantified by 34 climate models under a strong emissions scenario. Since 1950, the viscosity increase at cruising altitudes (∼200 hPa) reaches ∼1.5% per century, corresponding to a total drag increment of ∼0.22% per century for commercial aircraft. Meridional gradients and regional disparities exist, with low-mid latitudes experiencing greater increases in skin friction drag. The North Atlantic Corridor (NAC) is moderately affected, but its high traffic volume generates additional fuel cost of ∼3.8×107 gallons annually by 2100, compared to 2010. Globally, a normal year after 2100 would consume an extra ∼4×106 barrels per year. Inter model spread is <5% of the ensemble mean, due to high inter-climate model consensus for warming trends at cruising altitudes in the tropics and subtropics. Because temperature is a well simulated parameter in the IPCC archive, with only a moderate inter-model spread, the conclusions drawn here are statistically robust. Notably, additional fuel costs are likely from the increased vertical shear and related turbulence at NAC cruising altitudes. Increased flight log availability is required to confirm this apparent increasing turbulence trend.
Zhuang, Y., Fu, R. & Wang, H. Large-Scale Atmospheric Circulation Patterns Associated with US Great Plains Warm Season Droughts Revealed by Self-Organizing Maps. Journal of Geophysical Research - Atmospheres 125, 5, (2020).Abstract
This study uses a multivariate self‐organizing map approach to diagnose precipitation anomalies over the United States' Great Plains during the warm season (April–August) and the associated anomalous large‐scale atmospheric patterns, as represented by standardized anomalies of 500 hPa geopotential (Z500′), integrated vapor transport (IVT′), and convective inhibition index (CINi′). Circulation patterns favoring dryness identified by the method are generally consistent with those shown in previous studies, but this study provides a more comprehensive and probabilistic characterization of those that favor drought over the Southern Great Plains (SGP) and the Central Great Plains (CGP) and their temporal evolutions. Six circulation types that are associated with warm season rainfall variability over the Great Plains are identified. The SGP droughts are attributable to more frequent and persistent northern low‐southern high as well as dominant high circulation types and are connected to larger negative CINi′. In contrast, CGP droughts are attributable to more frequent and persistent western low‐eastern high, or northern high‐southern low, or dominant high patterns, and are linked to a larger negative IVT′, but not larger CINi′. Thus, these results suggest that land surface dryness and a stable atmospheric boundary layer may play a more important role over the SGP than reduced moisture transport in warm season droughts, but reduced moisture transport may play a more important role than thermodynamic stability in droughts over the CGP.
Chakraborty, S., Jiang, J., Su, H. & Fu, R. Deep Convective Evolution from Shallow Clouds over the Amazon and Congo Rainforests. Journal of Geophysical Research - Atmospheres 125, 1, (2020).Abstract
Using satellite measurements from A‐Train constellation and Global PrecipitationMeasurement mission, we investigate the relationships between the afternoon time shallow convectivetop height (CTHafternoon) and the evening time deep convective storm top height (CTHevening) and rain rate(RRevening) over the Amazon and Congo regions. We use CloudSat cloud type stratus and stratocumulus asthe shallow afternoon clouds. Our results indicate that the afternoon shallow clouds over the Congoregion are associated with suppressed and weakened evening time deep convection, whereas shallow cloudsover the Amazon region are associated with the growth of the evening time deep convection. Over the Congoregion, wefind that as CTHafternoonincreases, shallow convective rain rate in the afternoon (RRafternoon)increases. As a result, the evening time convective available potential energy (CAPE) as well as freetropospheric humidity (RH700‐300) decrease. Consequently, condensation occurring inside deep convectionreduces and CTHeveningas well as RReveningdecrease over there. Over the Amazon region, however,RRafternoondoes not vary significantly with CTHafternoon. As CTHafternoonincreases, CAPE, RH700‐300, andcondensation occurring inside deep convection increase in the evening. As a result, deep convectiveCTHeveningand RReveningincrease with CTHafternoonover the Amazon basin. These dissimilarities in theambient condition drive the shallow to deep convective evolution differently over these two rainforests.On the other hand, shallow clouds that remain shallow in the evening are associated with less CAPE andRH700‐300,RRafternoon,and RRevening. Although CAPE and RH700‐300promote deep convection to a heightcloud top height, high vertical wind shear inhibits deep convection.
Grimm, A.M., et al. South and North American Monsoons: characteristics, life cycle, variability, modelling and prediction. The Multi-scale Global Monsoon System (2020). Publisher's Version
Fernando, D.N., Chakraborty, S., Fu, R. & Mace, R.E. A process-based statistical seasonal prediction of May–July rainfall anomalies over Texas and the Southern Great Plains of the United States. Climate Services (2019).Abstract

With the aim of providing actionable drought early warning information that water managers and reservoir oper- ators in Texas could use to implement drought contingency triggers on water supply sources, we have developed a statistical seasonal prediction system using a canonical correlation analysis prediction model to predict rainfall from May through July (MJJ), the main rainfall season over much of Texas and the Southern Great Plains. The statistical model is trained with data between 1982 and 2005 using standardized anomalous geopotential height at 500 hPa, convective inhibition energy, and soil moisture content in April as the predictors to generate tercile categorical forecasts of MJJ rainfall. Based on commonly used forecast skill metrics, this statistical prediction sys- tem provides 20–60% higher skill than that obtained from dynamical seasonal forecasts, and the exceeds skill due to the persistence of MJJ rainfall anomalies over Texas, western Louisiana, Oklahoma and the Southern Kansas. 2011 hindcast shows that below-normal MJJ rainfall anomalies comparable to those observed over most of the region. The forecasts for 2014 captured the above-normal MJJ rainfall anomalies as observed in that year. The forecasts since 2014 have shown acceptable prediction skills at one-to-three months’ lead-time. We have also ex- tended the lead-time to generate probabilistic MJJ rainfall forecasts from January through March using a hybrid dynamical-statistical forecast scheme. The predictions have been used by the Texas Water Development Board to inform the Texas State Drought Preparedness Council and to support the implementation of drought contingency triggers for water supply sources by stakeholders, such as river authorities.

Costa, M.H., et al. Climate risks to Amazon agriculture raise awareness to conserve local forests. Frontier of Ecology and the environment 17, 10, 584-590 (2019).Abstract
In southern Amazonia, more than half of all cropland is devoted to the production of two rainfed crops per year, an agricultural practice known as “double cropping” (DC). Climate change, including feedbacks between changes in land use and the local cli-mate, is shortening the extent of the historical rainy season in southern Amazonia, increasing the risk of future detrimental envi-ronmental conditions, and posing a threat to the intensive DC agriculture that is currently practiced in that region, with potential negative consequences at regional, national, and even global scales. We argue that the conservation of undeveloped forests and savannas in southern Amazonia is supported by socioeconomic justifications and is in the best interests of agribusiness, local gov-ernments, and the public.
Shi, M.J., et al. The 2005 Amazon Drought Legacy Effect Delayed the 2006 Wet Season Onset. Geophysical Research Letters banner 46, 9082-9090 (2019).Abstract
While the long‐term drought effect on tropical forests has been observed in ground‐based and remote sensing measurements, the feedback of reduced forest biomass on subsequent rainfall is not well understood. We evaluate the impact of slow forest recovery after the 2005 Amazonian drought on local evapotranspiration (ET) and wet season onset (WSO) using remotely sensed precipitation, deuterium retrievals, reanalysis data, and a new ET product. A comparison to the 2009 rainy season, which exhibits similar large‐scale moisture flux convergence, shows that 2006 experienced a 25% ET reduction and 20 days of postponed WSO in the dry‐to‐wet transition. Our results imply that ET reduction due to drought‐driven legacy effect on the Amazon rainforest could be a crucial factor triggering WSO delay in the transitional season following drought events.
Gentine, P., et al. Land-atmospheric Interactions in the tropics - a review. Hydrology and Earth System Sciences 23, 4171–4197 (2019).Abstract
The continental tropics play a leading role in the terrestrial water and carbon cycles. Land–atmosphere interactions are integral in the regulation of surface energy, water and carbon fluxes across multiple spatial and temporal scales over tropical continents. We review here some of the important characteristics of tropical continental climates and how land–atmosphere interactions regulate them. Along with a wide range of climates, the tropics manifest a diverse array of land–atmosphere interactions. Broadly speaking, in tropical rainforests, light and energy are typically more limiting than precipitation and water supply for photosynthesis and evapotranspiration; whereas in savanna and semi-arid regions water is the critical regulator of surface fluxes and land–atmosphere interactions. We discuss the impact of the land surface, how it affects shallow clouds and how these clouds can feedback to the surface by modulating surface radiation. Some results from recent research suggest that shallow clouds may be especially critical to land–atmosphere interactions as these regulate the energy budget and moisture transport to the lower troposphere, which in turn affects deep convection. On the other hand, the impact of land surface conditions on deep convection appear to occur over larger, non-local, scales and might be critically affected by transitional regions between the climatologically dry and wet tropics.
Leite-Filho, A.T., Costa, M.H. & Fu, R. The southern Amazon rainy season: the role of deforestation and its interactions with large-scale mechanisms. International J. of Climatology (2019). Publisher's VersionAbstract

Past studies presented evidence that deforestation may affect the precipitation seasonality in southern Amazon. This study uses daily rainfall data from TRMM 3B42 product and a recent yearly 1‐km land use dataset to evaluate the quantitative effects of deforestation on the onset, demise and length of the rainy season in Southern Amazon for a period of 15 years (1998‐2012). Additionally, we use the Niño4 index, zonal wind data and deforestation data to explain and predict the interannual variability of the onset of the rainy season. During this period, onset has delayed ~0.38±0.05 days per year (5.7±0.75 days in 15 years), demise has advanced 1.34±0.76 days per year (20±11.4 days in 15 years) and the rainy season has shortened by 1.81±0.97 days per year (27±14.5 days in 15 years). Onset, demise and length also present meridional and zonal gradients linked to large‐scale climate mechanisms. After removing the effects related to geographical position and year, we verified a relationship between onset, demise and length and deforestation: Onset delays ~0.4±0.12 day, demise advances ~1.0±0.22 day and length decreases ~0.9±0.34 day per each 10% deforestation increase relative to existing forested area. We also present empirical evidence of the interaction between large‐scale and local scale processes, with interannual variation of the onset in the region explained by Niño4 sea surface temperature anomalies, Southern Hemisphere subtropical jet position, deforestation and their interactions (r2 = 69%, p < 0.001, MAE = 2.7 days).

Erfanian, A. & Fu, R. The role of spring dry zonal advection in summer drought onset over the US Great Plains. Atmospheric Chemistry and Physics 19, 15199–15216 (2019).Abstract
This study addresses the role of the atmospheric moisture budget in determining the onset and development of summer droughts over the North American Great Plains (GP) using two state-of-the-art reanalysis datasets. We identified zonal moisture advection as the main cause of severe tropospheric drying during the extreme droughts in the southern GP in 2011 and northern GP in 2012. For both events, the eastward advection of anomalously dry and warm air in the free troposphere in spring set the stage for summer drought. This led to a sharp drop in relative humidity above the boundary layer, enhancing dry entrainment and suppressing deep convection. Further breakdown of the zonal advection into dynamic (caused by circulation anomalies) and thermodynamic (caused by moisture anomalies) contributions reveals dominance of thermodynamic advection in the tropospheric drying observed during the onset of both 2011 and 2012 droughts. The dependence of thermodynamic advection on the moisture gradient links springtime precipitation in the Rockies and southwestern US, the source region of the anomalous dry advection, to the GP summer precipitation (with correlations > 0.4 using gauge-based data). Identifying this previously overlooked precursor of the GP summer droughts improves our predictive understanding of drought onset mechanisms over the region.
Chakraborty, S., Fu, R., Rosenfeld, D. & Massie, S.T. The influence of aerosols and meteorological conditions on the total rain volume. Geophysical Research Letter 45, 23, 13009-13106 (2018).Abstract
This study provides an observational assessment of the variations of the total rain volume (TRV) with aerosols through the entire lifetime of mesoscale convective systems (MCSs) over tropics. Using 70,000 MCSs' samples, we show that TRV increases with aerosols from clean to moderately heavy polluted conditions (aerosol optical depth [AOD] similar to 0.0-0.4). TRV decreases when AOD exceeds 0.5. The TRV change with AOD is strongest under favorable meteorological conditions, such as high total precipitable water (45-75 kg/m(2)), high convective available potential energy (1,200-2,400 J/kg), and intermediate vertical wind shear (9-21 x 10(-4)/s). TRV of MCSs increases from 2 to 4 km(3) (rain depth similar to 20-40 mm) when AOD < 0.15 or > 0.5, to more than 12 km(3) (similar to 120 mm) when 0.2 < AOD < 0.4 under above the mentioned optimal meteorological conditions. The basic response of TRV to aerosol concentrations is similar under all the meteorological conditions and during all stages of the MCS lifecycle.
Plain language summary Mesoscale convective systems (MCSs) contribute to the largest fraction of global rainfall and are often responsible for devastating flood events. It has long been hypothesized that aerosols can enhance rainfall of MCSs by suppressing rainfall during the early stage of the convection, enabling more cloud droplets to rise to higher altitude and so freeze. Freezing releases more latent heat, which drives strong rising motion and so enables formation of large hydrometeors for heavy rainfall. Thus, it is central to evaluate rainfall changes with aerosols through the entire lifetime of the MCCs. This work provides a first observational assessment of the variation of the total rain generated by MCSs through their lifetime with ambient aerosols, under various ambient meteorological conditions over the global tropical continents. Our results show that aerosols have a strong invigoration effect on MCSs' total rainfall volume. Total rainfall volume increases as AOD increases up to 0.4 and decreases as AOD increases beyond 0.5. Such effects are similar throughout different phases of their convective lifecycle and under various meteorological conditions.
Ren, D., et al. Impacts of climate warming on maximum aviation payloads. Climate Dynamics (2018).Abstract
The increasing importance of aviation activities in modern life coincides with a steady warming climate. However, the effect of climate warming on maximum aircraft carrying capacity or payload has been unclear. Here we clarify this issue using primary atmospheric parameters from 27 fully coupled climate models from the Coupled Model Inter-comparison Project 5 (CMIP5) archive, utilizing the direct proportionality of near-surface air density (NSAD) to maximum take-off total weight (MTOW). Historical (twentieth century) runs of these climate models showed high credibility in reproducing the reanalysis period (1950–2015) of NSAD. In particular, the model simulated trends in NSAD are highly aligned with the reanalysis values. This reduction in NSAD is a first order global signal, just as is the warming itself, that continues into the future. To examine the statistical significance of the density reduction, a t-test was performed for two 20-year periods 75 years apart (2080–2100 vs. 2005–2025), using the Representative Concentration Pathways (RCP) 8.5 emission scenario of the Intergovernmental Panel on Climate Change (IPCC). Most continental areas easily passed the test at a P-value of 0.05. These future changes of NSAD will likely have significant economic impacts on the aviation industry. For these two 20-year periods that we examined, the most extreme changes are in the Northern hemisphere in high latitudes, i.e., a 5% decrease in MTOW, or ~8.5–19% (aircraft-dependent) reduction in payload. The global average change is about 1%. For the busy North Atlantic Corridor (NAC), the reduction in MTOW is generally greater than 1% and that of payload several times larger.
Chakraborty, S., Schiro, K.A., Fu, R. & Neelin, J.D. On the role of aerosols, humidity, and vertical wind shear in the transition of shallow to deep convection at the Green Ocean Amazon 2014/5 site. Atmospheric Chemistry and Physics 18, 11135-11148 (2018).Abstract

The preconditioning of the atmosphere for a shallow-to-deep convective transition during the dry-to-wet season transition period (August–November) is investigated using Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) GoAmazon2014/5 campaign data from March 2014 to November 2015 in Manacapuru, Brazil. In comparison to conditions observed prior to shallow convection, anomalously high humidity in the free troposphere and boundary layer is observed prior to a shallow-to-deep convection transition. An entraining plume model, which captures this leading dependence on lower tropospheric moisture, is employed to study indirect thermodynamic effects associated with vertical wind shear (VWS) and cloud condensation nuclei (CCN) concentration on preconvective conditions. The shallow-to-deep convective transition primarily depends on humidity, especially that from the free troposphere, which tends to increase plume buoyancy. Conditions preceding deep convection are associated with high relative humidity, and low-to-moderate CCN concentration (less than the 67th percentile, 1274 cm−3 ). VWS, however, shows little relation to moisture and plume buoyancy. Buoyancy estimates suggest that the latent heat release due to freezing is important to deep convective growth under all conditions analyzed, consistent with potential pathways for aerosol effects, even in the presence of a strong entrainment. Shallow-only convective growth, however, shows an association with a strong (weak) low (deep) level VWS and with higher CCN concentration.

Zhuang, Y., Fu, R. & Wang, H. How do environmental conditions influence vertical buoyancy structure and shallow-to-deep convection transition across different climate regimes?. Journal of the Atmospheric Sciences 75, 6, 1909-1932 (2018).Abstract

We developed an entraining parcel approach that partitions parcel buoyancy into contributions from different processes, e.g. adiabatic cooling, condensation, freezing, and entrainment. Applying this method to research quality radiosonde profiles provided by the Atmospheric Radiation Program (ARM) at six sites, we evaluated how atmospheric thermodynamic conditions and entrainment influence various physical processes that determine the vertical buoyancy structure across different climate regimes as represented by these sites. The differences of morning buoyancy profiles between the deep convection/transition cases (DC) and shallow convection/non-transition cases (SC) were used to assess pre-conditions important for shallow-to-deep convection transition. Our results show that for continental sites such as the U.S. Southern Great Plains (SGP) and the West-Central Africa, surface condition alone is enough to account for the buoyancy difference between DC and SC cases, although entrainment further enhances the buoyancy difference at SGP. For oceanic sites in the Tropical West Pacific, humidity dilution in the lower-to-mid free troposphere (~1-6km) and temperature mixing in the mid-to-upper troposphere (>4km) have the most important influences on the buoyancy difference between DC and SC cases. For the humid Central Amazon region, entrainment in both the boundary layer and the lower free troposphere (~0-4km) have significant contributions to the buoyancy difference; the upper tropospheric influence seems unimportant. In addition, the integral of the condensation term, which represents the parcel's ability to transform available water vapor into heat through condensation, provides a better discrimination between DC and SC cases than the integral of buoyancy or the Convective Available Potential Energy (CAPE).

Zhao, B., et al. Impact of aerosols on ice crystal size. Atmospheric Chemistry and Physics 18, 2, 1065-1078 (2018).Abstract
The interactions between aerosols and ice clouds represent one of the largest uncertainties in global radiative forcing from pre-industrial time to the present. In particular, the impact of aerosols on ice crystal effective radius (Rei), which is a key parameter determining ice clouds’ net radiative effect, is highly uncertain due to limited and conflicting observational evidence. Here we investigate the effects of aerosols on Rei under different meteorological conditions using 9-year satellite observations. We find that the responses of Rei to aerosol loadings are modulated by water vapor amount in conjunction with several other meteorological parameters. While there is a significant negative correlation between Rei and aerosol loading in moist conditions, consistent with the “Twomey effect” for liquid clouds, a strong positive correlation between the two occurs in dry conditions. Simulations based on a cloud parcel model suggest that water vapor modulates the relative importance of different ice nucleation modes, leading to the opposite aerosol impacts between moist and dry conditions. When ice clouds are decomposed into those generated from deep convection and formed in situ, the water vapor modulation remains in effect for both ice cloud types, although the sensitivities of Rei to aerosols differ noticeably between them due to distinct formation mechanisms. The water vapor modulation can largely explain the difference in the responses of Rei to aerosol loadings in various seasons. A proper representation of the water vapor modulation is essential for an accurate estimate of aerosol–cloud radiative forcing produced by ice clouds.
Koster, R.D., et al. Hydroclimatic variability and predictability: a survey of recent research. Hydrology and Earth System Sciences 21, 3777-3798 (2017).Abstract
Recent research in large-scale hydroclimatic variability is surveyed, focusing on five topics: (i) variability in general, (ii) droughts, (iii) floods, (iv) land–atmosphere coupling, and (v) hydroclimatic prediction. Each surveyed topic is supplemented by illustrative examples of recent research, as presented at a 2016 symposium honoring the career of Professor Eric Wood. Taken together, the recent literature and the illustrative examples clearly show that current research into hydroclimatic variability is strong, vibrant, and multifaceted.
Bowerman, A.R., et al. An influence of extreme southern hemispheric cold surges on the North Atlantic Subtropical High through a shallow atmospheric circulation. Journal of Geophysical Research: Atmospheres 122, (2017).Abstract
Previous studies have attributed interhemisphere influences of the atmosphere to the latitudinal propagation of planetary waves crossing the equator, to the triggering of equatorial Kelvin waves, or to monsoonal circulation. Over the American‐Atlantic sector, such cross‐equatorial influences rarely occur during boreal summer due to unfavorable atmospheric conditions. We have observed that an alternative mechanism provides an interhemisphere influence. When episodes of extreme cold surges and upper tropospheric westerly winds occur concurrently over southern hemisphere Amazonia, cold surges from extratropical South America can penetrate deep into southern Amazonia. Although they do not appear to influence upper tropospheric circulation of the northern hemisphere, extremely strong southerly cross‐equatorial advection (>2σ standard deviations, or 2) of cold and dense air in the lower troposphere can reach as least 10°N. Such cold advection increases the northward cross‐equatorial pressure gradient in the lower to middle troposphere, thus shallow northerly return flow below 500 hPa. This return flow and the strong lower tropospheric southerly cross‐equatorial flow form an anomalous shallow meridional circulation spanning from southern Amazonia to the subtropical North Atlantic, with increased geopotential height anomalies exceeding +1σ to at least 18°N. It projects onto the southern edge of the North Atlantic Subtropical High (NASH), increasing its pressure and leading to equatorward expansion of NASH's southern boundary. These anomalies enhance the NASH, leading to its equatorward expansion. These extreme cold surges can potentially improving the predictability of weather patterns of the tropical and subtropical Atlantic, including the variability of the NASH's southern edge.