ALBERT

Description: Continued advancement in the realm of tropical cyclone (TC) forecasting requires a more accurate depiction of these storms at model initialization. This study examines the impact of precipitation assimilation on the representation of TCs in the North American Regional Reanalysis (NARR) before and after the 2004 introduction of precipitation assimilation over ocean in the vicinity of TCs. The probability distribution function of rainfall rates indicates that light (heavy) precipitation was overforecast (underforecast) in the early time period. Since the precipitation assimilation is applied through an adjustment to the latent heating distribution, the data assimilation system in the later time period initializes a low-level moisture and heating profile that is more conducive to the initiation of deep convection and the generation of precipitation. Consequently, the deep convection and enhanced latent heat release lead to a more robust warm core temperature perturbation and a better developed secondary circulation, which supplies the TC with larger quantities of moisture from the large-scale environment. Furthermore, the evolution of TC size, which was objectively estimated though the radius of outermost closed isobar (ROCI), is significantly more skillful ( p 〈0.05) in post-2003 storms. Based on this study, precipitation assimilation leads to a better analysis of temperature, winds, and moisture in the vicinity of TCs, resulting in improved representations of the water budget and storm lifecycle. Therefore, we conclude that efforts towards the development of precipitation assimilation techniques from radar and satellite datasets will be valuable toward the construction of improved TC forecasting tools with more authentic TC representation.

Description: Six years of aerosol size distribution measurements between 20 and 600 nm diameters and total aerosol concentration above 10 nm from March 2008 to February 2014 at the high-alpine site Jungfraujoch are presented. The size distribution was found to be typically bimodal with mode diameters and widths relatively stable throughout the year and the observation period. New particle formation was observed on 14.5% of all days without a seasonal preference. Particles typically grew only into the Aitken mode and did not reach cloud condensation nuclei (CCN) sizes on the time scale of several days. Growth of pre-existing particles in the Aitken mode, on average, contributed very few CCN. We concluded that the dominant fraction of CCN at Jungfraujoch originated in the boundary layer. A number of approaches were used to distinguish free tropospheric (FT) conditions and episodes with planetary boundary layer (PBL) influence. In the absence of PBL injections, the concentration of particles larger than 90 nm ( N 90 , roughly corresponding to the CCN concentration) reached a value ~40 cm −3 while PBL influence caused N 90 concentrations of several hundred or even 1000 cm −3 . Comparing three criteria for free tropospheric conditions, we found FT prevalence for 39% of the time with over 60% during winter and below 20% during summer. It is noteworthy that a simple criterion based on standard trace gas measurements appeared to outperform alternative approaches.

Description: Tropical convection has been observed to contain three cloud modes, the middle of which is cumulus congestus clouds. Congestus clouds act to moisten the tropical atmosphere, may be mixed-phase, and on occasion surpass the freezing level inversion from where they may develop into deeper convection. This study investigates the impacts of enhanced aerosol concentrations on the growth of congestus clouds produced in idealized cloud-resolving model simulations run under a state of radiative convective equilibrium (RCE). High resolution, long duration simulations were completed using the Regional Atmospheric Modeling System (RAMS). Aerosol concentrations between 2 and 4 km AGL were varied from clean to polluted conditions in order to represent the advection of Saharan dust over the Atlantic Ocean. The congestus populations within each aerosol simulation are statistically analyzed using ten days of model output after the simulation reaches RCE. Results indicate that congestus in more polluted conditions produce greater amounts of cloud water and ice mass, enhanced updraft strengths and an increase in the number of congestus cloud tops that extend above the freezing level. Enhanced vapor depositional growth on the populations of more numerous, smaller cloud droplets in the polluted conditions, and the subsequent increase in latent heat release in the warm phase regions of the cloud, are found to be important factors in convective invigoration of these cloud systems. Aerosol feedbacks associated with cold pools and condensate loading also influence the updraft strength, and act in opposition to the warm phase invigoration processes.

Description: With water vapor and clouds expected to effect significant feedbacks on climate, moisture transport through convective processes has important implications for future temperature change. The precipitation efficiency—the ratio of the rates at which precipitation and condensation form ( e  =  P / C )—is useful for characterizing how much boundary layer moisture recycles through precipitation versus mixes into the free troposphere through cloud detrainment. Yet, it is a difficult metric to constrain with traditional observational techniques. This analysis characterizes the precipitation efficiency of convection near the Big Island of Hawaii, USA, using a novel tracer: isotope ratios in water vapor. The synoptic circulation patterns associated with high and low precipitation efficiency are identified, and the importance of large-scale dynamics and local convective processes in regulating vertical distributions of atmospheric constituents important for climate is evaluated. The results suggest high e days are correlated with plume-like transport originating from the relatively clean tropics, while low e days are associated with westerly transport, generated by a branching of the jet stream. Differences in transport pathway clearly modify background concentrations of water vapor and other trace gases measured at Mauna Loa Observatory; however, local convective processes appear to regulate aerosols there. Indeed, differences between observed and simulated diurnal cycles of particle number concentration indicate precipitation scavenges aerosols and possibly facilitates new particle formation when e is high. As measurements of isotope ratios in water vapor expand across the subtropics, the techniques presented here can further our understanding of how synoptic weather, precipitation processes, and climate feedbacks interrelate.

Description: 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 (YOTC). 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 vs. non-precipitating 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 954–930 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.

Description: Solar influences on spatial patterns of Eurasian winter climate and possible mechanisms are investigated based on a multiple linear regression method, multi-sources observational and reanalysis data. Robust and significant solar signals are detected in Eurasian surface air temperature (SAT) and strong solar activity evidently warms most area of the continent. The spatial pattern of sea level pressure (SLP) responses to solar activity is similar but not identical to that of the North Atlantic Oscillation (NAO). Compared to the NAO, geographic distribution of solar-induced SLP anomalies shifts eastward, with significantly enhanced influences over northern Eurasia. Relatively weaker solar signals were also found in mid-to-upper troposphere. The spatial pattern of 500 hPa geopotential anomalies resembles a negative Scandinavia Teleconnection pattern and the 200 hPa subtropical jet is weakened, while zonal wind at high latitudes is enhanced due to strong solar activity. The anomalous zonal circulations can be attributed to the “top-down” mechanism. During high solar activity winters, an enhanced stratospheric zonal wind anomaly propagates downward, causing zonal wind anomalies in the troposphere. However, the “bottom-up” mechanisms may provide more reasonable explanations of the distinct solar influences on Eurasian climate. Solar-induced strong warm advection in lower atmosphere tends to increase SAT but decrease SLP, resulting in enhanced solar influences over northern Eurasia. Meanwhile, change in the land-ocean thermal contrast (LOTC) could also amplify the circulation anomaly. Inhomogeneous surface heating caused by anomalous solar activity modifies LOTC, which probably enhances the solar-induced circulation patterns. Such a positive feedback may potentially strengthen the solar influences.

Description: This study reports the chemical composition of particles present along Greenland's North Greenland Eemian Ice Drilling (NEEM) ice core, back to 110,000 years before present. Insoluble and soluble particles larger than 0.45 µm were extracted from the ice core by ice sublimation and their chemical composition was analyzed using scanning electron microscope and energy dispersive X-ray spectroscopy and micro-Raman spectroscopy. We show that the dominant insoluble components are silicates, whereas NaCl, Na 2 SO 4 , CaSO 4 , and CaCO 3 represent major soluble salts. For the first time, particles of CaMg(CO 3 ) 2 and Ca(NO 3 ) 2 •4H 2 O are identified in a Greenland ice core. The chemical speciation of salts varies with past climatic conditions. Whereas the fraction of Na-salts (NaCl + Na 2 SO 4 ) exceeds that of Ca-salts (CaSO 4 + CaCO 3 ) during the Holocene (0.6–11.7 kyr BP), the two fractions are similar during the Bølling-Allerød period (12.9–14.6 kyr BP). During cold climate such as over the Younger Dryas (12.0–12.6 kyr BP) and the Last Glacial Maximum (15.0–26.9 kyr BP), the fraction of Ca-salts exceeds that of Na-salts, showing that the most abundant ion generally controls the salt budget in each period. High-resolution analyses reveal changing particle compositions: those in Holocene ice show seasonal changes, and those in LGM ice show a difference between cloudy bands and clear layers, which again can be largely explained by the availability of ionic components in the atmospheric aerosol body of air masses reaching Greenland.

Description: Characterizing black carbon (BC) concentrations in the seasonal snowpack is of interest because BC deposition on snow can reduce albedo and accelerate melt. In Washington State, USA snowmelt from the seasonal snowpack provides an important source of water resources, but minimal work has been done characterizing BC concentrations in snow in this region. BC concentrations in snow were monitored over two winters (2012 and 2013) at Tronsen Meadow, located near Blewett Pass in the eastern Cascade Mountains in Central Washington, to characterize spatial and temporal variations in BC concentrations, and the processes affecting BC concentrations in the snowpack. BC concentrations were measured using a Single Particle Soot Photometer (SP2). Snowpit BC concentrations at spatial scales ranging from cm to 100 m scales were fairly homogenous during the accumulation season, with greater spatial variability during the melt season due to variable melt patterns. BC concentrations in snow increased in late winter-spring due to an increase in atmospheric BC concentrations and trapping of BC on the snow surface during melt. However, during a period of intense melt in 2013 BC concentrations decreased, likely caused by melt water scavenging. In summer 2012 the Table Mountain forest fire burned adjacent to the study site, and BC concentrations in the snowpack in 2013 were far higher than in previous years, with charred trees post-fire the likely source of the elevated BC.

Description: The interactions between idealized binary tropical cyclones (TCs) on f - and β -planes with different separation distance and thermodynamic soundings obtained from the NCEP/NCAR reanalysis data averaged over the western North Pacific (WNP) are investigated through ensemble three-dimensional numerical simulations with a horizontal resolution of 10 km in a single domain. In the simulations on the f -plane, two TCs show mutual cyclonic rotations with symmetric structures. Two TCs with thermodynamic profiles of larger convective available potential energy (CAPE) and maximum potential intensity (MPI) show greater interaction than those with a smaller CAPE and MPI due to the stronger tangential velocity near the TC center. In the simulations on the β -plane, the two TCs do not merge, because the beta effect prevents the attraction of the two TCs by generating asymmetric motions of the TC with northwestward forcing. The relative strengths of the two TCs change with time and depend on the low-level inflow influenced by the Coriolis parameter. Similar to the results on the f -plane, the two TCs only merge with the thermodynamic soundings of large CAPE and MPI.

Description: The tropical rain belt is a narrow band of clouds near the equator, where the most intense rainfall on the planet occurs. On seasonal timescales, the rain moves across the equator following the sun, resulting in wet and dry seasons in the tropics. The position of the tropical rain belt also varies on longer time scales. Through the latter half of the 20th century, for example, shifts in tropical rainfall have been associated with severe droughts, including the African Sahel and Amazon droughts. Here, I show that climate models project a northward migration of the tropical rain belt through the 21st century, with future anthropogenic aerosol reductions driving the bulk of the shift. Models that include both aerosol indirect effects yield significantly larger northward shifts than models that lack aerosol indirect effects. Moreover, the rate of the shift corresponds to the rate of the decrease of anthropogenic aerosol emissions across different time periods and future emission scenarios. This response is consistent with relative warming of the Northern Hemisphere, a decrease in northward cross-equatorial moist static energy transport, and a northward shift of the Hadley circulation, including the tropical rain belt. The shift is relatively weak in the Atlantic sector, consistent with both a smaller decrease in aerosol emissions and a larger reduction in northward cross equatorial ocean heat flux. Although aerosol effects remain uncertain, I conclude that future reductions in anthropogenic aerosol emissions may be the dominant driver of a 21st century northward shift of the tropical rain belt.