Browsing by Subject "AIRCRAFT"

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  • Zona, Donatella; Gioli, Beniamino; Commane, Roisin; Lindaas, Jakob; Wofsy, Steven C.; Miller, Charles E.; Dinardo, Steven J.; Dengel, Sigrid; Sweeney, Colm; Karion, Anna; Chang, Rachel Y. -W.; Henderson, John M.; Murphy, Patrick C.; Goodrich, Jordan P.; Moreaux, Virginie; Liljedahl, Anna; Watts, Jennifer D.; Kimball, John S.; Lipson, David A.; Oechel, Walter C. (2016)
    Arctic terrestrial ecosystems are major global sources of methane (CH4); hence, it is important to understand the seasonal and climatic controls on CH4 emissions from these systems. Here, we report year-round CH4 emissions from Alaskan Arctic tundra eddy flux sites and regional fluxes derived from aircraft data. We find that emissions during the cold season (September to May) account for >= 50% of the annual CH4 flux, with the highest emissions from noninundated upland tundra. A major fraction of cold season emissions occur during the "zero curtain" period, when subsurface soil temperatures are poised near 0 degrees C. The zero curtain may persist longer than the growing season, and CH4 emissions are enhanced when the duration is extended by a deep thawed layer as can occur with thick snow cover. Regional scale fluxes of CH4 derived from aircraft data demonstrate the large spatial extent of late season CH4 emissions. Scaled to the circumpolar Arctic, cold season fluxes from tundra total 12 +/- 5 (95% confidence interval) Tg CH4 y(-1), similar to 25% of global emissions from extratropical wetlands, or similar to 6% of total global wetland methane emissions. The dominance of late-season emissions, sensitivity to soil environmental conditions, and importance of dry tundra are not currently simulated in most global climate models. Because Arctic warming disproportionally impacts the cold season, our results suggest that higher cold-season CH4 emissions will result from observed and predicted increases in snow thickness, active layer depth, and soil temperature, representing important positive feedbacks on climate warming.
  • Sinclair, Victoria A.; Moisseev, Dmitri; von Lerber, Annakaisa (2016)
    In this paper it is discussed how dual-polarization radar observations can be used to verify model representations of secondary ice production. An event where enhanced specific differential phase, K-dp, signatures in snow occur at the altitudes where temperatures lie in the range between -8 and -3 degrees C is investigated. By combining radar and surface-based precipitation observations it is shown that these dual-polarization radar signatures are most likely caused by ice with concentrations exceeding those expected from primary ice parameterizations. It is also shown that the newly formed ice particles readily aggregate, which may explain why K-dp values seem to be capped at 0.2-0.3 degrees/km for a Cband radar. For the event of interest, multiple high-resolution (1km) Weather Research and Forecasting (WRF) model simulations are conducted. When the default versions of the Morrison microphysics schemes were used, the simulated number concentration of frozen hydrometeors is much lower than observed and the simulated ice particle concentrations are comparable with values expected from primary ice parameterizations. Higher ice concentrations, which exceed values expected from primary ice parameterizations, were simulated when adhoc thresholds for rain and cloud water mixing ratio in the Hallett-Mossop part of the Morrison scheme were removed. These results suggest that the parameterization of secondary ice production in operational weather prediction models needs to be revisited and that dual-polarization radar observations, in conjunction with ancillary observations, can be used to verify them.
  • Lampilahti, Janne; Manninen, Hanna; Leino (os. Paananen), Katri; Väänänen, Riikka; Manninen, Antti; Buenrostro Mazon, Stephany N.; Nieminen, Tuomo; Leskinen, Matti; Enroth, Joonas; Bister, Marja; Zilitinkevich, Sergej; Kangasluoma, Juha; Järvinen, Heikki; Kerminen, Veli-Matti; Petäjä, Tuukka; Kulmala, Markku (2020)
    Recent studies have shown the importance of new particle formation (NPF) to global cloud condensation nuclei (CCN) production, as well as to air pollution in megacities. In addition to the necessary presence of low-volatility vapors that can form new aerosol particles, both numerical and observational studies have shown that the dynamics of the planetary boundary layer (BL) plays an important role in NPF. Evidence from field observations suggests that roll vortices might be favorable for inducing NPF in a convective BL. However, direct observations and estimates of the potential importance of this phenomenon to the production of new aerosol particles are lacking. Here we show that rolls frequently induce NPF bursts along the horizontal circulations and that the small clusters and particles originating from these localized bursts grow in size similar to particles typically ascribed to atmospheric NPF that occur almost homogeneously at a regional scale. We outline a method to identify roll-induced NPF from measurements and, based on the collected data, estimate the impact of roll vortices on the overall aerosol particle production due to NPF at a boreal forest site (83% +/- 34% and 26% +/- 8% overall enhancement in particle formation for 3 and 10 nm particles, respectively). We conclude that the formation of roll vortices should be taken into account when estimating particle number budgets in the atmospheric BL.
  • Li, Haoran; Moisseev, Dmitri (2020)
    Dual-frequency dual-polarization radar observations of the melting of two ice populations in a stratiform rainfall event are presented. The observed phenomenon occurs as a two-layer linear depolarization ratio (LDR) signature in a single radar bright band. Doppler spectra observations show that the upper LDR layer is caused by the melting of ice needles, potentially generated by the rime-splintering process, while the lower one is mainly due to the melting of background ice particles formed at the cloud top. The melting signal of small needles acts as a unique benchmark for detecting the onset of melting and is used to verify the current methods for the identification of melting layer boundaries. The radar-derived characteristics of the melting layer are found to be dependent on the radar variable and frequency used. The implications of the presented findings for radar-based studies of precipitation properties in and above the melting layer are also discussed.
  • Tiira, Jussi; Moisseev, Dmitri (2020)
    Vertical profiles of polarimetric radar variables can be used to identify fingerprints of snow growth processes. In order to systematically study such manifestations of precipitation processes, we have developed an unsupervised classification method. The method is based on k-means clustering of vertical profiles of polarimetric radar variables, namely reflectivity, differential reflectivity and specific differential phase. For rain events, the classification is applied to radar profiles truncated at the melting layer top. For the snowfall cases, the surface air temperature is used as an additional input parameter. The proposed unsupervised classification was applied to 3.5 years of data collected by the Finnish Meteorological Institute Ikaalinen radar. The vertical profiles of radar variables were computed above the University of Helsinki Hyytiala station, located 64 km east of the radar. Using these data, we show that the profiles of radar variables can be grouped into 10 and 16 classes for rainfall and snowfall events, respectively. These classes seem to capture most important snow growth and ice cloud processes. Using this classification, the main features of the precipitation formation processes, as observed in Finland, are presented.