Journal of Geophysical Research: Atmospheres

Kerns, Brandon W.; Chen, Shuyi S.

doi: 10.1002/2015JD024661pmid: N/A

A large‐scale precipitation tracking (LPT) method is developed to track convection and precipitation associated with the Madden‐Julian oscillation (MJO) using the Tropical Rainfall Measurement Mission 3B42 rainfall data from October to March 1998–2015. LPT uses spatially smoothed 3 day rainfall accumulation to identify and track precipitation features in time with a minimum size of 300,000 km2 and time continuity at least 10 days. While not all LPT systems (LPTs) are attributable to the MJO, among the 199 LPTs, there were 42 with a mean eastward propagation of at least 2 m s−1, which are considered to be MJO convective initiation events. These LPTs capture the diversity of the MJO convection, which is not well depicted by the Real‐time Multivariate MJO (RMM) index or the outgoing longwave radiation MJO index. During the 17 years, there were 17 instances out of 45 with a MJO signature in the RMM without eastward propagating LPTs. Among the 42 eastward propagating LPTs, 24 propagated across the Maritime Continent (MC), which confirms the MC barrier effect. Among the cases that crossed the MC from the Indian Ocean to the western Pacific (MC crossing), 18 (75%) had a significant MJO signature in the RMM index. In contrast, only six (33%) of the non‐MC‐crossing cases occurred with a RMM MJO signal. There is a significant seasonal and interannual variability with MC‐crossing LPTs occurring in December more commonly than other months. More MC‐crossing events were observed during La Niña than El Niño, which is consistent with the observations of stronger and more frequent MJO events identified by RMM during La Niña years.

Gu, Hongping; Ma, Zhuguo; Li, Mingxing

doi: 10.1002/2015JD024098pmid: N/A

Lake Taihu is the third largest freshwater lake in China and is situated in the Middle and lower Yangtze River delta. It is characterized by its shallowness (~1.9 m), large area (~2338 km2), and high turbidity in recent years. The lake's effect on local summer precipitation is first studied in this paper through the use of an atmosphere‐lake coupled model. By enlarging the light extinction coefficient, modifying the radiation scheme, and setting the roughness length to constants, the coupled model after adjustment realistically reproduces the thermal stratification and magnitude of diurnal variation over Lake Taihu, with mean biases of 0.7°C for lake surface temperature and 0.4°C for near‐surface air temperature, respectively. Based on this calibrated coupled model, two high‐resolution numerical simulations with and without the lake (lake grid cells replaced by cropland) were conducted to identify the lake effects. The results show that an overall effect of Lake Taihu on local summer precipitation is negative during daytime and positive during nighttime and the precipitation pattern may be modified to some extent. The lake effect varies between areas and with time of day and occurs primarily on the downwind shore. A composite analysis for a representative decreased precipitation region reveals that during daytime in the summer, the combination of decreased air temperature and latent heat flux, along with intensified divergence and downdraft, acts together to stabilize the lower atmosphere and suppress thermal convective activities, ultimately resulting in less precipitation over this region.

Geller, Marvin A.; Zhou, Tiehan; Yuan, Wei

doi: 10.1002/2015JD024125pmid: N/A

By means of theory, a simplified cartoon illustrating wave forcing of the stratospheric quasi‐biennial oscillation (QBO), and general circulation modeling of the QBO, it is argued that the period of the QBO is mainly controlled by the magnitude of the gravity wave (GW) vertical fluxes of horizontal momentum (GWMF) forcing the QBO, while the QBO amplitude is mainly determined by the phase speeds of the GWs that make up this momentum flux. It is furthermore argued that it is the zonally averaged GWMF that principally determines the QBO period irrespective of the longitudinal distribution of this GW momentum flux. These concepts are used to develop a hypothesis for the cause of a previously reported El Niño–Southern Oscillation (ENSO) modulation of QBO periods and amplitudes. Some observational evidence is reported for the ENSO modulation of QBO amplitudes to have been different before the 1980s than after about 1990. A hypothesis is also given to explain this in terms of the different ENSO modulation of tropical deep convection that took place before the 1980s from that which occurred after about 1990. The observational evidence, while consistent with our hypotheses, does not prove that our hypotheses are correct given the small number of El Niños and La Niñas that occurred in the early and later periods. Further research is needed to support or refute our hypotheses.

Varma, Atul Kumar; Piyush, D. N.; Gohil, B. S.; Pal, P. K.; Srinivasan, J.

doi: 10.1002/2016JD024907pmid: N/A

The Megha‐Tropiques, an Indo‐French satellite, carries on board a microwave sounder, Sondeur Atmosphérique du Profil d'Humidité Intertropical par Radiométrie (SAPHIR), and a microwave radiometer, Microwave Analysis and Detection of Rain and Atmospheric Structures (MADRAS), along with two other instruments. Being a Global Precipitation Measurement constellation satellite MT‐MADRAS was an important sensor to study the convective clouds and rainfall. Due to the nonfunctioning of MADRAS, the possibility of detection and estimation of rain from SAPHIR is explored. Using near‐concurrent SAPHIR and precipitation radar (PR) onboard Tropical Rainfall Measuring Mission (TRMM) observations, the rain effect on SAPHIR channels is examined. All the six channels of the SAPHIR are used to calculate the average rain probability (PR) for each SAPHIR pixel. Further, an exponential rain retrieval algorithm is developed. This algorithm explains a correlation of 0.72, RMS error of 0.75 mm/h, and bias of 0.04 mm/h. When rain identification and retrieval algorithms are applied together, it explains a correlation of 0.69 with an RMS error of 0.47 mm/h and bias of 0.01 mm/h. On applying the algorithm to the independent SAPHIR data set and compared with TRMM‐3B42 rain on monthly scale, it explains a correlation of 0.85 and RMS error of 0.09 mm/h. Further distribution of rain difference of SAPHIR with other rain products is presented on global scale as well as for the climatic zones. For examining the capability of SAPHIR to measure intense rain, instantaneous rain over Phailin cyclone from SAPHIR is compared with other standard satellite‐based rain products such as 3B42, Global Satellite Mapping of Precipitation, and Precipitation Estimation from Remote Sensing Information using Artificial Neural Network.

Archer, Cristina L.; Colle, Brian A.; Veron, Dana L.; Veron, Fabrice; Sienkiewicz, Matthew J.

doi: 10.1002/2016JD024896pmid: N/A

The marine boundary layer of the northeastern U.S. is studied with focus on wind speed, atmospheric stability, and turbulent kinetic energy (TKE), the three most relevant properties in the context of offshore wind power development. Two long‐term observational data sets are analyzed. The first one consists of multilevel meteorological variables measured up to 60 m during 2003–2011 at the offshore Cape Wind tower, located near the center of the Nantucket Sound. The second data set comes from the 2013–2014 IMPOWR campaign (Improving the Modeling and Prediction of Offshore Wind Resources), in which wind and wave data were collected with new instruments on the Cape Wind platform, in addition to meteorological data measured during 19 flight missions offshore of New York, Connecticut, Rhode Island, and Massachusetts. It is found that, in this region: (1) the offshore wind resource is remarkable, with monthly average wind speeds at 60 m exceeding 7 m s−1 all year round, highest winds in winter (10.1 m s−1) and lowest in summer (7.1 m s−1), and a distinct diurnal modulation, especially in summer; (2) the marine boundary layer is predominantly unstable (61% unstable vs. 21% neutral vs. 18% stable), meaning that mixing is strong, heat fluxes are positive, and the wind speed profile is often nonlogarithmic (~40% of the time); and (3) the shape of the wind speed profile (log versus nonlog) is an effective qualitative proxy for atmospheric stability, whereas TKE alone is not.

Adachi, Kouji; Moteki, Nobuhiro; Kondo, Yutaka; Igarashi, Yasuhito

doi: 10.1002/2016JD025153pmid: N/A

Light‐absorbing atmospheric aerosols such as carbonaceous particles influence the climate through absorbing sunlight. The mixing states of these aerosol particles affect their optical properties. This study examines the changes in the mixing states and abundance of strongly light absorbing carbonaceous particles by using transmission electron microscopy (TEM) and single‐particle soot photometer (SP2), as well as of iron oxide particles, in Tokyo, Japan. TEM and SP2 use fundamentally different detection techniques for the same light‐absorbing particles. TEM allows characterization of the morphological, chemical, and structural features of individual particles, whereas SP2 optically measures the number, size, and mixing states of black carbon (BC). A comparison of the results obtained using these two techniques indicates that the peaks of high soot (nanosphere soot (ns‐soot)) concentration periods agree with those of the BC concentrations determined by SP2 and that the high Fe‐bearing particle fraction periods measured by TEM agree with that of high number concentrations of iron oxide particles measured using SP2 during the first half of the observation campaign. The results also show that the changes in the ns‐soot/BC mixing states primarily correlate with the air mass sources, wind speed, precipitation, and photochemical processes. Nano‐sized, aggregated, iron oxide particles mixed with other particles were commonly observed by using TEM during the high iron oxide particle periods. We conclude that although further quantitative comparison between TEM and SP2 data will be needed, the morphologically and optically defined ns‐soot and BC, respectively, are essentially the same substance and that their mixing states are generally consistent across the techniques.

Neuman, J. A.; Trainer, M.; Brown, S. S.; Min, K.‐E.; Nowak, J. B.; Parrish, D. D.; Peischl, J.; Pollack, I. B.; Roberts, J. M.; Ryerson, T. B.; Veres, P. R.

doi: 10.1002/2016JD025197pmid:

Rao, Jian; Ren, Rongcai

doi: 10.1002/2015JD024521pmid: N/A

Long‐term simulations from the Community Earth System Model with the Whole Atmosphere Community Climate Model (CESM‐WACCM) as its atmospheric component are used to investigate the asymmetry and the nonlinearity of the influences of El Niño–Southern Oscillation (ENSO) on the northern winter stratosphere. As in Part 1 of this study, four different types of ENSO are considered. The composite CESM‐WACCM results first confirm the conclusions drawn from the observations, that the stratospheric polar jet responses to “moderate El Niño” and “strong La Niña” are stronger than those to “strong El Niño” and “moderate La Niña”. In association with the ENSO sea surface temperature (SST) patterns that are reproduced well in the model, the tropical rainfall response centers exhibit an asymmetric east‐west shift between El Niño and La Niña, which directly leads to the nonlinear and asymmetric Pacific‐North America responses in the extratropics. Accordingly, the strengthening (weakening) planetary wave response in the stratosphere during warm (cold) ENSO also exhibits nonlinear and asymmetric features. When the ENSO SST forcing is prescribed to be linear and symmetric in WACCM, the nonlinearity and asymmetry of the stratospheric responses to moderate ENSO reveal the dominant role of the inherent properties of the atmosphere. However, the absence of asymmetry and nonlinearity in the stratospheric responses to strong ENSO in our sensitivity experiments indicates that the asymmetry in SST forcing between strong El Niño and La Niña still plays an important role in the asymmetric and nonlinear influences of ENSO on the extratropics.

McKinnon, Karen A.; Rhines, Andrew; Tingley, Martin P.; Huybers, Peter

doi: 10.1002/2016JD025292pmid: N/A

The occurrence of recent summer temperature extremes in the midlatitudes has raised questions about whether and how the distributions of summer temperature are changing. While it is clear that in most regions the average temperature is increasing, there is less consensus regarding the presence or nature of changes in the shape of the distributions, which can influence the probability of extreme events. Using data from over 4000 weather stations in the Global Historical Climatology Network‐Daily database, we quantify the changes in daily maximum and minimum temperature distributions for peak summer in the Northern Hemisphere midlatitudes during 1980–2015 using quantile regression. A large majority (87–88%) of the trends across percentiles and stations can be explained by a shift of the distributions with no change in shape. The remaining variability is summarized through projections onto orthogonal basis functions that are closely related to changes in variance, skewness, and kurtosis. North America and Eurasia show significant shifts in the estimated distributions of daily maximum and minimum temperatures. Although no general change in summer variance is found, variance has regionally increased in Eurasia and decreased in most of North America. Changes in shape that project onto the skewness and kurtosis basis functions have a much smaller spatial scale and are generally insignificant.

Minder, Justin R.; Letcher, Theodore W.; Skiles, S. McKenzie

doi: 10.1002/2016JD024995pmid: N/A

The snow‐albedo feedback (SAF) strongly influences climate over midlatitude mountainous regions. However, over these regions the skill of regional climate models (RCMs) at simulating properties such as snow cover and surface albedo is poorly characterized. These properties are evaluated in a pair of 7 year long high‐resolution RCM simulations with the Weather Research and Forecasting model over the central Rocky Mountains. Key differences between the simulations include the computational domain (regional versus continental) and land surface model used (Noah versus Noah‐MP). Simulations are evaluated against high‐resolution satellite estimates of snow cover and albedo from the Moderate Resolution Imaging Spectroradiometer. Both simulations generally reproduce the observed seasonal and spatial variability of snow cover and also exhibit important biases. One simulation substantially overpredicts subpixel fractional snow cover over snowy pixels (by up to 0.4) causing large positive biases in surface albedo, likely due in part to inadequate representation of canopy effects. The other simulation exhibits a negative bias in areal snow extent (as much as 19% of the analysis domain). Surface measurements reveal large positive biases in snow albedo (exceeding 0.2) during late spring caused by neglecting radiative effects of impurities deposited onto snow. Semi‐idealized climate change experiments show substantially different magnitudes of SAF‐enhanced warming in the two simulations that can be tied to the differences in snow cover in their control climates. More confident projections of regional climate change over mountains will require further work to evaluate and improve representation of snow cover and albedo in RCMs.