Journal of Geophysical Research: Atmospheres

doi: 10.1002/jgrd.51650pmid: N/A

Dennison, Fraser W.; McDonald, Adrian J.; Morgenstern, Olaf

doi: 10.1002/2014JD023009pmid: N/A

The aim of this study is to investigate the influence of ozone depletion and recovery on the Southern Annular Mode (SAM) and stratosphere‐troposphere coupling. Using the National Institute of Water and Atmospheric Research‐United Kingdom Chemistry and Aerosols chemistry‐climate model, we compare reference runs that include forcing due to greenhouse gases and ozone‐depleting substances to sensitivity simulations in which ozone‐depleting substances are fixed at their 1960 levels. We find that ozone depletion leads to an increased frequency of extreme anomalies and increased persistence of the SAM in the stratosphere as well as stronger, more persistent stratosphere‐troposphere coupling. Currently, the stratosphere provides an appreciable amount of predictability to the troposphere on timescales of 1 or 2 months; however, we find that this effect reduces over time as stratospheric ozone recovers to preozone hole levels toward the latter part of this century.

Ma, Jing; Xu, Haiming; Dong, Changming; Lin, Pengfei; Liu, Yu

doi: 10.1002/2014JD022930pmid: N/A

We examined atmospheric responses to 35,000+ oceanic eddies in the Kuroshio Extension region during the period of 2006–2009. Using satellite data, we showed that cold (warm) eddies cause surface winds to decelerate (accelerate) and reduce (increase) latent and sensible heat fluxes, cloud liquid water, water vapor content, and rain rate; all of these changes are quantified. Both the linear correlation between wind divergence and downwind sea surface temperature (SST) gradient and the correspondence between vorticity and crosswind SST gradient support the vertical momentum mixing mechanism, which indicates that SST perturbations modify surface winds by changing the vertical turbulent mixing in the marine atmospheric boundary layer (MABL). High‐resolution National Centers for Environmental Prediction Climate Forecast System Reanalysis (CFSR) data can reproduce the atmospheric responses to the oceanic eddies in the MABL albeit with some differences in intensity. In addition, the CFSR data reveal that the atmospheric responses to these oceanic eddies are not confined in the MABL. MABL deepens (shoals) over the warm (cold) eddies; enhanced (reduced) vertical transport of transient zonal momentum occurs over the warm (cold) eddies from the sea surface to about 850 hPa level; vertical velocity anomalies over oceanic eddies penetrate beyond the MABL into free atmosphere; there exists a positive correlated relationship between SST and convective rain rate anomalies, indicative of ocean eddies' impact on the free troposphere. However, the composites of cloud liquid water and rain rate are different from the results based on the satellite data.

De Conti, Alberto; Silveira, Fernando H.; Visacro, Silvério

doi: 10.1002/2015JD023132pmid: N/A

A theoretical study is presented to investigate lightning strikes to towers, with focus on wave interactions occurring at the return‐stroke front due to the arrival of current pulses that propagate upward on the channel after being transmitted from the tower. The lightning channel is represented as a transmission line including corona and nonlinear losses. Analyses for the hypothetical case of a lossless channel considering matched tower and channel impedances show that the arrival of current pulses at the upward moving return‐stroke front leads to an increase in corona currents leaving the channel. This transient process generates current pulses whose arrival at the tower top can be interpreted as the effect of a current reflection at the upward moving front, even though no impedance discontinuity exists at that point if return‐stroke and leader channel properties are assumed the same. In the more realistic case of a lossy channel considering unmatched channel and tower impedances, the nonlinear channel resistance modifies the current pulses that propagate along the channel so that they merge smoothly with the return‐stroke front. In this case, the interaction of the current pulses transmitted from the tower to the channel with the upward moving return‐stroke front does not lead to features that can be clearly interpreted as the result of current reflections at that point in the evaluated conditions. Finally, it is argued that the current reflection coefficient at the tower top should be viewed as a current dependent parameter as opposed to a constant, linear value.

Rajsekhar, Deepthi; Singh, Vijay P.; Mishra, Ashok K.

doi: 10.1002/2014JD022670pmid: N/A

Drought properties and the socioeconomic impact it makes are expected to increase in the coming years due to climate change. Here we review the possible impacts of changes in climate variability on the properties of different drought types. The downscaled and bias‐corrected data from five general circulation models (GCMs) were used to produce an ensemble of precipitation, temperature, and wind speed, through a relative entropy approach and were used for drought analysis. A novel Multivariate Drought Index was then employed for an integrated quantification of all physical forms of drought. We studied the spatial patterns of drought properties and performed multivariate frequency analysis for each planning region in Texas to recognize the distribution of potential drought hazard areas under climate change impact by formulating a Drought Hazard Index. A drought vulnerability assessment was also carried out by taking into consideration various socioeconomic factors, leading to the development of socioeconomic Drought Vulnerability Index. A set of composite drought risk maps that combines hazard and vulnerability analysis were developed. This study also explored the cause‐effect relationship between the drought events and several hydroclimatic triggers. A transfer entropy measure was used to quantify the causal relationships, thus indicating the predominant future drought triggers. Overall, the findings are expected to help achieve an effective drought mitigation strategy for the state of Texas.

Kinoshita, Takenari; Murayama, Yasuhiro; Kawamura, Seiji

doi: 10.1002/2014JD022647pmid: N/A

The interactions between gravity waves and atmospheric tidal waves have been observationally studied, although the phase relation between them has not been fully understood. In this study, the long‐term wind velocity data observed with the Poker Flat MF radar (65∘N, 147∘W) were analyzed for the period of 1999–2008 to show local time dependence and seasonal climatologies of the 12 h and 24 h components in the mesospheric winds and their modulations of gravity wave kinetic energy. We made climatological 1 day composite plots of the kinetic energy of gravity waves for wave periods of 1–4 h and harmonic components of horizontal wind for each month. The results show that the kinetic energy of gravity waves peaks twice at 3–6 LT and 18–21 LT, which tend to coincide with the transition of the 12 h component of zonal wind from westward to eastward flow. On the other hand, a 2 month case study revealed that the gravity wave kinetic energy and the 12 h components of zonal wind appear to keep their phase difference constant (like a “phase locked”) for more than 10 days. Events of this kind are also found in other years. To examine whether this relation can be explained by interaction between the 12 h component of zonal wind and gravity waves, we applied a gravity wave drag model to the background state defined as the sum of observed monthly mean and harmonic components of zonal wind. It is suggested that the orographic gravity wave drag has a 12 h periodicity and that the time of the drag enhancement changes in time following change in the phase of harmonic components of winds.

Wang, Jianhua; Wang, Qingming; Zhao, Yong; Li, Haihong; Zhai, Jiaqi; Shang, Yizi

doi: 10.1002/2014JD022874pmid: N/A

Pan evaporation is an important indicator of atmospheric evaporative demand, and its long‐term variation is of much concern in studies of climate change. Based on data from 33 meteorological stations from 1962 to 2012, this work considered the temporal and spatial trends of pan evaporation and the meteorological variables that affect them in the Three‐River Source Region (TRSR) of southwestern China. Pan evaporation in the TRSR has decreased significantly since 1988 with an obvious abrupt change from 1993 to 2003. Furthermore, a 27 year period of oscillation has existed over the past 51 years. Pan evaporation reflects the combined effects of four meteorological variables: net radiation (Rn), wind speed (u2), actual vapor pressure (ea), and air temperature (Ta). Based on this research, a number of conclusions were drawn. (1) The pace of climate change increased after 1980 and pan evaporation decreased at a rate of −13.3 mm/a2 from 1980 to 2012, which is much faster than the rate of −1.2 mm/a2 from 1962 to 1979. (2) For the decrease of pan evaporation from 1980 to 2012, the quantifying contributions of Rn, u2, ea, and Ta were −8.7, −6.4, −1.8, and +3.6 mm/a2, respectively. Thus, it was established for the TRSR that “global dimming” was the main reason, and “wind stilling” was a close second to global dimming for the decrease in pan evaporation. (3) Different regions of the TRSR are affected differently by the effects of the meteorological variables. Low‐elevation regions in the TRSR are more susceptible to the effects of net radiation and wind speed, whereas high‐elevation regions are affected more by actual vapor pressure and air temperature.

Xu, Guobao; Liu, Xiaohong; Wu, Guoju; Chen, Tuo; Wang, Wenzhi; Zhang, Qiong; Zhang, Youfu; Zeng, Xiaomin; Qin, Dahe; Sun, Weizhen; Zhang, Xuanwen

doi: 10.1002/2014JD023027pmid:

Zhang, Xuejun; Tang, Qiuhong

doi: 10.1002/2015JD023400pmid: N/A

Satellite real‐time precipitation enables hydrological monitoring in China where the near‐real‐time ground observations are not readily available. However, the inconsistency between the real‐time satellite precipitation and gauge‐based retrospective data may introduce large systematic bias in near‐real‐time hydrological monitoring. Here we attempted to integrate the Tropical Rainfall Measuring Mission (TRMM) real‐time precipitation (3B42RTV7) into a 62 year gauge‐based retrospective product, the IGSNRR (Institute of Geographical Sciences and Natural Resources Research) dataset through matching their cumulative probability functions toward a near‐real‐time hydrological monitoring consistent with the long‐term retrospective simulations. A nearly 11 year period from March 2000 to December 2010 was taken as the training period to establish the satellite‐gauge precipitation relationship, which was employed in the period of 2011–2013 to evaluate the performance of the adjustment. The results show that the adjusted 3B42RTV7 matches well with IGSNRR precipitation, while the unadjusted data tend to overestimate precipitation. Forced by the adjusted 3B42RTV7, the Variable Infiltration Capacity model can reproduce the IGSNRR‐derived hydrographs and high/low flows better than the model forced by the unadjusted data. The percentiles of the adjusted hydrological estimates in the 62 year estimates from IGSNRR are used for near‐real‐time assessment of hydrological extremes. The hydrological monitoring assisted by the adjusted satellite precipitation, which enables the employment of the long‐term ground observations, is able to capture more detailed drought information than that before adjustment. Our experiment suggests that the satellite real‐time precipitation, after adjustment, can generate the current hydrological conditions which can be directly compared with the long‐term climatology, and thus facilitates near‐real‐time diagnosis and detection of hydrological extremes.

Bi, Xueyan; Gao, Zhiqiu; Liu, Yangang; Liu, Feng; Song, Qingtao; Huang, Jian; Huang, Huijun; Mao, Weikang; Liu, Chunxia

doi: 10.1002/2015JD023172pmid: N/A

This paper investigates the relationships between friction velocity, 10 m drag coefficient, and 10 m wind speed using data collected at two offshore observation towers (one over the sea and the other on an island) from seven typhoon episodes in the South China Sea from 2008 to 2014. The two towers were placed in areas with different water depths along a shore‐normal line. The depth of water at the tower over the sea averages about 15 m, and the depth of water near the island is about 10 m. The observed maximum 10 min average wind speed at a height of 10 m is about 32 m s−1. Momentum fluxes derived from three methods (eddy covariance, inertial dissipation, and flux profile) are compared. The momentum fluxes derived from the flux profile method are larger (smaller) over the sea (on the island) than those from the other two methods. The relationship between the 10 m drag coefficient and the 10 m wind speed is examined by use of the data obtained by the eddy covariance method. The drag coefficient first decreases with increasing 10 m wind speed when the wind speeds are 5–10 m s−1, then increases and reaches a peak value of 0.002 around a wind speed of 18 m s−1. The drag coefficient decreases with increasing 10 m wind speed when 10 m wind speeds are 18–27 m s−1. A comparison of the measurements from the two towers shows that the 10 m drag coefficient from the tower in 10 m water depth is about 40% larger than that from the tower in 15 m water depth when the 10 m wind speed is less than 10 m s−1. Above this, the difference in the 10 m drag coefficients of the two towers disappears.