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J. L. Stanford
,
J. R. Ziemke
, and
S. Y. Gao

Abstract

Stratospheric circulation is investigated by further analyses of three years of Stratospheric And Mesospheric Sounder (SAMS) data. Eddy effects on constituent transport are investigated with two transport formulations the transformed Eulerian mean formulation and the effective transport formulation. The transformed Eulerian mean formulation, together with calculated residual mean winds (*, *), is used to delineate regions, or times, of significant eddy contributions to constituent transport. Significant regions are found in the stratosphere in all seasons, not only in the Northern Hemisphere (NH) winter high latitudes (where contributions from nonlinear and nonsteady perturbations in sudden and final warming events are expected), but also in the midlatitude, middle stratosphere in autumn and at winter-summer low latitudes near the stratopause. Questionably large calculated ρ0−1Δ · M magnitudes near the stratopause during solstice suggest that the use of residual winds calculated here may be inadequate for describing mass transport at these heights. The effective transport formulation is used, involving individual calculations of the effective-transport velocity (†, †) for both CH4 and N2O. With CH4, the tracer continuity equation is diagnosed term-by-term for January, April, July, and October months. Both gases reveal a descent in the tropics during northern spring (associated with a “double peak” in mixing ratio measured along latitude) and upwelling in the summer tropics of the stratosphere during solstice. The NH summer upwelling appears to cause larger CH4 increases than corresponding Southern Hemisphere (SH) summer tropical upwelling.

The effective transport calculations involving CH4 show that in July most local change in mixing ratio in the upper stratosphere is attributable to mean advection. In the upper stratosphere and lower mesosphere during NH summer-autumn, pulses of enhanced mixing ratio (related to an apparent coupling of semiannual and annual circulation components) propagate from low northern latitudes into both hemispheres. The behavior is similar to that predicted by diabatic models, but under equinox conditions. The calculated (†, †) fields for June–July exhibit this cellular feature for both CH4 and N2O. In the extratropics at stratopause heights, particularly in the SH, large local amplitudes of the semiannual component may be attributable in part to this mean meridional advection from summer to winter hemisphere. Each year, both CH4 and N2O show features consistent with significant NH autumnal vertical and meridional transport at high latitudes in the lower mesosphere.

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Zexia Duan
,
C. S. B. Grimmond
,
Chloe Y. Gao
,
Ting Sun
,
Changwei Liu
,
Linlin Wang
,
Yubin Li
, and
Zhiqiu Gao

Abstract

Quantitative knowledge of the water and energy exchanges in agroecosystems is vital for irrigation management and modeling crop production. In this study, the seasonal and annual variabilities of evapotranspiration (ET) and energy exchanges were investigated under two different crop environments—flooded and aerobic soil conditions—using three years (June 2014–May 2017) of eddy covariance observations over a rice–wheat rotation in eastern China. Across the whole rice–wheat rotation, the average daily ET rates in the rice paddies and wheat fields were 3.6 and 2.4 mm day−1, respectively. The respective average seasonal ET rates were 473 and 387 mm for rice and wheat fields, indicating a higher water consumption for rice than for wheat. Averaging for the three cropping seasons, rice paddies had 52% more latent heat flux than wheat fields, whereas wheat had 73% more sensible heat flux than rice paddies. This resulted in a lower Bowen ratio in the rice paddies (0.14) than in the wheat fields (0.4). Because eddy covariance observations of turbulent heat fluxes are typically less than the available energy (R n − G; i.e., net radiation minus soil heat flux), energy balance closure (EBC) therefore does not occur. For rice, EBC was greatest at the vegetative growth stages (mean: 0.90) after considering the water heat storage, whereas wheat had its best EBC at the ripening stages (mean: 0.86).

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F. Giorgi
,
E.-S. Im
,
E. Coppola
,
N. S. Diffenbaugh
,
X. J. Gao
,
L. Mariotti
, and
Y. Shi

Abstract

Because of their dependence on water, natural and human systems are highly sensitive to changes in the hydrologic cycle. The authors introduce a new measure of hydroclimatic intensity (HY-INT), which integrates metrics of precipitation intensity and dry spell length, viewing the response of these two metrics to global warming as deeply interconnected. Using a suite of global and regional climate model experiments, it is found that increasing HY-INT is a consistent and ubiquitous signature of twenty-first-century, greenhouse gas–induced global warming. Depending on the region, the increase in HY-INT is due to an increase in precipitation intensity, dry spell length, or both. Late twentieth-century observations also exhibit dominant positive HY-INT trends, providing a hydroclimatic signature of late twentieth-century warming. The authors find that increasing HY-INT is physically consistent with the response of both precipitation intensity and dry spell length to global warming. Precipitation intensity increases because of increased atmospheric water holding capacity. However, increases in mean precipitation are tied to increases in surface evaporation rates, which are lower than for atmospheric moisture. This leads to a reduction in the number of wet days and an increase in dry spell length. This analysis identifies increasing hydroclimatic intensity as a robust integrated response to global warming, implying increasing risks for systems that are sensitive to wet and dry extremes and providing a potential target for detection and attribution of hydroclimatic changes.

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S. Sorooshian
,
X. Gao
,
K. Hsu
,
R. A. Maddox
,
Y. Hong
,
H. V. Gupta
, and
B. Imam

Abstract

Recent progress in satellite remote-sensing techniques for precipitation estimation, along with more accurate tropical rainfall measurements from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) and precipitation radar (PR) instruments, have made it possible to monitor tropical rainfall diurnal patterns and their intensities from satellite information. One year (August 1998–July 1999) of tropical rainfall estimates from the Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks (PERSIANN) system were used to produce monthly means of rainfall diurnal cycles at hourly and 1° × 1° scales over a domain (30°S–30°N, 80°E–10°W) from the Americas across the Pacific Ocean to Australia and eastern Asia.

The results demonstrate pronounced diurnal variability of tropical rainfall intensity at synoptic and regional scales. Seasonal signals of diurnal rainfall are presented over the large domain of the tropical Pacific Ocean, especially over the ITCZ and South Pacific convergence zone (SPCZ) and neighboring continents. The regional patterns of tropical rainfall diurnal cycles are specified in the Amazon, Mexico, the Caribbean Sea, Calcutta, Bay of Bengal, Malaysia, and northern Australia. Limited validations for the results include comparisons of 1) the PERSIANN-derived diurnal cycle of rainfall at Rondonia, Brazil, with that derived from the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) radar data; 2) the PERSIANN diurnal cycle of rainfall over the western Pacific Ocean with that derived from the data of the optical rain gauges mounted on the TOGA-moored buoys; and 3) the monthly accumulations of rainfall samples from the orbital TMI and PR surface rainfall with the accumulations of concurrent PERSIANN estimates. These comparisons indicate that the PERSIANN-derived diurnal patterns at the selected resolutions produce estimates that are similar in magnitude and phase.

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X. Liang
,
S. Miao
,
J. Li
,
R. Bornstein
,
X. Zhang
,
Y. Gao
,
F. Chen
,
X. Cao
,
Z. Cheng
,
C. Clements
,
W. Dabberdt
,
A. Ding
,
D. Ding
,
J. J. Dou
,
J. X. Dou
,
Y. Dou
,
C. S. B. Grimmond
,
J. E. González-Cruz
,
J. He
,
M. Huang
,
X. Huang
,
S. Ju
,
Q. Li
,
D. Niyogi
,
J. Quan
,
J. Sun
,
J. Z. Sun
,
M. Yu
,
J. Zhang
,
Y. Zhang
,
X. Zhao
,
Z. Zheng
, and
M. Zhou

Abstract

Urbanization modifies atmospheric energy and moisture balances, forming distinct features [e.g., urban heat islands (UHIs) and enhanced or decreased precipitation]. These produce significant challenges to science and society, including rapid and intense flooding, heat waves strengthened by UHIs, and air pollutant haze. The Study of Urban Impacts on Rainfall and Fog/Haze (SURF) has brought together international expertise on observations and modeling, meteorology and atmospheric chemistry, and research and operational forecasting. The SURF overall science objective is a better understanding of urban, terrain, convection, and aerosol interactions for improved forecast accuracy. Specific objectives include a) promoting cooperative international research to improve understanding of urban summer convective precipitation and winter particulate episodes via extensive field studies, b) improving high-resolution urban weather and air quality forecast models, and c) enhancing urban weather forecasts for societal applications (e.g., health, energy, hydrologic, climate change, air quality, planning, and emergency response management). Preliminary SURF observational and modeling results are shown (i.e., turbulent PBL structure, bifurcating thunderstorms, haze events, urban canopy model development, and model forecast evaluation).

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