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Kevin E. Trenberth and Yongxin Zhang

Abstract

The perception about whether a place is a nice place to live often depends on how often it rains (or snows). The frequency relates to how dreary the weather appears, and it is the duration much more than the amount that clouds perceptions. Yet, information about the frequency of rainfall, or precipitation in general, is spotty at best. Here, we analyze a new near-global (60°N–60°S) dataset at hourly time scales and 0.25° resolution. The dataset, the newly calibrated Climate Prediction Center morphing technique (CMORPH), enables comparison of results with 3-hourly and daily data, which is what has previously been available, and seasonal aspects are also examined. The results are quite sensitive to both the spatial scales of the data and their temporal resolutions, and it is important to get down to hourly values to gain a proper appreciation of the true frequency. At 1° resolution, values are 35% higher than for 0.25°. At 3-hourly resolution, they are about 25% higher than hourly, and at daily resolution, they are about 150% higher than hourly on average. Overall, near-global (60°N–60°S) precipitation occurs 11.0% ± 1.1% (1 sigma) of the time or, alternatively, 89.0% of the time it is not precipitating. But outside of the intertropical and South Pacific convergence zones, where values exceed 30%, and the arid and desert regions, where values are below 4%, the rates are more like 10% or so, and over land where most people live, values are closer to about 8%.

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Kevin E. Trenberth and Yongxin Zhang

Abstract

A detailed analysis of hourly precipitation from 60°N to 60°S for the covariability is performed at 0.25° resolution using the new CMORPH dataset. For all points, correlations are computed with surrounding points both concurrently and for various leads and lags up to a day. Results are more coherent over the oceans than land; the contours of constant correlation tend to be elliptical, oriented northeast–southwest in the northern extratropics and southeast–northwest in the southern extratropics. An ellipse is fitted to the correlation pattern, and major and minor axis vectors and eccentricity are mapped. Based upon both the isotropic correlations and ellipse, points are allocated to one of 20 clusters, and 16 are documented. Over the main extratropical ocean storm tracks, correlations exceed 0.8 for points 50 km distant and fall to about 0.3 at about 5° radius. In the tropics values drop to 0.65 within 50 km and 0.2 at 5° radius. Over land, values are lower in summer and drop to 0.1 at 5° radius. Decorrelation e-folding distances range from less than 50 km over land to 200 km over extratropical ocean storm tracks. The movement of precipitation is compared with mean atmospheric winds. The lead–lag relationships indicate movement of systems but reveal the relatively short lifetimes of precipitation, of less than 12 h, even taking movement into account. The orientation of the ellipse reflects the structures of rain phenomena (fronts, etc.) rather than movement. These statistics demonstrate that daily averages fail to capture the essential character of precipitation.

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Yanhong Gao, Lan Cuo, and Yongxin Zhang

Abstract

Changes in moisture as represented by PE (precipitation − evapotranspiration) and the possible causes over the Tibetan Plateau (TP) during 1979–2011 are examined based on the Global Land Data Assimilation Systems (GLDAS) ensemble mean runoff and reanalyses. It is found that the TP is getting wetter as a whole but with large spatial variations. The climatologically humid southeastern TP is getting drier while the vast arid and semiarid northwestern TP is getting wetter. The Clausius–Clapeyron relation cannot be used to explain the changes in PE over the TP.

Through decomposing the changes in PE into three major components—dynamic, thermodynamic, and transient eddy components—it is noted that the dynamic component plays a key role in the changes of PE over the TP. The thermodynamic component contributes positively over the southern and central TP whereas the transient eddy component tends to reinforce (offset) the dynamic component over the southern and parts of the northern TP (central TP).

Seasonally, the dynamic component contributes substantially to changes in PE during the wet season, with small contributions from the thermodynamic and transient eddy components. Further analyses reveal the poleward shift of the East Asian westerly jet stream by 0.7° and poleward moisture transport as well as the intensification of the summer monsoon circulation due to global warming, which are shown to be responsible for the general wetting trend over the TP. It is further demonstrated that changes in local circulations that occur due to the differential heating of the TP and its surroundings are responsible for the spatially varying changes in moisture over the TP.

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Yongxin Zhang, Yi-Leng Chen, and Kevin Kodama

Abstract

A high wind event (14–15 February 2001) over the Hawaiian Islands associated with a cold front is simulated using the National Centers for Environmental Prediction (NCEP) Mesoscale Spectral Model (MSM) coupled with an advanced land surface model (LSM). During this period, a strong high pressure cell moved to the northeast of the Hawaiian Islands following the passage of the cold front. The cell then merged with the semipermanent subtropical high and resulted in windy conditions across the state of Hawaii. Analyses of soundings from Lihue on Kauai and Hilo on the Big Island reveal a mean-state critical level below 400 hPa, a strong cross-barrier flow (∼13 m s−1), and the presence of a trade wind inversion.

The MSM–LSM predicts downslope windstorms on the lee sides of mountains or ridges with tops beneath the trade wind inversion and within ocean channels between islands. In the case of high mountains with a peak height above the trade wind inversion, weak winds are simulated on the lee side. Around the corners of the islands and in gaps between mountains, gap winds and downslope windstorms are both important for the development of localized leeside windstorms.

The localized windstorms over the Hawaiian Islands develop as a result of interactions between large-scale airflow and the complex local topography. Since the terrain is not adequately resolved by the 10-km RSM–LSM, it is no surprise that these windstorms are better simulated by the high-resolution nonhydrostatic MSM–LSM than the 10-km RSM–LSM.

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Kevin E. Trenberth and Yongxin Zhang

Abstract

The net surface energy flux is computed as a residual of the energy budget using top-of-atmosphere radiation combined with the divergence of the column-integrated atmospheric energy transports, and then used with the vertically integrated ocean heat content tendencies to compute the ocean meridional heat transports (MHTs). The mean annual cycles and 12-month running mean MHTs as a function of latitude are presented for 2000–16. Effects of the Indonesian Throughflow (ITF), associated with a net volume flow around Australia accompanied by a heat transport, are fully included. Because the ITF-related flow necessitates a return current northward in the Tasman Sea that relaxes during El Niño, the reduced ITF during El Niño may contribute to warming in the south Tasman Sea by allowing the East Australian Current to push farther south even as it gains volume from the tropical waters not flowing through the ITF. Although evident in 2015/16, when a major marine heat wave occurred, these effects can be overwhelmed by changes in the atmospheric circulation. Large interannual MHT variability in the Pacific is 4 times that of the Atlantic. Strong relationships reveal influences from the southern subtropics on ENSO for this period. At the equator, northward ocean MHT arises mainly in the Atlantic (0.75 PW), offset by the Pacific (−0.33 PW) and Indian Oceans (−0.20 PW) while the atmosphere transports energy southward (−0.35 PW). The net equatorial MHT southward (−0.18 PW) is enhanced by −0.1 PW that contributes to the greater warming of the southern (vs northern) oceans.

Open access
Kevin E. Trenberth, Yongxin Zhang, and Maria Gehne

Abstract

Intermittency is a core characteristic of precipitation, not well described by data and very poorly modeled. Detailed analyses are made of near-global gridded (about 1°) hourly or 3-hourly precipitation rates from two updated observational datasets [3-hourly TRMM 3B42, version 7, and hourly CMORPH, version 1.0, bias corrected (CRT)] and from special runs of CESM from January 1998 to December 2013 to obtain hourly values. The analyses explore the intermittency of precipitation: the frequency, intensity, duration, and amounts. A comparison is made for all products using several metrics with a focus on the duration of events, and a new metric is proposed based on the ratio of the frequency of precipitation at certain rates (0.2–2 mm h−1) for hourly versus 3-hourly versus daily data. For all seasons and rain rates, TRMM values are similar in pattern to CMORPH, but durations are about 80%–85%. It is mainly over land in the monsoons that CMORPH exceeds TRMM rain durations. Observed duration of precipitation events in CMORPH over oceans are 12–15 h in the tropics and subtropics, much less than the ~20 h for CESM. Hence, the observational results differ somewhat but both are considerably different from the model, which has too much precipitation overall, and it precipitates far too often at low rates and not enough for intense rates, with the divide about 1–2 mm h−1. There is a need to properly represent precipitation phenomena and processes either explicitly or implicitly (parameterized).

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Valérie Dulière, Yongxin Zhang, and Eric P. Salathé Jr.

Abstract

Trends in extreme temperature and precipitation in two regional climate model simulations forced by two global climate models are compared with observed trends over the western United States. The observed temperature extremes show substantial and statistically significant trends across the western United States during the late twentieth century, with consistent results among individual stations. The two regional climate models simulate temporal trends that are consistent with the observed trends and reflect the anthropogenic warming signal. In contrast, no such clear trends or correspondence between the observations and simulations is found for extreme precipitation, likely resulting from the dominance of the natural variability over systematic climate change during the period. However, further analysis of the variability of precipitation extremes shows strong correspondence between the observed precipitation indices and increasing oceanic Niño index (ONI), with regionally coherent patterns found for the U.S. Northwest and Southwest. Both regional climate simulations reproduce the observed relationship with ONI, indicating that the models can represent the large-scale climatic links with extreme precipitation. The regional climate model simulations use the Weather Research and Forecasting (WRF) Model and Hadley Centre Regional Model (HadRM) forced by the ECHAM5 and the Hadley Centre Climate Model (HadCM) global models for the 1970–2007 time period. Comparisons are made to station observations from the Historical Climatology Network (HCN) locations over the western United States. This study focused on temperature and precipitation extreme indices recommended by the Expert Team on Climate Change Detection Monitoring and Indices (ETCCDMI).

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Valérie Dulière, Yongxin Zhang, and Eric P. Salathé Jr.

Abstract

Extreme precipitation and temperature indices in reanalysis data and regional climate models are compared to station observations. The regional models represent most indices of extreme temperature well. For extreme precipitation, finer grid spacing considerably improves the match to observations. Three regional models, the Weather Research and Forecasting (WRF) at 12- and 36-km grid spacing and the Hadley Centre Regional Model (HadRM) at 25-km grid spacing, are forced with global reanalysis fields over the U.S. Pacific Northwest during 2003–07. The reanalysis data represent the timing of rain-bearing storms over the Pacific Northwest well; however, the reanalysis has the worst performance at simulating both extreme precipitation indices and extreme temperature indices when compared to the WRF and HadRM simulations. These results suggest that the reanalysis data and, by extension, global climate model simulations are not sufficient for examining local extreme precipitations and temperatures owing to their coarse resolutions. Nevertheless, the large-scale forcing is adequately represented by the reanalysis so that regional models may simulate the terrain interactions and mesoscale processes that generate the observed local extremes and frequencies of extreme temperature and precipitation.

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Yanhong Gao, L. Ruby Leung, Yongxin Zhang, and Lan Cuo

Abstract

Net precipitation [precipitation minus evapotranspiration (PE)] changes between 1979 and 2011 from a high-resolution regional climate simulation and its reanalysis forcing are analyzed over the Tibetan Plateau (TP) and compared to the Global Land Data Assimilation System (GLDAS) product. The high-resolution simulation better resolves precipitation changes than its coarse-resolution forcing, which contributes dominantly to the improved PE change in the regional simulation compared to the global reanalysis. Hence, the former may provide better insights about the drivers of PE changes. The mechanism behind the PE changes is explored by decomposing the column integrated moisture flux convergence into thermodynamic, dynamic, and transient eddy components. High-resolution climate simulation improves the spatial pattern of PE changes over the best available global reanalysis. High-resolution climate simulation also facilitates new and substantial findings regarding the role of thermodynamics and transient eddies in PE changes reflected in observed changes in major river basins fed by runoff from the TP. The analysis reveals the contrasting convergence/divergence changes between the northwestern and southeastern TP and feedback through latent heat release as an important mechanism leading to the mean PE changes in the TP.

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Yongxin Zhang, Yi-Leng Chen, Thomas A. Schroeder, and Kevin Kodama

Abstract

Sea-breeze cases during 23–28 June 1978 over northwest Hawaii are simulated using the National Centers for Environmental Prediction (NCEP) Mesoscale Spectral Model (MSM) coupled with an advanced Land Surface Model (LSM) with 3-km horizontal resolution. Subjective analyses show that except for 27 June, the MSM–LSM-predicted onset time, duration, and vertical extent of the sea breezes agree well with observations. The largest mean absolute errors for surface air temperature occur at the coastal stations under strong trade wind conditions (e.g., 23 and 27 June). The model-simulated rainfall distribution in association with sea-breeze fronts is consistent with observations. Sensitivity tests demonstrate the modulation of sea-breeze behavior by surface properties. High-resolution (1 km) MSM–LSM simulations for 23 and 27 June show improvements over the 3-km MSM–LSM in reproducing the observed sea breezes through a better representation of local terrain and a better simulation of orographically enhanced trades channeling through the Waimea Saddle. Deficiencies noted in the model simulations include 1) sea-breeze speeds are more than 2–3 m s−1 weaker than observations, and 2) horizontal penetration of sea breezes is generally overestimated. These deficiencies in the model simulations are primarily related to two factors: one is the underestimation of the trade wind speeds in the initialization from the NCEP–NCAR reanalysis data that is favoring the farther penetration of the sea breezes, and the other is the uncertainties in the thermal properties of the lava rocks that affect the surface temperature and the sea-breeze speed.

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