Browse

You are looking at 101 - 110 of 118,415 items for :

  • All content x
Clear All
Elizabeth A. Maroon, Stephen G. Yeager, Gokhan Danabasoglu, and Nan Rosenbloom

Abstract

The subpolar North Atlantic (SPNA) experienced extreme cold during 2015, an event often called the “cold blob.” The evolution of this event in the Community Earth System Model version 1 Decadal Prediction Large Ensemble (CESM1-DPLE) hindcast initialized in November 2014 is compared to observations. This CESM1-DPLE hindcast failed to predict cold conditions during 2015 despite already cold SPNA initial conditions and despite having high sea surface temperature skill in the SPNA in all other years. The goal of this paper is to understand what led to this prediction failure in order to provide insight for future decadal prediction efforts. Our analysis shows that strongly positive North Atlantic Oscillation (NAO) conditions during winter and spring 2015 likely sustained the cold blob but were not simulated in any CESM1-DPLE members. We examine the rarity of the 2015 event using the CESM1-DPLE’s uninitialized counterpart, the CESM1 Large Ensemble (CESM1-LE). Results from the CESM1-LE indicate that the exceptional state of the observed NAO in the winter of 2015 is at least part of the explanation for why this event was not encompassed in the CESM1-DPLE spread. To test another possibility—namely, that deficiencies in the initial conditions degraded the prediction—we performed additional hindcasts using the CESM1-DPLE protocol but different initial conditions. Altering the initial conditions did not improve the simulation of the 2015 cold blob, and in some cases, degraded it. Given the difficulty of predicting this event, this case could be a useful test bed for future prediction system development.

Open access
Abdulrahman Khamaj, Amin G. Alhashim, Vincent T. Ybarra, and Azham Hussain

Abstract

Communicating weather forecasts from the public perspective is essential for meeting people’s needs and enhancing their overall experiences. Because of the lack of cited work on the public’s behavior and perception of weather data and delivery sources in Middle Eastern countries such as Saudi Arabia (KSA), this study employs a cross-sectional questionnaire to fill the gap and apply the protective action decision model to non-Western individuals. The questionnaire examined respondents’ opinions about 1) the importance of weather forecast accessibility, 2) crucial weather features, and 3) available features on existing smartphone weather applications (apps) in KSA. The results showed that nearly all participants reported that their decisions of daily lives and activities were highly dependent on weather forecasts. Most participants thought weather forecast features are necessary. Although the most commonly used source for weather forecasts in KSA was smartphone apps, many participants responded that these apps were lacking specific weather functionalities (e.g., giving weather alerts to their exact location). Regression analyses found that KSA individuals who do not believe that weather forecasts are important are predicted by 1) not wanting any new features added to weather applications and 2) thinking that weather forecasts do not impact lives or property. This study’s findings can guide governmental and private weather agencies in KSA and other Middle Eastern or developing countries to better understand how to meet and communicate people’s weather needs.

Restricted access
W. D. Smyth, S. J. Warner, J. N. Moum, H. T. Pham, and S. Sarkar

Abstract

Factors thought to influence deep cycle turbulence in the equatorial Pacific are examined statistically for their predictive capacity using a 13-yr moored record that includes microstructure measurements of the turbulent kinetic energy dissipation rate. Wind stress and mean current shear are found to be most predictive of the dissipation rate. Those variables, together with the solar buoyancy flux and the diurnal mixed layer thickness, are combined to make a pair of useful parameterizations. The uncertainty in these predictions is typically 50% greater than the uncertainty in present-day in situ measurements. To illustrate the use of these parameterizations, the record of deep cycle turbulence, measured directly since 2005, is extended back to 1990 based on historical mooring data. The extended record is used to refine our understanding of the seasonal variation of deep cycle turbulence.

Open access
Clara Sophie Draper

Abstract

The ensembles used in the NOAA National Centers for Environmental Prediction (NCEP) global data assimilation and numerical weather prediction (NWP) system are under-dispersed at and near the land surface, preventing their use in ensemble-based land data assimilation. Comparison to offline (land-only) data assimilation ensemble systems suggests that while the relevant atmospheric fields are under-dispersed in NCEP’s system, this alone cannot explain the under-dispersed land component, and an additional scheme is required to explicitly account for land model error. This study then investigates several schemes for perturbing the soil (moisture and temperature) states in NCEP’s system, qualitatively comparing the induced ensemble spread to independent estimates of the forecast error standard deviation in soil moisture, soil temperature, 2m temperature, and 2m humidity. Directly adding perturbations to the soil states, as is commonly done in offline systems, generated unrealistic spatial patterns in the soil moisture ensemble spread. Application of a Stochastically Perturbed Physics Tendencies scheme to the soil states is inherently limited in the amount of soil moisture spread that it can induce. Perturbing the land model parameters, in this case vegetation fraction, generated a realistic distribution in the ensemble spread, while also inducing perturbations in the land (soil states) and atmosphere (2m states) that are consistent with errors in the land/atmosphere fluxes. The parameter perturbation method is then recommended for NCEP’s ensemble system, and it is currently being refined within the development of an ensemble-based coupled land/atmosphere data assimilation for NCEP’s NWP system.

Restricted access
Chongran Zhang, Jing Zhang, and Qigang Wu

Abstract

Enhanced surface melt over the ice shelves of the Antarctic Peninsula (AP) is one of the precursors to their collapse, which can be proceeded by accelerated ground glacier flow and increased contribution to sea level rise. With the collapse of Larsen A and B, and the major 2017 calving event from Larsen C, whether Larsen C is bound for a similar fate has received increasing attention. Here, the interannual variation of regional circulation over the AP region is studied using the Empirical Orthogonal Function (EOF) / Principal Component (PC) analysis on the sea level pressure of ERA5 reanalysis. The EOF modes capture the variations of depth, location and extent of Amundsen Sea Low and Weddell Sea Low in each season. Statistically significant positive correlations exist between Larsen C surface temperature and the PC time series of EOF mode 1 in winter and spring through northerly/northwesterly wind anomalies west of the AP. The PC time series of EOF mode 2 is negatively correlated with Larsen C surface temperature in autumn and summer and surface melt in summer, all due to southerly wind anomalies east of the AP. Surface energy budget analysis associated with EOF mode 2 shows that downwelling longwave radiation over Larsen C is negatively statistically significantly, correlated with EOF mode 2 and is the major atmospheric forcing regulating the variation of Larsen C surface melt. Positively enhanced EOF mode 2 since 2004 is responsible for the recent cooling and decline of surface melt over Larsen C.

Restricted access
BINGTIAN LI, ZEXUN WEI, YONGGANG WANG, XINYU GUO, TENGFEI XU, and XIANQING LV

Abstract

An enhanced harmonic analysis (S_TIDE) approach is adopted to examine the seasonal variations of internal tidal amplitudes in the northern South China Sea (SCS). Results of idealized experiments reveal that the seasonality can be captured by S_TIDE. By applying S_TIDE to mooring data, observed seasonality of internal tidal amplitudes in the northern SCS are explored. Not diurnal and semidiurnal internal tides (ITs), but overtides and long-period constituents of ITs exhibit clear seasonal cycles. However, differences between amplitudes of the eastward velocity and the northward counterpart are evident for K1, M2 and MK3, which may be caused by the intensification of background currents. Amplitudes of those ITs are stronger at intersection time between spring and summer in the eastward direction, but weaker in the northward direction. EOF analysis reveals that modes of diurnal ITs are higher than those of seimidiurnal ITs, which induces relatively more complicated seasonal variations. In addition to intensification of background currents, influences of surface tides and stratification will also induce variations of internal tidal amplitudes, introducing tremendous difficulty in predicting variation trends of internal tidal amplitudes, which greatly reduces predictability of ITs.

Restricted access
Xudong Wang, Shang-Ping Xie, Zhaoyong Guan, and Minyang Wang

Abstract

The summer intraseasonal oscillation (ISO) is characterized by a northward-moving rain band in the Indo-western Pacific warm pool region. Physical origin of the ISO is not fully understood, masked by strong interaction of convection and circulation. This study examines intraseasonal to interannual variability during June to August over the Indo-western Pacific warm pool region. The results show that the tropical Northwest Pacific anomalous anticyclone (NWP-AAC) is a fundamental mode on both intraseasonal and interannual timescales, destabilized by the monsoon mean state, specifically through barotropic energy conversion and convective feedback in the low-level confluence between the monsoon westerlies and easterly trade winds. On the interannual timescale, the NWP-AAC shows a biennial tendency, reversing phase from the summer of El Niño to the summer that follows; the AAC in post-El Niño summer is excited indirectly through sea surface temperature anomalies in the Indo-NWP. On the intraseasonal timescale, the column-integrated moisture advection causes the NWP-AAC-related convection propagates northward. Our results provide a unifying view of multi-scale Asian summer monsoon variability, with important implications for subseasonal to seasonal prediction.

Restricted access
Carsten S. Frederiksen, Xiaogu Zheng, and Simon Grainger

Abstract

Decadal and multi-decadal variability in the ERSSTv5 global SST dataset are studied in terms of implicit fast (noise) and slow (signal) processes that affect variability on decadal time scales. Using a new method that better estimates the fast, or noise, component of decadal variability, estimates of the modes of variability in the slow component are possible. The fast component of decadal variability has a leading fast mode, which explains 62% of the variance, and it is shown that this fast variability, or decadal climate noise, is well represented by any of the indices associated with intra-decadal or interannual variability in the tropical Pacific Ocean.

Three slow modes are identified, representing 69% of the slow multi-decadal variance, after removing the radiative forcing trend. These modes are shown to be related to variability in the Atlantic Multi-decadal Oscillation (AMO), and SST multi-decadal variability in the Central Western Pacific and in the Indian Ocean gyre region, respectively. The first and third slow modes represent two phases of a propagating mode with a period of about 80 years. The second slow mode represents multi-decadal variability of the Western Pacific Warm Pool which is less robust than the other two and shown to be weakly related to the AMO with a lag of about 30 years; fast variability in this region is related to the leading fast mode. Three regions of significant slow variability are identified south of Australia, south of Africa and near the Drake Passage in association with the Antarctic Circumpolar Current.

Restricted access
MORIO NAKAYAMA, HISASHI NAKAMURA, and FUMIAKI OGAWA

Abstract

As a major mode of annular variability in the Southern Hemisphere, the baroclinic annular mode (BAM) represents the pulsing of extratropical eddy activity. Focusing mainly on sub-weekly disturbances, this study assesses the impacts of a midlatitude oceanic frontal zone on the BAM and its dynamics through a set of “aqua-planet” atmospheric general circulation model experiments with zonally uniform sea-surface temperature (SST) profiles prescribed. Though idealized, one experiment with realistic frontal SST gradient reasonably well reproduces observed BAM-associated anomalies as a manifestation of a typical lifecycle of migratory baroclinic disturbances. Qualitatively, these BAM features are also simulated in the other experiment where the frontal SST gradient is removed. However, the BAM-associated variability weakens markedly and shifts equatorward, in association with the corresponding modifications in the climatological-mean stormtrack activity. The midlatitude oceanic frontal zone amplifies and anchors the BAM variability by restoring near-surface baroclinicity through anomalous sensible heat supply from the ocean and moisture supply to cyclones, although the BAM is essentially a manifestation of atmospheric internal dynamics. Those experiments and observations further indicate that the BAM modulates momentum flux associated with transient disturbances to induce a modest but robust meridional shift of the polar-front jet, suggesting that the BAM can help maintain the southern annular mode. They also indicate that the quasi-periodic behavior of the BAM is likely to reflect internal dynamics in which atmospheric disturbances on both sub-weekly and longer time scales are involved.

Restricted access
Thomas A. Green Jr., Daniel Leins, Gary M. Lackmann, James Morrow, and Jonathan Blaes

Abstract

Nearly 100 North Carolina State University students have participated in a unique, highly structured internship course conducted by the National Weather Service Forecast Office in Raleigh, NC. Here, we explore the impact that this course has had on their professional development and career trajectories. The course has now been running for 17 years, and this paper provides an update on how the course has changed over time, including an evolution of the interview process to participate in the course, the number of students enrolled each semester has systematically been lowered to allow for more individual attention, and additional experiences outside of the WFO have been added. There are benefits for the students, with about half of the students now employed by the NWS, and nearly universal praise for how the course impacted their career progression. The university benefits from the course because the course serves as a compelling selling point for the MEAS department when recruiting students and the department also ensures that the curriculum is adequately preparing potential students for the job market. Finally, the NWS gains by creating a pool of potential employees that will require less spin-up time if hired, and graduates of the NCSU program have gone on to be involved with similar student volunteer programs at their respective offices once hired.

Full access