Browse

You are looking at 1 - 10 of 121,337 items for

  • Refine by Access: All Content x
Clear All
Jason M. Apke, Yoo-Jeong Noh, and Kristopher Bedka

Abstract

This study introduces a validation technique for quantitative comparison of algorithms which retrieve winds from passive detection of cloud- and water vapor-drift motions, also known as Atmospheric Motion Vectors (AMVs). The technique leverages airborne wind-profiling lidar data collected in tandem with 1-min refresh rate geostationary satellite imagery. AMVs derived with different approaches are used with accompanying numerical weather prediction model data to estimate the full profiles of lidar-sampled winds which enables ranking of feature tracking, quality control, and height-assignment accuracy and encourages meso-scale, multi-layer, multi-band wind retrieval solutions. The technique is used to compare the performance of two brightness motion, or “optical flow,” retrieval algorithms used within AMVs, 1) Patch Matching (PM; used within operational AMVs) and 2) an advanced Variational Optical Flow (VOF) method enabled for most atmospheric motions by new-generation imagers. The VOF AMVs produce more accurate wind retrievals than the PM method within the benchmark in all imager bands explored. It is further shown that image regions with low texture and multi-layer-cloud scenes in visible and infrared bands are tracked significantly better with the VOF approach, implying VOF produces representative AMVs where PM typically breaks down. It is also demonstrated that VOF AMVs have reduced accuracy where the brightness texture does not advect with the mean wind (e.g. gravity waves), where the image temporal noise exceeds the natural variability, and when the height-assignment is poor. Finally, it is found that VOF AMVs have improved performance when using fine-temporal refresh rate imagery, such as 1-min versus 10-min data.

Restricted access
Yuval Yevnin and Yaron Toledo

Abstract

The paper presents a combined numerical–deep learning (DL) approach for improving wind and wave forecasting. First, a DL model is trained to improve wind velocity forecasts by using past reanalysis data. The improved wind forecasts are used as forcing in a numerical wave forecasting model. This novel approach, used to combine physics-based and data-driven models, was tested over the Mediterranean. The correction to the wind forecast resulted in ∼10% RMSE improvement in both wind velocity and wave height over reanalysis data. This significant improvement is even more substantial at the Aegean Sea when Etesian winds are dominant, improving wave height forecasts by over 35%. The additional computational costs of the DL model are negligible compared to the costs of either the atmospheric or wave numerical model by itself. This work has the potential to greatly improve the wind and wave forecasting models used nowadays by tailoring models to localized seasonal conditions, at negligible additional computational costs.

Significance Statement

Wind and wave forecasting models solve a set of complicated physical equations. Improving forecasting accuracy is usually achieved by using a higher-resolution, empirical coefficients calibration or better physical formulations. However, measurements are rarely used directly to achieve better forecasts, as their assimilation can prove difficult. The presented work bridges this gap by using a data-driven deep learning model to improve wind forecasting accuracy, and the resulting wave forecasting. Testing over the Mediterranean Sea resulted in ∼10% RMSE improvement. Inspecting the Aegean Sea when the Etesian wind is dominant shows an outstanding 35% improvement. This approach has the potential to improve the operational atmospheric and wave forecasting models used nowadays by tailoring models to localized seasonal conditions, at negligible computational costs.

Restricted access
Dingchi Zhao, Wenhao Dong, Yanluan Lin, Yang Hu, and Dianbin Cao

Abstract

Using abundant rainfall gauge measurements and Global Precipitation Mission (GPM) data, spatial patterns of rainfall diurnal cycles and their seasonality over high mountain Asia (HMA) were examined. Spatial distributions of rainfall diurnal cycles over the HMA have a prominent seasonality regulated by circulations at different spatiotemporal scales, within which large regional contrasts are embedded. Rainfall diurnal variability is relatively weak in the pre-monsoon season, with larger amplitude over the western HMA, the southeastern HMA as well as southern periphery regions, characterized by a dominant late afternoon to morning rainfall preference. The pattern of rainfall spatial distributions is closely related to the mid-latitude westerlies. Both the mean rainfall and amplitudes of diurnal cycles become more pronounced with the advance of monsoon season but weaken during post-monsoon. The widespread late afternoon to night pattern over HMA migrating with seasonal atmospheric circulation is consistent with the lifetime of convective systems, which become active from the afternoon due to radiative heating and decay during the night. Stationary terrain-dependent night to morning rainfall patterns are visible in those east-west orientated valleys over HMA and the Qaidam Basin throughout the seasons. This salient geographical dependence is associated with local circulation produced by the strong differential thermal conditions over mountains and valleys, which can lift the warm moist air at the mouth of the valley and trigger nocturnal convection.

Restricted access
Tianyi Wang and Tim Li

Abstract

The diversity of the Madden–Julian oscillation (MJO) initiation (i.e., initial onset of active convection before moving eastward) regions was explored using a clustering method. Regions favorable for MJO initiations are grouped into four longitude zones: the Atlantic and Africa (AA), the Indian Ocean (IO), the Maritime Continent (MC), and the western Pacific (WP). The region-dependent dominant initiation mechanisms are explored using a composite procedure. The AA initiation is attributed to a circumnavigating process associated with a preceding MJO. As upper-tropospheric westerly anomalies move into the AA region, the associated descending motion leads to suppressed convection over the IO, which further triggers convection onset to its west through anomalous westward moisture advection. The IO initiation arises from the downstream forcing of a preceding suppressed phase of MJO. A delayed air–sea interaction process also plays a role. The MC initiation is triggered by a westward-propagating dry equatorial Rossby wave in the Pacific. The low-level poleward flows associated with the anticyclonic Rossby wave gyres advect high mean moisture, promoting the convection onset over the MC. The WP initiation is triggered by a preceding suppressed phase of MJO that moves eastward, in a way similar to the downstream scenario in the IO. The AA initiation is usually associated with a La Niña–like background sea surface temperature pattern, which favors the decoupling of upper-tropospheric westerly anomalies from the preceding MJO. The MC and WP initiations are more frequent during El Niño, as the relevant meridional moisture gradient is sharper and the eastern Pacific is moister.

Restricted access
Mi-Kyung Sung, Seok-Woo Son, Changhyun Yoo, Jaeyoung Hwang, and Soon-Il An

Abstract

This study investigates the growth mechanisms of the North Pacific Oscillation (NPO), one of the primary causes of winter temperature extremes at midlatitudes. We find that the NPO has two distinct origins. One is the Rossby wave propagating across Eurasia that strengthens during cold or warm surges over East Asia. The corresponding vorticity and thermal anomalies grow into the NPO as they move eastward. The other is a local disturbance at the Asian Pacific jet stream exit that amplifies when propagating westward. The dynamical processes behind these two types of NPO growth are investigated by conducting quasigeostrophic geopotential tendency budget analysis, focusing on the relative importance of the vorticity flux and differential heat flux. It is revealed that the contribution of high-frequency eddies is greater in the NPO that grows from a local disturbance. However, NPO growth in both cases is primarily steered by low-frequency vorticity flux, which facilitates eastward or westward propagation through relative or planetary vorticity advections, respectively. Differential heat flux promotes the growth of the NPO before the onset, but dissipates the NPO anomalies afterward. The net effect of the heat flux, however, allows the NPO to amplify and persist through baroclinic instability by constraining the NPO anomalies to have westward vertical tilt. Accordingly, the NPO grows conforming to the dissipative destabilizing mechanism. From the perspective of potential vorticity (PV), the eastward growth of the NPO originates from the downstream advection of PV anomalies accumulated over East Asia, while westward growth is favored by a strong PV gradient near the jet stream.

Significance Statement

This study examines the origins and growth mechanism of the North Pacific Oscillation (NPO), which causes cold or warm spells in the midlatitudes during winter. Two distinct origins of the NPO are found, which are abnormal temperature events over East Asia and local atmospheric disturbances at the Asian Pacific jet stream exit. Evolutions of the NPO are traced by analyzing the roles of the vorticity and heat fluxes in addition to the contributions of synoptic eddies, which turns out to be more significant for the NPO that grows from a local disturbance. Vorticity flux overall dominates the growth of the NPO, but heat flux also helps the NPO amplify owing to background instability.

Open access
Lei Liu and Huijie Xue

Abstract

Observational surface data are utilized to reconstruct the subsurface density and geostrophic velocity fields via the “interior + surface quasigeostrophic” (isQG) method in a subdomain of the Antarctic Circumpolar Current (ACC). The input variables include the satellite-derived sea surface height (SSH), satellite-derived sea surface temperature (SST), satellite-derived or Argo-based sea surface salinity (SSS), and a monthly estimate of the stratification. The density reconstruction is assessed against a newly released high-resolution in situ dataset that is collected by a southern elephant seal. The results show that the observed mesoscale structures are reasonably reconstructed. In the Argo-SSS-based experiment, pattern correlations between the reconstructed and observed density mostly exceed 0.8 in the upper 300 m. Uncertainties in the SSS products notably influence the isQG performance, and the Argo-SSS-based experiment yields better density reconstruction than the satellite-SSS-based one. Through the two-dimensional (2D) omega equation, we further employ the isQG reconstructions to diagnose the upper-ocean vertical velocities (denoted w isQG2D), which are then compared against the seal-data-based 2D diagnosis of w seal. Notable discrepancies are found between w isQG2D and w seal, primarily because the density reconstruction does not capture the seal-observed smaller-scale signals. Within several subtransects, the Argo-SSS-based w isQG2D reasonably reproduce the spatial structures of w seal, but present smaller magnitude. We also apply the isQG reconstructions to the 3D omega equation, and the 3D diagnosis of w isQG3D is very different from w isQG2D, indicating the limitations of the 2D diagnostic equation. With reduced uncertainties in satellite-derived products in the future, we expect the isQG framework to achieve better subsurface estimations.

Restricted access
Kevin Gallo and Praveena Krishnan

Abstract

Satellite-derived observations of land surface temperature (LST) are being utilized in a growing number of land surface studies; however, these observations are generally obtained from optical sensors that exclude cloudy observations of the land surface. The impact of using only clear-sky observations of land surfaces on monthly and annual estimates of daytime LST over two U.S. Climate Reference Network (USCRN) sites was evaluated over five years with daily in situ LST observations available for all-sky (clear and cloudy) conditions. The in situ LST observations were obtained for the nominal daytime observations associated with the MODIS sensors on board the Terra and Aqua satellites and were identified as all-sky or clear-sky conditions by utilizing cloud information provided with the MODIS LST product. Both monthly/annual mean and monthly/annual maximum values of daytime LST were significantly different when only clear-sky values were utilized, in comparison with all-sky values. Monthly averaged differences between the mean clear- and all-sky daytime LST (dLST) values ranged from −0.1° ± 1.5°C for January to 5.6° ± 1.8°C for May. Annually averaged dLST values, over the five years of the study, were 2.58°C, and differences between the maximum values of clear- and all-sky daytime LST values were −1.03°C. Although significant differences between mean annual clear-sky and all-sky daytime LST values were more frequent than differences observed for the annual maximum daytime LST values, the results suggest that the exclusive use of either mean or maximum clear-sky daytime LST values is not advisable for applications in which the use of daytime all-sky LST values would be more applicable.

Restricted access
Junhong Wei, Fuqing Zhang, Jadwiga H. Richter, M. Joan Alexander, and Y. Qiang Sun

Abstract

Based on 20-day control forecasts by the 9-km Integrated Forecasting System (IFS) at the European Centre for Medium-Range Weather Forecasts (ECMWF) for selected periods of summer and winter events, this study investigates global distributions of gravity wave momentum fluxes resolved by the highest-resolution-ever global operational numerical weather prediction model. Two supplementary datasets, including 18-km ECMWF IFS experiments and the 30-km ERA5, are included for comparison. In the stratosphere, there is a clear dominance of westward momentum fluxes over the winter extratropics with strong baroclinic instability, while eastward momentum fluxes are found in the summer tropics. However, meridional momentum fluxes, locally as important as the above zonal counterpart, show different behaviors of global distribution characteristics, with northward and southward momentum fluxes alternating with each other especially at lower altitudes. Both events illustrate conclusive evidence that stronger stratospheric fluxes are found in the ECMWF forecast with finer resolution, and that ERA5 datasets have the weakest signals in general, regardless of whether regridding is applied. In the troposphere, probability distributions of vertical motion perturbations are highly asymmetric with more strong positive signals especially over latitudes covering heavy rainfall, likely caused by convective forcing. With the aid of precipitation accumulation, a simple filtering method is proposed in an attempt to eliminate those tropospheric asymmetries by convective forcing, before calculating tropospheric wave-induced fluxes. Furthermore, this research demonstrates promising findings that the proposed filtering method could help in reducing the potential uncertainties with respect to estimating tropospheric wave-induced fluxes. Finally, absolute momentum flux distributions with proposed approaches are presented, for further assessment in the future.

Restricted access
Zhixing Chen, Changwei Bian, Wensheng Jiang, Youyu Lu, Xinyan Mao, Xiaolei Liu, and Tao Wang

Abstract

A series of laboratory experiments are carried out to demonstrate the impacts of instrumented bottom frame legs on flow and turbulence. The magnitudes of vertical velocity, turbulent kinetic energy, dissipation, and shear stress induced by the frame legs depend on several factors, including the diameter and number of the frame legs, distances between the legs and the observational location, and the magnitude of the incoming flow and its direction with respect to the layout of the frame. In situ observations were carried out near the mouth of the Yellow River using two acoustic Doppler velocimeters mounted on a bottom frame. The estimated vertical velocity and turbulent kinetic energy dissipation rate show a significant asymmetry with flood and ebb tidal flows. This asymmetry can be partly explained by the influences of the bottom frame legs. Finally, the design and deployment principles of bottom frames are discussed for the purpose of reducing the impacts of the frame legs.

Significance Statement

Instrumented bottom frames are widely used for observations in the oceanic bottom boundary layer and above. However, the impacts of frame legs on the observed flow and turbulence have rarely been investigated. A series of laboratory experiments demonstrate that frame legs can induce vertical flow and enhanced turbulence, and the magnitudes of these influences vary with the size and layout of the frame legs and the magnitude and direction of the background flow. The results of the laboratory experiments can partially explain an “asymmetry” behavior of the vertical flow and turbulent kinetic energy with the flood and ebb tidal flows, derived from in situ observations near the mouth of the Yellow River.

Restricted access
Ying Yang, Zhiwei Zhu, Xinyong Shen, Leishan Jiang, and Tim Li

Abstract

As the most dominant tropical climate mode on the interannual timescale, El Niño-Southern Oscillation (ENSO) is suggested to significantly influence the interannual variation of East Asian summer monsoon rainfall (IEASMR). However, the leading mode of IEASMR remains almost untouched when the impacts of preceding ENSO events are linearly removed, suggesting the existence of alternative impact factors and predictability sources of IEASMR.

After removing the impact of ENSO, the sea surface temperature anomalies (SSTAs) over both the tropical Atlantic and extratropical North Atlantic are found to be related with IEASMR via atmospheric teleconnections. Positive SSTA over the tropical Atlantic could induce tropical diabatic heating, which triggers an equivalent barotropic Rossby wavetrain emanating from the Atlantic, going across the Eurasian continent and ending with a cyclonic anomaly over northeast Asia. The tropical diabatic heating could also induce western North Pacific anomalous anticyclone via tropical routes. The dipole SSTA pattern with cooling in the west and warming in the east over the extratropical North Atlantic induces local circulation anomalies via heat flux exchange, which could further perturb a Rossby wavetrain with a cyclonic anomaly over northeast Asia, thus modulating IEASMR. Numerical experiments with prescribed atmospheric heating associated with Atlantic SSTAs could realistically reproduce these teleconnections towards IEASMR.

By adding the predictability sources of Atlantic SSTAs, the seasonal hindcast skills of IEASMR could be significantly improved over both the tropical western North Pacific and subtropical land regions such as central China and Japan.

Restricted access