scholarly journals Structure of the Western Tibetan Vortex inconsistent with a thermally-direct circulation

2021 ◽  
Author(s):  
Xiao-Feng Li ◽  
Jingjing Yu ◽  
Shaofeng Liu ◽  
Jingzhi Wang ◽  
Lei Wang
Keyword(s):  
Large Scale ◽  
Surface Air Temperature ◽  
Circulation Model ◽  
Circulation Pattern ◽  
Net Radiation ◽  
Near Surface ◽  
Radiative Balance ◽  
Air Temperatures ◽  
Western Tibetan Plateau ◽  
Global Atmospheric Circulation

AbstractThe Western Tibetan Vortex (WTV) is a large-scale circulation pattern identified from year-to-year circulation variability, which was used to understand the causal mechanisms for slowdown of the glacier melting over the western Tibetan Plateau (TP). A recent argument has suggested the WTV is the set of wind field anomalies resulting from variability in near-surface air temperatures over the western TP (above 1500 m), which, in turn, is likely driven by the surface net radiation. This study thereby evaluates the above putative thermal-direct mechanism. By conducting numerical sensitivity experiments using a global atmospheric circulation model, SAMIL, we find a WTV-like structure cannot be generated from a surface thermal forcing imposed on the western TP. A thermally-direct circulation generated by the surface or near surface heating is expect to cause upward motions and a baroclinic structure above it. In contrast, downward motions and a quasi-barotropic are observed in the vertical structure of the WTV. Besides, we find variability of the surface net radiation (sum of the surface shortwave and longwave net radiation) over the western TP can be traced back to the WTV variability based on ERA5 data. The anticyclonic (cyclonic) WTV reduces (increases) the cloudiness through the anomalous downward (upward) motions, causes more (less) input shortwave net radiation and thereby more (less) surface net radiations, resulting in the warmer (cooler) surface and near-surface air temperature over the western TP. The argument is constructive in encouraging examination of the radiative balance processes that complements previous studies.

scholarly journals Polar Amplification and Ice Free Conditions under 1.5, 2 and 3 °C of Global Warming as Simulated by CMIP5 and CMIP6 Models

Atmosphere ◽  
2021 ◽  
Vol 12 (11) ◽  
pp. 1494
Author(s):  
Fernanda Casagrande ◽  
Francisco A. B. Neto ◽  
Ronald B. de Souza ◽  
Paulo Nobre
Keyword(s):  
Global Warming ◽  
Temperature Response ◽  
Surface Air Temperature ◽  
The Arctic ◽  
Polar Regions ◽  
High Latitudes ◽  
Near Surface ◽  
Polar Amplification ◽  
Air Temperatures ◽  
Average Rise

One of the most visible signs of global warming is the fast change in the polar regions. The increase in Arctic temperatures, for instance, is almost twice as large as the global average in recent decades. This phenomenon is known as the Arctic Amplification and reflects several mutually supporting processes. An equivalent albeit less studied phenomenon occurs in Antarctica. Here, we used numerical climate simulations obtained from CMIP5 and CMIP6 to investigate the effects of +1.5, 2 and 3 °C warming thresholds for sea ice changes and polar amplification. Our results show robust patterns of near-surface air-temperature response to global warming at high latitudes. The year in which the average air temperatures brought from CMIP5 and CMIP6 models rises by 1.5 °C is 2024. An average rise of 2 °C (3 °C) global warming occurs in 2042 (2063). The equivalent warming at northern (southern) high latitudes under scenarios of 1.5 °C global warming is about 3 °C (1.8 °C). In scenarios of 3 °C global warming, the equivalent warming in the Arctic (Antarctica) is close to 7 °C (3.5 °C). Ice-free conditions are found in all warming thresholds for both the Arctic and Antarctica, especially from the year 2030 onwards.

scholarly journals An evaluation of the impact of aerosol particles on weather forecasts from a biomass burning aerosol event over the Midwestern United States: observational-based analysis of surface temperature

2016 ◽  
Vol 16 (10) ◽  
pp. 6475-6494 ◽  
Author(s):  
Jianglong Zhang ◽  
Jeffrey S. Reid ◽  
Matthew Christensen ◽  
Angela Benedetti
Keyword(s):  
United States ◽  
Air Temperature ◽  
Surface Air Temperature ◽  
Cooling Efficiency ◽  
Surface Cooling ◽  
Near Surface ◽  
Smoke Aerosol ◽  
Weather Forecasts ◽  
Air Temperatures ◽  
Daily Changes

Abstract. A major continental-scale biomass burning smoke event from 28–30 June 2015, spanning central Canada through the eastern seaboard of the United States, resulted in unforecasted drops in daytime high surface temperatures on the order of 2–5  °C in the upper Midwest. This event, with strong smoke gradients and largely cloud-free conditions, provides a natural laboratory to study how aerosol radiative effects may influence numerical weather prediction (NWP) forecast outcomes. Here, we describe the nature of this smoke event and evaluate the differences in observed near-surface air temperatures between Bismarck (clear) and Grand Forks (overcast smoke), to evaluate to what degree solar radiation forcing from a smoke plume introduces daytime surface cooling, and how this affects model bias in forecasts and analyses. For this event, mid-visible (550 nm) smoke aerosol optical thickness (AOT, τ) reached values above 5. A direct surface cooling efficiency of −1.5 °C per unit AOT (at 550 nm, τ550) was found. A further analysis of European Centre for Medium-Range Weather Forecasts (ECMWF), National Centers for Environmental Prediction (NCEP), United Kingdom Meteorological Office (UKMO) near-surface air temperature forecasts for up to 54 h as a function of Moderate Resolution Imaging Spectroradiometer (MODIS) Dark Target AOT data across more than 400 surface stations, also indicated the presence of the daytime aerosol direct cooling effect, but suggested a smaller aerosol direct surface cooling efficiency with magnitude on the order of −0.25 to −1.0 °C per unit τ550. In addition, using observations from the surface stations, uncertainties in near-surface air temperatures from ECMWF, NCEP, and UKMO model runs are estimated. This study further suggests that significant daily changes in τ550 above 1, at which the smoke-aerosol-induced direct surface cooling effect could be comparable in magnitude with model uncertainties, are rare events on a global scale. Thus, incorporating a more realistic smoke aerosol field into numerical models is currently less likely to significantly improve the accuracy of near-surface air temperature forecasts. However, regions such as eastern China, eastern Russia, India, and portions of the Saharan and Taklamakan deserts, where significant daily changes in AOTs are more frequent, are likely to benefit from including an accurate aerosol analysis into numerical weather forecasts.

scholarly journals Suitability of a constant air temperature lapse rate over an Alpine glacier: testing the Greuell and Böhm model as an alternative

2013 ◽  
Vol 54 (63) ◽  
pp. 120-130 ◽  
Author(s):  
Lene Petersen ◽  
Francesca Pellicciotti ◽  
Inge Juszak ◽  
Marco Carenzo ◽  
Ben Brock
Keyword(s):  
Air Temperature ◽  
Surface Air Temperature ◽  
Ambient Air ◽  
Lapse Rate ◽  
Spatial And Temporal Variability ◽  
Near Surface ◽  
Alpine Glacier ◽  
Temperature Lapse Rate ◽  
Boundary Layer Height ◽  
Air Temperatures

AbstractNear-surface air temperature, typically measured at a height of 2 m, is the most important control on the energy exchange and the melt rate at a snow or ice surface. It is distributed in a simplistic manner in most glacier melt models by using constant linear lapse rates, which poorly represent the actual spatial and temporal variability of air temperature. In this paper, we test a simple thermodynamic model proposed by Greuell and Böhm in 1998 as an alternative, using a new dataset of air temperature measurements from along the flowline of Haut Glacier d’Arolla, Switzerland. The unmodified model performs little better than assuming a constant linear lapse rate. When modified to allow the ratio of the boundary layer height to the bulk heat transfer coefficient to vary along the flowline, the model matches measured air temperatures better, and a further reduction of the root-mean-square error is obtained, although there is still considerable scope for improvement. The modified model is shown to perform best under conditions favourable to the development of katabatic winds – few clouds, positive ambient air temperature, limited influence of synoptic or valley winds and a long fetch – but its performance is poor under cloudy conditions.

scholarly journals The typhoon-induced drying of the Maritime Continent

2020 ◽  
Vol 117 (8) ◽  
pp. 3983-3988 ◽  
Author(s):  
Enrico Scoccimarro ◽  
Silvio Gualdi ◽  
Alessio Bellucci ◽  
Daniele Peano ◽  
Annalisa Cherchi ◽  
...  
Keyword(s):  
Tropical Cyclones ◽  
Numerical Experiments ◽  
General Circulation ◽  
Circulation Model ◽  
Circulation Pattern ◽  
Maritime Continent ◽  
Global Circulation ◽  
Global Atmospheric Circulation ◽  
Definition Of ◽  
Convective Condensation

The Maritime Continent plays a role in the global circulation pattern, due to the energy released by convective condensation over the region which influences the global atmospheric circulation. We demonstrate that tropical cyclones contribute to drying the Maritime Continent atmosphere, influencing the definition of the onset of the dry season. The process was investigated using observational data and reanalysis. Our findings were confirmed by numerical experiments using low- and high-resolution versions of the CMCC-CM2 General Circulation Model contributing to the HighResMIP CMIP6 effort.

Which hydro-meteorological variables control large-scale photosynthesis?

2020 ◽  
Author(s):  
Wantong Li ◽  
Mirco Migliavacca ◽  
Yunpeng Luo ◽  
René Orth
Keyword(s):  
Soil Moisture ◽  
Land Surface ◽  
Vegetation Dynamics ◽  
Large Scale ◽  
Global Analysis ◽  
Spatial Scales ◽  
Meteorological Variables ◽  
Climate Projections ◽  
Net Radiation ◽  
Near Surface

<p>Vegetation dynamics are determined by a multitude of hydro-meteorological variables, and this interplay changes in space and time. Due to its complexity, it is still not fully understood at large spatial scales. This knowledge gap contributes to increased uncertainties in future climate projections because large-scale photosynthesis is influencing the exchange of energy and water between the land surface and the atmosphere, thereby potentially impacting near-surface weather. In this study, we explore the relative importance of several hydro-meteorological variables for vegetation dynamics. For this purpose, we infer the correlations of anomalies in temperature, precipitation, soil moisture, VPD, surface net radiation and surface downward solar radiation with respective anomalies of photosynthetic activity as inferred from Sun-Induced chlorophyll Fluorescence (SIF). To detect changing hydro-meteorological controls across different climate conditions, this global analysis distinguishes between climate regimes as determined by long-term mean aridity and temperature. The results show that soil moisture was the most critical driver with SIF in the simultaneous correlation with dry and warm conditions, while temperature and VPD was both influential on cold and wet regimes during the study period 2007-2018. We repeat our analysis by replacing the SIF data with NDVI, as a proxy for vegetation greenness, and find overall similar results, except for surface net radiation expanding controlled regions on cold and wet regimes. As the considered hydro-meteorological variables are inter-related, spurious correlations can occur. We test different approaches to investigate and account for this phenomenon. The results can provide new insight into mechanisms of vegetation-water-energy interactions and contribute to improve dynamic global vegetation models.</p>

scholarly journals Comparison of Machine-Learning Algorithms for Near-Surface Air-Temperature Estimation from FY-4A AGRI Data

2020 ◽  
Vol 2020 ◽  
pp. 1-14
Author(s):  
Ke Zhou ◽  
Hailei Liu ◽  
Xiaobo Deng ◽  
Hao Wang ◽  
Shenglan Zhang
Keyword(s):  
Machine Learning ◽  
Air Temperature ◽  
Large Scale ◽  
Weather Prediction ◽  
Surface Air Temperature ◽  
Machine Learning Algorithms ◽  
Gradient Boosting ◽  
Temporal And Spatial Distribution ◽  
Near Surface ◽  
Extreme Gradient Boosting

Six machine-learning approaches, including multivariate linear regression (MLR), gradient boosting decision tree, k-nearest neighbors, random forest, extreme gradient boosting (XGB), and deep neural network (DNN), were compared for near-surface air-temperature (Tair) estimation from the new generation of Chinese geostationary meteorological satellite Fengyun-4A (FY-4A) observations. The brightness temperatures in split-window channels from the Advanced Geostationary Radiation Imager (AGRI) of FY-4A and numerical weather prediction data from the global forecast system were used as the predictor variables for Tair estimation. The performance of each model and the temporal and spatial distribution of the estimated Tair errors were analyzed. The results showed that the XGB model had better overall performance, with R2 of 0.902, bias of −0.087°C, and root-mean-square error of 1.946°C. The spatial variation characteristics of the Tair error of the XGB method were less obvious than those of the other methods. The XGB model can provide more stable and high-precision Tair for a large-scale Tair estimation over China and can serve as a reference for Tair estimation based on machine-learning models.

scholarly journals Role of Large-Scale Circulation and Terrain in Causing Extreme Heat in Western North China

2016 ◽  
Vol 29 (7) ◽  
pp. 2511-2527 ◽  
Author(s):  
Ruidan Chen ◽  
Riyu Lu
Keyword(s):  
Large Scale ◽  
North China ◽  
Surface Air Temperature ◽  
Extreme Heat ◽  
Large Scale Circulation ◽  
Air Temperatures ◽  
Higher Temperature ◽  
Upper Level ◽  
Topographic Relief

Abstract Previous studies have suggested that, because of its particular location on the southeastern lee side of mountains, extreme heat (EH) over western north China (WNC) is affected by the foehn phenomenon. In this study, the EH days during summer over this region are categorized into foehn-favorable EH and no-foehn EH, according to whether there are anomalous northwesterlies over mountains, and composite analyses are performed on them. The analyzed results indicate that the no-foehn EH is characterized by an anticyclonic anomaly and a large-scale higher surface air temperature, while the foehn-favorable EH is featured by a cyclonic anomaly to the northeast and a localized higher temperature. Associated with the cyclonic anomaly, northwesterlies prevail over the mountain surface and provide a favorable environment for the occurrence of the foehn effect over WNC, which is located on the southeastern lee side of mountains. That is, both cyclonic and anticyclonic anomalies can induce EH over WNC (i.e., foehn-favorable EH and no-foehn EH, respectively). Further investigation indicates that large-scale cyclonic and anticyclonic anomalies tend to favor local descent and ascent anomalies over the lee side, respectively, through interaction with the particular terrain. Therefore, large-scale circulations and local terrain-induced winds play an offsetting role in affecting the surface air temperatures over WNC, and EH occurs when anomalous large-scale anticyclone or terrain-induced descent dominate. This study implies that attention should be paid to not only the upper-level/large-scale circulations but also to their impact on lower-level/local winds for temperature variability over the places with great topographic relief worldwide.

scholarly journals Urban-Induced Changes on Local Circulation in Complex Terrain: Central Mexico Basin

Atmosphere ◽  
2021 ◽  
Vol 12 (7) ◽  
pp. 904
Author(s):  
Lourdes P. Aquino-Martínez ◽  
Arturo I. Quintanar ◽  
Carlos A. Ochoa-Moya ◽  
Erika Danaé López-Espinoza ◽  
David K. Adams ◽  
...  
Keyword(s):  
Large Scale ◽  
Meridional Circulation ◽  
Circulation Pattern ◽  
Heat Fluxes ◽  
Local Circulation ◽  
Pressure System ◽  
Near Surface ◽  
Zonal Circulation ◽  
Downslope Wind ◽  
Mexico Basin

Land use land cover (LULC) significantly impacts local circulation in the Mexico Basin, particularly wind field circulations such as gap winds, convergence lines, and thermally induced upslope/downslope wind. A case study with a high-pressure system over the Mexico Basin isolates the influence of large-scale synoptic forcing. Numerical simulations reveal a wind system composed of meridional circulation and a zonal component. Thermal pressure gradients between the Mexico basin and its colder surroundings create near-surface convergence lines as part of the meridional circulation. Experiments show that the intensity and organization of meridional circulations and downslope winds increase when LULC changes from natural and cultivated land to urban. Zonal circulation exhibits a typical circulation pattern with the upslope flow and descending motion in the middle of the basin. Large values of moist static energy are near the surface where air parcels pick up energy from the surface either as fluxes of enthalpy or latent heat. Surface heat fluxes and stored energy in the ground are drivers of local circulation, which is more evident in zonal circulation patterns.

Relative contributions of internal atmospheric variability and surface processes to the interannual variations in wintertime Arctic surface air temperatures

2021 ◽  
pp. 1-48
Author(s):  
Fengmin Wu ◽  
Wenkai Li ◽  
Peng Zhang ◽  
Wei Li
Keyword(s):  
Sea Ice ◽  
Large Scale ◽  
Surface Air Temperature ◽  
Surface Processes ◽  
The Arctic ◽  
Interannual Variations ◽  
Atmospheric Variability ◽  
Near Surface ◽  
Atmospheric Circulations ◽  
Downward Longwave Radiation

AbstractSuperimposed on a warming trend, Arctic winter surface air temperature (SAT) exhibits substantial interannual variability, whose underlying mechanisms are unclear, especially regarding the role of sea-ice variations and atmospheric processes. Here, atmospheric reanalysis data and idealized atmospheric model simulations are used to reveal the mechanisms by which sea-ice variations and atmospheric anomalous conditions affect interannual variations in wintertime Arctic SAT. Results show that near-surface interannual warming in the Arctic is accompanied by comparable warming throughout large parts of the Arctic troposphere and large-scale anomalous atmospheric circulation patterns. Within the Arctic, changes in large-scale atmospheric circulations due to internal atmospheric variability explain a substantial fraction of interannual variation in SAT and tropospheric temperatures, which lead to an increase in moisture and downward longwave radiation, with the rest likely coming from sea ice-related and other surface processes. Arctic winter sea-ice loss allows the ocean to release more heat and moisture, which enhances Arctic warming; however, this effect on SAT is confined to the ice-retreat area and has a limited influence on large-scale atmospheric circulations.

scholarly journals How Frequent is Precipitation over the Contiguous United States? Perspectives from Ground-Based and Spaceborne Radars

2017 ◽  
Vol 18 (6) ◽  
pp. 1657-1672 ◽  
Author(s):  
Mark Smalley ◽  
Pierre-Emmanuel Kirstetter ◽  
Tristan L’Ecuyer
Keyword(s):  
United States ◽  
Complex Terrain ◽  
Surface Air Temperature ◽  
Precipitation Frequency ◽  
Near Surface ◽  
Flat Surfaces ◽  
Air Temperatures ◽  
Temporal And Spatial ◽  
Beam Height ◽  
Precipitation Events

Abstract High temporal and spatial resolution observations of precipitation occurrence from the NEXRAD-based Multi-Radar Multi-Sensor (MRMS) system are compared to matched observations from CloudSat for 3 years over the contiguous United States (CONUS). Across the CONUS, precipitation is generally reported more frequently by CloudSat (7.8%) than by MRMS (6.3%), with dependence on factors such as the NEXRAD beam height, the near-surface air temperature, and the surface elevation. There is general agreement between ground-based and satellite-derived precipitation events over flat surfaces, especially in widespread precipitation events and when the NEXRAD beam heights are low. Within 100 km of the nearest NEXRAD site, MRMS reports a precipitation frequency of 7.54% while CloudSat reports 7.38%. However, further inspection reveals offsetting biases between the products, where CloudSat reports more snow and MRMS reports more rain. The magnitudes of these discrepancies correlate with elevation, but they are observed in both the complex terrain of the Rocky Mountains and the relatively flat midwestern areas of the CONUS. The findings advocate for caution when using MRMS frequency and accumulations in complex terrain, when temperatures are below freezing, and at ranges greater than 100 km. A multiresolution analysis shows that no more than 1.88% of CloudSat pixels over flat terrain are incorrectly identified as nonprecipitating as a result of shallow showers residing the CloudSat clutter-filled blind zone when near-surface air temperatures are above 15°C.

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