The Western Tibetan Vortex as an emergent feature of near-surface temperature variations

2020 ◽  
Author(s):  
Remco de Kok ◽  
Walter Immerzeel
Keyword(s):  
Surface Temperature ◽  
20Th Century ◽  
Large Scale ◽  
High Mountain ◽  
Net Radiation ◽  
Near Surface ◽  
Temperature Variations ◽  
Surface Temperatures ◽  
Emergent Feature ◽  
Main Driver

<p>Glaciers are growing in a part of High Mountain Asia (HMA), contrary to the demise of glaciers worldwide. A proposed explanation for this behaviour is the decreasing strength of the "Western Tibetan Vortex" (WTV), a circular motion of air in the troposphere around northwestern High Mountain Asia, which is proposed to drive near-surface temperatures. Here, we show that the WTV is the change of wind field resulting from changes in near-surface temperature, and that it is not unique to northwestern HMA, but is generally applicable to large parts of the globe. Instead, we argue that net radiation is likely the main driver of near-surface temperatures in Western HMA in summer and autumn, and that the WTV is the response of the atmosphere to changes in temperature. The decreasing strength of the WTV, as seen during summer in the 20th century, is thus likely the result of changing net radiation, and not the main driver of cooling itself. We do argue that the WTV is a useful concept to understand large scale climate variability in the region, and that such an approach could yield important insights in other mid-latitude regions as well.</p>

scholarly journals Sensitivity of Attribution of Anthropogenic Near-Surface Warming to Observational Uncertainty

2017 ◽  
Vol 30 (12) ◽  
pp. 4677-4691 ◽  
Author(s):  
Gareth S. Jones ◽  
John J. Kennedy
Keyword(s):  
Twentieth Century ◽  
Surface Temperature ◽  
Greenhouse Gas ◽  
Large Scale ◽  
Near Surface ◽  
Bias Adjustment ◽  
Detection Analysis ◽  
Surface Temperatures ◽  
Observational Uncertainty ◽  
The Impact

The impact of including comprehensive estimates of observational uncertainties on a detection and attribution analysis of twentieth-century near-surface temperature variations is investigated. The error model of HadCRUT4, a dataset of land near-surface air temperatures and sea surface temperatures, provides estimates of measurement, sampling, and bias adjustment uncertainties. These uncertainties are incorporated into an optimal detection analysis that regresses simulated large-scale temporal and spatial variations in near-surface temperatures, driven by well-mixed greenhouse gas variations and other anthropogenic and natural factors, against observed changes. The inclusion of bias adjustment uncertainties increases the variance of the regression scaling factors and the range of attributed warming from well-mixed greenhouse gases by less than 20%. Including estimates of measurement and sampling errors has a much smaller impact on the results. The range of attributable greenhouse gas warming is larger across analyses exploring dataset structural uncertainty. The impact of observational uncertainties on the detection analysis is found to be small compared to other sources of uncertainty, such as model variability and methodological choices, but it cannot be ruled out that on different spatial and temporal scales this source of uncertainty may be more important. The results support previous conclusions that there is a dominant anthropogenic greenhouse gas influence on twentieth-century near-surface temperature increases.

scholarly journals The influence of the forest on night-time snow surface temperature

2001 ◽  
Vol 32 ◽  
pp. 217-222 ◽  
Author(s):  
Peter Höller
Keyword(s):  
Surface Temperature ◽  
Open Field ◽  
Air Temperature ◽  
Forest Canopy ◽  
Small Range ◽  
Snow Surface ◽  
Net Radiation ◽  
Night Time ◽  
Near Surface ◽  
Snow Surface Temperature

AbstractSnow surface temperature (Ts) plays an important role in the formation of surface hoar or near-surface faceted crystals The goal of this study was to obtain detailed information on Ts in different forest stands nelr the timberline. The investigations were conducted during clear nights and showed that the snow surface temperature is influenced very strongly by the forest canopy. While the air temperature was very similar on the different experimental sites, Ts was higher in the forest than in the open field; on the south-facing slope the difference between the forest and the open field was 3–4.5°C, and on the north-facing slope approximately 3–7°C. Taking into account that εair is 0.7 and εtree is 0.94, the incoming radiation (I ↓) for the different experimental sites was calculated by the equation of Brunt (the canopy density was estimated using photographs taken with an 8 mm fish-eye). To calculate Ts, air temperature and averaged values of the net radiation (because the net radiation (I) has only a small range of variation during clear nights) were used. The results show that the calculated values were higher than the measured values (by approximately 2°C). However, a better correlation was found by using lower values of the emissivity (εair0.67 and εtree0.91).

A DUAL‐WAVELENGTH THERMAL INFRARED SCANNER AS A POTENTIAL AIRBORNE GEOPHYSICAL EXPLORATION TOOL

Geophysics ◽  
1976 ◽  
Vol 41 (6) ◽  
pp. 1318-1336 ◽  
Author(s):  
Leonard A. LeSchack ◽  
Nancy Kerr Del Grande
Keyword(s):  
New York ◽  
Soil Moisture ◽  
Surface Temperature ◽  
Absolute Temperature ◽  
Surface Emissivity ◽  
Field Site ◽  
Ir Radiation ◽  
Temperature Variations ◽  
Heat Flows ◽  
Surface Temperatures

We are investigating a new airborne method for measuring surface temperatures that may be useful for identifying thermal anomalies of geologic origin. From Planck’s equation we derive the valuable approximation that, for small temperature variations, the radiant emittance is proportional to the emissivity times the absolute temperature to the power of (50/wavelength in μm). From this, expressions are obtained for the emitted infrared (ir) radiation measured simultaneously in the 5 and 10 μm bands. Ratios of these expressions are shown to have the following useful properties at 288 K: (a) they are insensitive to surface emissivity variations for vegetated terrain, (b) they vary nearly as the 5th power of the surface temperature, and (c) they distinguish emissivity‐related from temperature‐related effects. We have made preliminary tests of this methodology at a field site in Scipio Center, New York. We have characterized the observed surface temperature variations, the significant effects of soil moisture, and separated out the purely emissivity‐related features of vegetated terrain. Cluster analysis served to divide the ir data into groups that behave similarly as a function of the measured soil moisture. Two such distinct terrain groups were identified at the field site. The ir data were corrected for: (a) natural surface emissivity variations, (b) the intervening atmospheric path, and (c) the reflected sky radiation. The corrected surface temperature data were compared with calculated values computed from a model that simulates the surface temperature, using meteorological, hydrological, topographical, and soil thermal input parameters. The simulated mean surface temperatures, 291.9 K (group 1) and 291.6 K (group 2), differed only by, respectively, 0.0 K and 0.1 K from the measured mean surface temperatures. Our preliminary results suggest the potential for developing a new airborne geophysical method for isolating abnormal heat flows. Weak heat flows, about 10–20 times the terrestrial average, have the effect of raising the surface temperature about 0.1–0.2 K. These temperature anomalies would, with the methodology suggested, appear as a residual difference between the measured (corrected) surface temperature and the simulated surface temperature. Such surface temperature differences appear, from our research, to be measurable by airborne ir scanners when data over surface areas of [Formula: see text] or larger are averaged. Accordingly, our research appears to support the conclusion that surface temperature enhancements of geophysical origin between 0.1 and 0.2 K can be identified using airborne infrared methods.

Cessation of ice-wedge development during the 20th century in spruce forests of eastern Mackenzie Delta, Northwest Territories, Canada

2007 ◽  
Vol 44 (11) ◽  
pp. 1503-1515 ◽  
Author(s):  
S V Kokelj ◽  
M FJ Pisaric ◽  
C R Burn
Keyword(s):  
20Th Century ◽  
Ice Age ◽  
Spruce Forest ◽  
Mackenzie Delta ◽  
Study Region ◽  
Near Surface ◽  
Spruce Forests ◽  
Surface Temperatures ◽  
Forest Sites ◽  
Ice Wedge

Ice wedges are presently inactive in white spruce (Picea glauca) forests of eastern Mackenzie Delta as shown by the absence of vein ice above ice wedges, the maintenance of intact breaking cables, and the abundance of rootlets propagating across ridge–trough sequences. At spruce forest sites, near-surface ground cooling rates and minimum near-surface temperatures from the years 2003–2005 were above ice-wedge cracking thresholds. Ground thermal conditions associated with cracking were recorded at a tundra peatland with active ice wedges. Annual mean permafrost temperatures at the spruce forest sites ranged from –1.8 to –2.9 °C, whereas at the tundra peatland, the permafrost was colder than –6 °C. Although winter air temperatures are similar throughout the study region, deeper snow cover, thicker active layers, and warmer permafrost account for the more gradual seasonal cooling and warmer near-surface temperatures recorded at the subarctic forest sites. The subtle ridge to trough relief, 12–35 cm of permafrost above wedge ice, roots up to 80 years old grown across ice wedges, and negligible tritium levels in wedge ice indicate that thermal contraction cracking in the spruce forests has been infrequent throughout much of the last century. The proximity of wedge ice to the base of the aggrading permafrost table and the absence of old spruce roots spanning ice-wedge troughs suggest that ice-wedge cracking did occur in the forest environments during the cold and dry conditions associated with the Little Ice Age and early part of the 20th century. When these ice wedges cracked, minimum temperatures at the top of permafrost were probably at least 3–8 °C colder than presently observed and similar to present conditions at the tundra peatland.

Analyzing Near Surface Temperature Inversions Across the Greenland Ice Sheet Using In-situ, Remote Sensing, and Reanalysis Data

2020 ◽  
Author(s):  
Alden Adolph ◽  
Wesley Brown ◽  
Karina Zikan ◽  
Robert Fausto
Keyword(s):  
Surface Temperature ◽  
Energy Exchange ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Near Surface ◽  
Surface Temperatures ◽  
Air Temperatures ◽  
Exchange Processes ◽  
Temperature Inversions

<p>As Arctic temperatures have increased, the Greenland Ice Sheet has exhibited a negative mass balance, with a substantial and increasing fraction of mass loss due to surface melt. Understanding surface energy exchange processes in Greenland is critical for our ability to predict changes in mass balance. In-situ and remotely sensed surface temperatures are useful for monitoring trends, melt events, and surface energy balance processes, but these observations are complicated by the fact that surface temperatures and near surface air temperatures can significantly differ due to the presence of inversions that exist across the Arctic. Our previous work shows that even in the summer, very near surface inversions are present between the 2m air and surface temperatures a majority of the time at Summit, Greenland. In this study, we expand upon these results and combine a variety of data sources to quantify differences between surface snow/ice temperatures and 2m air temperatures across the Greenland Ice Sheet and investigate controls on the magnitude of these near surface temperature inversions. In-situ temperatures, wind speed, specific humidity, and albedo data are provided from automatic weather stations operated by the Programme for Monitoring of the Greenland Ice Sheet (PROMICE). We use the Clouds and the Earth's Radiant Energy System (CERES) cloud area fraction data to analyze effects of cloud presence on near surface temperature gradients. The in-situ temperatures are compared to Modern-Era Retrospective analysis for Research and Applications Version 2 (MERRA-2) and Moderate Resolution Imaging Spectrometer (MODIS) ice surface temperature data to extend findings across the ice sheet. Using PROMICE in-situ data from 2015, we find that these 2m temperature inversions are present 77% of the time, with a median strength of 1.7°C. The data confirm that the presence of clouds weakens inversions. Initial results indicate a RMSE of 3.9°C between MERRA-2 and PROMICE 2m air temperature, and a RMSE of 5.6°C between the two datasets for surface temperature. Improved understanding of controls on near surface inversions is important for use of remotely sensed snow surface temperatures and for modeling of surface mass and energy exchange processes.</p>

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 Impacts of Irrigation on Summertime Temperatures in the Pacific Northwest

2020 ◽  
Vol 24 (1) ◽  
pp. 1-26
Author(s):  
Patricia M. Lawston ◽  
Joseph A. Santanello ◽  
Brian Hanson ◽  
Kristi Arsensault
Keyword(s):  
Remote Sensing ◽  
Soil Moisture ◽  
Surface Temperature ◽  
Land Surface ◽  
Maximum Temperature ◽  
Daily Maximum Temperature ◽  
Climate Signal ◽  
Near Surface ◽  
Surface Temperatures ◽  
Land Surface Temperatures

AbstractIrrigation has the potential to modify local weather and regional climate through a repartitioning of water among the surface, soil, and atmosphere with the potential to drastically change the terrestrial energy budget in agricultural areas. This study uses local observations, satellite remote sensing, and numerical modeling to 1) explore whether irrigation has historically impacted summer maximum temperatures in the Columbia Plateau, 2) characterize the current extent of irrigation impacts to soil moisture (SM) and land surface temperature (LST), and 3) better understand the downstream extent of irrigation’s influence on near-surface temperature, humidity, and boundary layer development. Analysis of historical daily maximum temperature (TMAX) observations showed that the three Global Historical Climate Network (GHCN) sites downwind of Columbia Basin Project (CBP) irrigation experienced statistically significant cooling of the mean summer TMAX by 0.8°–1.6°C in the post-CBP (1968–98) as compared to pre-CBP expansion (1908–38) period, opposite the background climate signal. Remote sensing observations of soil moisture and land surface temperatures in more recent years show wetter soil (~18%–25%) and cooler land surface temperatures over the irrigated areas. Simulations using NASA’s Land Information System (LIS) coupled to the Weather Research and Forecasting (WRF) Model support the historical analysis, confirming that under the most common summer wind flow regime, irrigation cooling can extend as far downwind as the locations of these stations. Taken together, these results suggest that irrigation expansion may have contributed to a reduction in summertime temperatures and heat extremes within and downwind of the CBP area. This supports a regional impact of irrigation across the study area.

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