Intrusion Of Warm Air Research Articles

Warm and moist air intrusions into the winter Arctic: a Lagrangian view on the near-surface energy budgets

Abstract. In this study, warm and moist air intrusions (WaMAIs) over the Arctic Ocean sectors of Barents Sea, Kara Sea, Laptev Sea, East Siberian Sea, Chukchi Sea, and Beaufort Sea in 40 recent winters (from 1979 to 2018) are identified from the ERA5 reanalysis using both Eulerian and Lagrangian views. The analysis shows that WaMAIs, fueled by Arctic blocking, cause a relative surface warming and hence a sea-ice reduction by exerting positive anomalies of net thermal irradiances and turbulent fluxes on the surface. Over Arctic Ocean sectors with land-locked sea ice in winter, such as Laptev Sea, East Siberian Sea, Chukchi Sea, and Beaufort Sea, the total surface energy-budget is dominated by net thermal irradiance. From a Lagrangian perspective, total water path (TWP) increases linearly with the downstream distance from the sea-ice edge over the completely ice-covered sectors, inducing almost linearly increasing net thermal irradiance and total surface energy-budget. However, over the Barents Sea, with an open ocean to the south, total net surface energy-budget is dominated by the surface turbulent flux. With the energy in the warm-and-moist air continuously transported to the surface, net surface turbulent flux gradually decreases with distance, especially within the first 2∘ north of the ice edge, inducing a decreasing but still positive total surface energy-budget. The boundary-layer energy-budget patterns over the Barents Sea can be categorized into three classes: radiation-dominated, turbulence-dominated, and turbulence-dominated with cold dome, comprising about 52 %, 40 %, and 8 % of all WaMAIs, respectively. Statistically, turbulence-dominated cases with or without cold dome occur along with 1 order of magnitude larger large-scale subsidence than the radiation-dominated cases. For the turbulence-dominated category, larger turbulent fluxes are exerted to the surface, probably because of stronger wind shear. In radiation-dominated WaMAIs, stratocumulus develops more strongly and triggers intensive cloud-top radiative cooling and related buoyant mixing that extends from cloud top to the surface, inducing a thicker well-mixed layer under the cloud. With the existence of cold dome, fewer liquid water clouds were formed, and less or even negative turbulent fluxes could reach the surface.

Exploring the Dynamics of an Arctic Sea Ice Melt Event Using a Coupled Atmosphere‐Ocean Single‐Column Model (AOSCM)

AbstractThe Arctic climate system is host to many processes which interact vertically over the tightly coupled atmosphere, sea ice and ocean. The coupled Atmosphere‐Ocean Single‐Column Model (AOSCM) allows to decouple local small‐scale and large‐scale processes to investigate the model performance in an idealized setting. Here, an observed Arctic warm air intrusion event is used to show how to identify model deficiencies using the AOSCM. The AOSCM allows us to effectively produce a large number of perturbation simulations, around 1,000, to map sensitivities of the model results due to changes in physical and model properties as well as to the large‐scale tendencies. The analysis of the summary diagnostics, that is, aggregated results from sensitivity experiments evaluated against modeled physical properties, such as surface energy budget and mean sea ice thickness, reveals sensitivities to the chosen parameters. Further, we discuss how the conclusions can be used to understand the behavior of the global host model. The simulations confirm that the horizontal advection of heat and moisture plays an important role for maintaining a low‐level cloud cover, as in earlier studies. The combined cloud layers increase the energy input to the surface, which in turn enhances the ongoing melt. The clouds present an additional sensitivity in terms of how they are represented but also their interaction with the large‐scale advection and the model time step. The methodology can be used for a variety of other regions, where the coupling to the ocean is important.

GNSS-based water vapor estimation and validation during the MOSAiC expedition

Abstract. Within the transpolar drifting expedition MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate), the Global Navigation Satellite System (GNSS) was used among other techniques to monitor variations in atmospheric water vapor. Based on 15 months of continuously tracked GNSS data including GPS, GLONASS and Galileo, epoch-wise coordinates and hourly zenith total delays (ZTDs) were determined using a kinematic precise point positioning (PPP) approach. The derived ZTD values agree to 1.1 ± 0.2 mm (root mean square (rms) of the differences 10.2 mm) with the numerical weather data of ECMWF's latest reanalysis, ERA5, computed for the derived ship's locations. This level of agreement is also confirmed by comparing the on-board estimates with ZTDs derived for terrestrial GNSS stations in Bremerhaven and Ny-Ålesund and for the radio telescopes observing very long baseline interferometry in Ny-Ålesund. Preliminary estimates of integrated water vapor derived from frequently launched radiosondes are used to assess the GNSS-derived integrated water vapor estimates. The overall difference of 0.08 ± 0.04 kg m−2 (rms of the differences 1.47 kg m−2) demonstrates a good agreement between GNSS and radiosonde data. Finally, the water vapor variations associated with two warm-air intrusion events in April 2020 are assessed.

Atmospheric Rivers, Warm Air Intrusions, and Surface Radiation Balance in the Amundsen Sea Embayment

AbstractOver recent decades the primary climatic forcing on the ice shelves in the Amundsen Sea Embayment (ASE) has been the influx of warm water into sub‐ice‐shelf ocean cavities. By contrast, the contribution of a globally warming atmosphere has been negligible but may play a more significant role in the future. Warm, moist air intrusions from the ocean onto the ice sheet surface play an essential role in the surface energy budget and constitute the primary driver for surface melting in West Antarctica. This study employs observations from automatic weather stations deployed on the Pine Island Glacier and numerical model outputs from the Antarctic Mesoscale Prediction System to investigate warm air intrusion (WAI) events and examine their impacts on surface glacier melt in the ASE. A deep low‐pressure center over the Amundsen/Ross Seas and a blocking ridge over the southeast Pacific created favorable conditions for developing an atmospheric river that directs warm and moist air toward the ASE. An analysis of meteorological observations through 2013–2014 shows that 2013 received two times higher frequency WAI events than 2014. 2013 year coincides with more substantial pressure gradients in the western Ross Sea and the eastern Amundsen Sea. The February and March 2013 WAI events induce about 3 days of surface melting over the Pine Island Glacier and the Thurston Island area. A continued increase in the large‐scale advection of warm air from the midlatitudes toward the ASE could lead to an increased surface melting frequency with implications for ice shelves in the study area.

Eulerian and Lagrangian views of warm and moist air intrusions into summer Arctic

In this study, warm and moist air intrusions (WaMAI) over the sea sectors of Kara, Laptev, East Siberian and Beaufort from 1979 to 2018 are identified in ERA5 reanalysis and their air-mass transformation is analysed using interpolation in ERA5 and satellite products along trajectories. The analysis shows that WaMAIs, driven by blocking high-pressure systems over the respective ocean sectors, induce surface warming (11–18 W m−2) and sea ice melt from positive anomalies of net longwave radiation (5–8 W m−2) and turbulent flux (8–13 W m−2) to the surface, although the anomaly of net shortwave radiation (−9 ~ +1 W m−2) is negative. From a Lagrangian perspective, the surface energy-budget anomaly decreases linearly, while total column cloud liquid water (TCLW) increases linearly with the downstream distance from the sea-ice edge. However, the cloud radiative effects of both longwave and shortwave radiation reach an equilibrium as TCLW increases in a much lower rate beyond 7 degrees north of the sea ice edge. The boundary-layer energy-budget pattern can be categorized into two classes: radiation-dominated and turbulence-dominated, comprised of 26% and 62% WaMAIs respectively. Statistically, turbulence-dominated cases occur with 3 times stronger large-scale subsidence, and also feature a larger anomaly in net shortwave radiation. In radiation-dominated WaMAIs, stratocumulus develops more strongly and hence exerts larger longwave and shortwave forcing to the surface. In both categories, a well-mixed boundary layer deepens by 500 m along the trajectories, from the continuous turbulent mixing.

Understanding the hot season dynamics and variability across India

Climate change has a detrimental impact on human health due to increasing temperature extremes. It is undoubtedly true over India, where in recent decades, a rise in the frequency and intensity of extreme events is witnessed. The present study primarily focuses on understanding the dynamics of the hot season over homogeneous regions of India. Using Canonical Correlation Analysis, five temperature homogeneous regions are defined. All regions showed a positive trend with less interannual variability, except Gangetic plains. The seasonal composites for extreme temperature years show that hot season over North India (NI) occurs mainly due to blocking high in upper atmospheres. Similarly, daily temperature anomalies for the heatwave days during hot years exhibit stationarity of such blocks centered over the region. Two global teleconnections have been found to be responsible for the NI seasonal anomaly, (i) a continuous anomalous low over Europe Granger cause anomalous high across the region, (ii) the subtropical jet stream and the polar jet stream help to maintain stationarity of anticyclonic blocks over the region. Sinking of air due to an upper atmospheric high over NI causes adiabatic warming near the surface. Southern India mainly gets heated due to warm air intrusion from northwest regions, contributing to anomalous low humidity over the southern coastal regions. This phenomenon blocks moisture transport from the adjacent ocean entering the region resulting in amplification of hotness due to moisture blocking, leading to an increased temperature anomaly.

Intensity Change of Typhoon Nancy (1961) during Landfall in a Moist Environment over Japan: A Numerical Simulation with Spectral Nudging

Abstract Intensity change of tropical cyclones (TCs) as they make landfall is closely linked to sustained periods of high surface winds and heavy precipitation. Few studies have investigated the intensity change of intense TCs that make landfall in middle latitudes such as Japan because few intense typhoons make landfall in middle latitudes. In this study, a numerical simulation of intense Typhoon Nancy (1961) was used to understand the intensity change that occurred when Nancy made landfall in Japan. A spectral nudging technique was introduced to reduce track errors in the simulation. During landfall, the simulated storm exhibited the salient asymmetric structure and rapid eyewall contraction. A tangential wind budget indicated that the maximum wind speed decreased concurrent with an increase in surface friction and advection associated with low-level asymmetric flows. Detailed evolution of the storm’s warm core was analyzed with a potential temperature budget. In the upper part of the warm core centered at a 12-km height, cooling due to ventilation by asymmetric flows and longwave radiation overcame heating due to condensation and shortwave radiation during the contraction of eyewall clouds. In the lower part of the warm core, adiabatic cooling more than offset warm-air intrusions associated with asymmetric flows and condensational heating. The condensation was supplied by an abundance of moisture due to evaporation from the ocean in the well-developed typhoon based on a moisture budget. Sensitivity experiments revealed that environmental baroclinicity in the midlatitudes, orography, and radiative processes made minor contributions to the weakening. The weakening was instead controlled by inner-core dynamics and interactions with land surfaces.

Comprehensive measures to enhance electric heater defrosting (EHD) performance for household frost-free refrigerators

The frost-free refrigerator typically uses a bottom-placed calrod heater to remove frost on evaporator surface. However, the mismatch between full coverage of frost and bottom-up heat transfer extends defrost duration, and warm air intrusion into freezing cabinets (FCs) during defrosting leads to increased FC-temperature rise and overall energy consumption. Therefore, comprehensive measures were applied in this article to enhance defrost performance of the frost-free refrigerator. A special fan cover was adopted which opened during cooling cycles to supply cold air into cabinets normally, but closed during defrosting to block warm air intrusion passage. Moreover, an extra heater (60 W) was inserted in the middle height of the evaporator besides the original bottom one (180 W) to form better frost-heat match. Results show the fan cover and dual heaters altogether reduced defrost duration by 4.5 min and FC-temperature rise by 2.7 °C. Lower FC-temperature after defrost further declined energy consumption for the subsequent recovery cycle, which compensated for its increase for the defrost cycle and dominated the 1.2% reduction in overall energy consumption of the refrigerator. Both measures proved effective in enhancing defrost performance. However, the middle heater may affect cooling cycle performance of the refrigerator, necessitating further investigation.

Saharan air intrusions as a relevant mechanism for Iberian heatwaves: The record breaking events of August 2018 and June 2019

The summers of 2018 and 2019 were characterized by unusually warm conditions over Europe. Here, we describe the intense heatwaves striking the Iberian Peninsula in early August 2018 and late June 2019. The 2018 episode was relatively short-lived but outstanding in amplitude, particularly in western Iberia. Similar to previous mega-heatwaves, the 2019 event was long-lasting and affected large areas of western and central Europe, including eastern Iberia. During these events, many absolute temperature records were broken in western and eastern Iberia, respectively (some of them standing since 2003). In both cases, a cyclonic circulation off the coast in the northeastern Atlantic and a strong subtropical ridge pattern over the affected area promoted the advection of an anomalously warm air mass. This paper highlights the role of these very warm, stable and dry air intrusions of Saharan origin in the western and eastern Iberia heatwave events. Using a thermodynamical classification based on the geopotential height thickness and potential temperature, we show how the magnitude and poleward extension of these Saharan intrusions were unprecedented in the period since 1948. The relationship between Iberian heatwaves and Saharan warm air intrusions is discussed in the long-term context, showing a closer link in southern sectors of the Peninsula. However, a consistent poleward trend in the latitudinal extension of these subtropical intrusions reveals their increasing relevance for heatwaves in northern sectors of Iberia and western Europe. This overall trend is accompanied by an apparent “see-saw” in the occurrence of subtropical intrusions between eastern and western Iberia on multi-decadal scales.

The 2018 North Greenland polynya observed by a newly introduced merged optical and passive microwave sea-ice concentration dataset

Abstract. Observations of sea-ice concentration are available from satellites year-round and almost weather-independently using passive microwave radiometers at resolutions down to 5 km. Thermal infrared radiometers provide data with a resolution of 1 km but only under cloud-free conditions. We use the best of the two satellite measurements and merge thermal infrared and passive microwave sea-ice concentrations. This yields a merged sea-ice concentration product combining the gap-free spatial coverage of the passive microwave sea-ice concentration and the 1 km resolution of the thermal infrared sea-ice concentration. The benefit of the merged product is demonstrated by observations of a polynya which opened north of Greenland in February 2018. We find that the merged sea-ice concentration product resolves leads at sea-ice concentrations between 60 % and 90 %. They are not resolved by the coarser passive microwave sea-ice concentration product. The benefit of the merged product is most pronounced during the formation of the polynya. Next, the environmental conditions during the polynya event are analysed. The polynya was caused by unusual southerly winds during which the sea ice drifted northward instead of southward as usual. The daily displacement was 50 % stronger than normal. The polynya was associated with a warm-air intrusion caused by a high-pressure system over the Eurasian Arctic. Surface air temperatures were slightly below 0 ∘C and thus more than 20 ∘C higher than normal. Two estimates of thermodynamic sea-ice growth yield sea-ice thicknesses of 60 and 65 cm at the end of March in the area opened by the polynya. This differed from airborne sea-ice thickness measurements, indicating that sea-ice growth processes in the polynya are complicated by rafting and ridging. A sea-ice volume of 33 km3 was produced thermodynamically.

Arctic Summer Airmass Transformation, Surface Inversions, and the Surface Energy Budget

During the Arctic Clouds in Summer Experiment (ACSE) in summer 2014 a weeklong period of warm-air advection over melting sea ice, with the formation of a strong surface temperature inversion and dense fog, was observed. Based on an analysis of the surface energy budget, we formulated the hypothesis that, because of the airmass transformation, additional surface heating occurs during warm-air intrusions in a zone near the ice edge. To test this hypothesis, we explore all cases with surface inversions occurring during ACSE and then characterize the inversions in detail. We find that they always occur with advection from the south and are associated with subsidence. Analyzing only inversion cases over sea ice, we find two categories: one with increasing moisture in the inversion and one with constant or decreasing moisture with height. During surface inversions with increasing moisture with height, an extra 10–25 W m−2of surface heating was observed, compared to cases without surface inversions; the surface turbulent heat flux was the largest single term. Cases with less moisture in the inversion were often cloud free and the extra solar radiation plus the turbulent surface heat flux caused by the inversion was roughly balanced by the loss of net longwave radiation.

Arctic warming by cloud radiation enhanced by moist air intrusion observed at Ny-Ålesund, Svalbard

At Ny-Ålesund, Svalbard, Arctic, two distinct states were seen in downward longwave radiation (LD) during winter 2015/16. After the cold state with the temperature around −10 to −20 °C, a warm state with plus temperature appeared and LD increased from about 170 to 320–330 W m−2. This abrupt and large increase of LD was due to the change of cloud condition, from clear to overcast, which was examined from the cloud radar data taken during GRENE Arctic Research Project. However, only the cloud change could not explain this large change of LD. During the warm state, the atmospheric circulation patterns changed, distortion of tropospheric polar vortex appeared with a high pressure ridge (blocking high), and strong cyclone and intrusion of warm and moist air from the Atlantic Ocean together with longwave radiation from clouds influenced the warming at Svalbard. Thus, the discussions of two Arctic winter states in LD was much effective than those in net longwave. This intrusion of warm-moist air from the lower latitude was one of the major processes contributing to the Arctic amplification, just as also proposed using GCM by Yoshimori et al. (2017). The paper provided the actual observational evidence of this warming process.

Moisture transport and Antarctic sea ice: austral spring 2016 event

Abstract. In austral spring 2016 the Antarctic region experienced anomalous sea ice retreat in all sectors, with sea ice extent in October and November 2016 being the lowest in the Southern Hemisphere over the observational period (1979–present). The extreme sea ice retreat was accompanied by widespread warming along the coastal areas as well as in the interior of the Antarctic continent. This exceptional event occurred along with a strong negative phase of the Southern Annular Mode (SAM) and the moistest and warmest spring on record, over large areas covering the Indian Ocean, the Ross Sea and the Weddell Sea. In October 2016, the positive anomalies of the totally integrated water vapor (IWV) and 2 m air temperature (T2m) over the Indian Ocean, western Pacific, Bellingshausen Sea and southern part of Ross Sea were unprecedented in the last 39 years. In October and November 2016, when the largest magnitude of negative daily sea ice concentration anomalies was observed, repeated episodes of poleward advection of warm and moist air took place. These results suggest the importance of moist and warm air intrusions into the Antarctic region as one of the main contributors to this exceptional sea ice retreat event.

June 2017: The Earliest European Summer Mega‐heatwave of Reanalysis Period

AbstractThis paper examines the characteristics of the heatwave that affected western and central Europe in June 2017. Using a novel algorithm, we show that its extension, intensity, and persistence were comparable to those of other European mega‐heatwaves, but it occurred earlier in the summer. The most affected area was Iberia, which experienced devastating forest fires with human casualties and the warmest temperatures of the reanalysis period from daily to seasonal scales. The peak of the mega‐heatwave displayed an unprecedented warm air intrusion due to a record‐breaking subtropical ridge with signatures closer to those of July and August. The atmospheric circulation was the main triggering factor of the event. However, thermodynamical changes of the last decades made a substantial contribution to the event, by increasing the likelihood of surpassing high‐temperature thresholds. This episode could be a good example of a coming future, with high‐summer mega‐heatwaves occurring earlier.

Upper air thermal inversion and their impact on the summer monsoon rainfall over Goa – A case study

Profiles of periodic GPS Radiosonde ascends collected from a station at the west coast of India (Goa) during summer monsoon months (June to September) of 2009 and 2013 have been used to analyze the thermal inversion statistics at various heights and their repercussions on the regional weather is studied. The interaction of contrasting air masses over the northern Arabian Sea often produces a two layer structure in the lower 5000 m close to the coastal station with warm and dusty air (Summer Shamal) occupying the space above the cool and moist Low Level Jet (LLJ) by virtue of their density differences. The warm air intrusion creates low lapse rate pockets above LLJ and modifies the gravitational stability strong enough to inhibit convection. It is observed that the inversion occurring in the lower 3000 m layer with an optimum layer thickness of 100–200 m has profound influence on the weather beneath it. We demonstrated the validity of the proposed hypothesis by analyzing the collocated data from radiosonde, lidar and the rain gauge during 16th July 2013 as a case study. The lidar depolarization ratio provides evidence to support the two layer structure in the lidar backscatter image. The presence of dust noticed in the two layer interface hints the intrusion of warm air that makes the atmosphere stable enough to suppress convection. The daily rainfall record of 2013 surprisingly coincides with the patterns of a regional break like situation centered at 16th July 2013 in Goa.

ENSO influence on the interannual variability of the Red Sea convergence zone and associated rainfall

ABSTRACTThe Red Sea convergence zone (RSCZ) is formed by opposite surface winds blowing from northwest to southeast directions at around 18°–19°N between October and January. A reverse‐oriented, low‐level monsoon trough at 850 hPa, known as the Red Sea trough (RST), transfers moisture from the southern Red Sea to RSCZ. The positions of the RSCZ and RST and the intensity of the RST have been identified as important factors in modulating weather and climatic conditions across the Middle East. Here, we investigate the influence of the El Niño southern oscillation (ENSO) on the interannual variability of RSCZ, RST, and regional rainfall during winter months. Our results indicate that El Niño (warm ENSO phase) favours a shift of the RSCZ to the north and a strengthening of the RST in the same direction. Conversely, during November and December of La Niña periods (cold ENSO phase), the RSCZ shift to the south and the RST strengthens in the same direction. During El Niño periods, southeasterly wind speeds increase (20–30%) over the southern Red Sea and northwesterly wind speeds decrease (10–15%) over the northern Red Sea. Noticeable increases in the number of rainy days and the intensity of rain events are observed during El Niño phases. These increases are associated with colder than normal air intrusion at lower levels from the north combined with warm air intrusion from the south over the RSCZ. Our analysis suggests that during El Niño winters, warmer sea surface temperatures and higher convective instability over the Red Sea favour local storms conditions and increase rainfall over the Red Sea and adjoining regions.

Classification and forecast of heavy rainfall in northern Kyushu during Baiu season using weather pattern recognition

AbstractIn this study, the Self‐Organizing Maps in combination with K‐means clustering technique are used for classification of synoptic weather patterns inducing heavy rainfall exceeding 100 mm day−1 during the Baiu season (June–July) of 1979–2010 over northern Kyushu, southwestern Japan. It suggests that these local extreme rainfall events are attributed to four clustered patterns, which are primarily related to the Baiu front and the extratropical/tropical cyclone/depression activities and represented by the intrusion of warm and moist air accompanied by the low‐level jet or cyclonic circulation. The classification results are then implemented with the analogue method to predict the occurrence (yes/no) of local heavy rainfall days in June–July of 2011–2016 by using the prognostic synoptic fields from the operational Japan Meteorological Agency (JMA) Global Spectral Model (GSM). In general, the predictability of our approach evaluated by the Equitable Threat Score up to 7‐day lead times is significantly improved than that from the conventional method using only the predicted rainfall intensity from GSM. Although the false alarm ratio is still high, it is expected that the new method will provide a useful guidance, particularly for ranges longer than 2 days, for decision‐making and preparation by weather forecasters or end‐users engaging in disaster‐proofing and water management activities.

Major cause of unprecedented Arctic warming in January 2016: Critical role of an Atlantic windstorm

In January 2016, the Arctic experienced an extremely anomalous warming event after an extraordinary increase in air temperature at the end of 2015. During this event, a strong intrusion of warm and moist air and an increase in downward longwave radiation, as well as a loss of sea ice in the Barents and Kara seas, were observed. Observational analyses revealed that the abrupt warming was triggered by the entry of a strong Atlantic windstorm into the Arctic in late December 2015, which brought enormous moist and warm air masses to the Arctic. Although the storm terminated at the eastern coast of Greenland in late December, it was followed by a prolonged blocking period in early 2016 that sustained the extreme Arctic warming. Numerical experiments indicate that the warming effect of sea ice loss and associated upward turbulent heat fluxes are relatively minor in this event. This result suggests the importance of the synoptically driven warm and moist air intrusion into the Arctic as a primary contributing factor of this extreme Arctic warming event.

Precipitation and synoptic regime in two extreme years 2009 and 2010 at Dome C, Antarctica – implications for ice core interpretation

Abstract. At the East Antarctic deep ice core drilling site Dome C, daily precipitation measurements were initiated in 2006 and are being continued until today. The amounts and stable isotope ratios of the precipitation samples as well as crystal types are determined. Within the measuring period, the two years 2009 and 2010 showed striking contrasting temperature and precipitation anomalies, particularly in the winter seasons. The reasons for these anomalies are analysed using data from the mesoscale atmospheric model WRF (Weather Research and Forecasting Model) run under the Antarctic Mesoscale Prediction System (AMPS). 2009 was relatively warm and moist due to frequent warm air intrusions connected to amplification of Rossby waves in the circumpolar westerlies, whereas the winter of 2010 was extremely dry and cold. It is shown that while in 2010 a strong zonal atmospheric flow was dominant, in 2009 an enhanced meridional flow prevailed, which increased the meridional transport of heat and moisture onto the East Antarctic plateau and led to a number of high-precipitation/warming events at Dome C. This was also evident in a positive (negative) SAM (Southern Annular Mode) index and a negative (positive) ZW3 (zonal wave number three) index during the winter months of 2010 (2009). Changes in the frequency or seasonality of such event-type precipitation can lead to a strong bias in the air temperature derived from stable water isotopes in ice cores.

The METCRAX II Field Experiment: A Study of Downslope Windstorm-Type Flows in Arizona’s Meteor Crater

Abstract The second Meteor Crater Experiment (METCRAX II) was conducted in October 2013 at Arizona’s Meteor Crater. The experiment was designed to investigate nighttime downslope windstorm−type flows that form regularly above the inner southwest sidewall of the 1.2-km diameter crater as a southwesterly mesoscale katabatic flow cascades over the crater rim. The objective of METCRAX II is to determine the causes of these strong, intermittent, and turbulent inflows that bring warm-air intrusions into the southwest part of the crater. This article provides an overview of the scientific goals of the experiment; summarizes the measurements, the crater topography, and the synoptic meteorology of the study period; and presents initial analysis results.