Fall Freeze Research Articles

Urban Warming Challenges Verification of Frost Advisories and Freeze Warnings in Madison, Wisconsin

Abstract Urban heat islands (UHIs) may increase the likelihood that frost sensitive plants will escape freezing nighttime temperatures in late spring and early fall. Using data from 151 temperature sensors in the Madison, Wisconsin, region during March 2012–October 2016, we found that during time periods when the National Weather Service (NWS) issued freeze warnings (threshold of 0.0°C) or frost advisories (threshold of 2.22°C) were valid, temperatures in Madison’s most densely populated, built-up areas often did not fall below the respective temperature thresholds. Urban locations had a mean minimum temperature of 0.72° and 1.39°C for spring and fall freeze warnings, respectively, compared to −0.52° and −0.53°C for rural locations. On average, 31% of the region’s land area experienced minimum temperatures above the respective temperature thresholds during freeze warnings and frost advisories, and the likelihood of temperatures falling below critical temperature thresholds increased as the distance away from core urban centers increased. The urban–rural temperature differences were greatest in fall compared to spring, and when sensor temperatures did drop below thresholds, the maximum time spent at or below thresholds was highest for rural locations during fall freeze warnings (6.2 h) compared to urban locations (4.8 h). These findings potentially have widely varying implications for the general public and industry. UHIs create localized, positive perturbations to nighttime temperatures that are difficult to account for in forecasts; therefore, freeze warnings and frost advisories may have varying degrees of verification in medium-sized cities like Madison, Wisconsin, that are surrounded by cropland and natural vegetation. Significance Statement The purpose of this study was to understand whether the urban heat island effect in Madison, Wisconsin, creates localized temperature patterns where county-scale frost advisories and freeze warnings may not verify. Approximately one-third of Madison’s urban core area and most densely populated region experienced temperatures that were consistently above critical low temperature thresholds. This is important because gardening and crop management decisions are responsive to the perceived risk of cold temperatures in spring and fall that can damage or kill plants. These results suggest that urban warming presents forecast challenges to the issuance of frost advisories and freeze warnings, supporting the need for improved numerical weather prediction at higher spatial resolution to account for complex urban meteorology.

Maternal carryover, winter severity, and brown bear abundance relate to elk demographics.

Ungulates are key components of ecosystems due to their effects on lower trophic levels, role as prey, and value for recreational and subsistence harvests. Understanding factors that drive ungulate population dynamics can inform protection of important habitat and successful management of populations. To ascertain correlates of ungulate population dynamics, we evaluated the effects of five non-exclusive hypotheses on ungulate abundance and recruitment: winter severity, spring nutritional limitation (spring bottleneck), summer-autumn maternal condition carryover, predation, and timber harvest. We used weather, reconstructed brown bear (Ursus arctos) abundance, and timber harvest data to estimate support for these hypotheses on early calf recruitment (calves per 100 adult females in July–August) and population counts of Roosevelt elk (Cervus canadensis roosevelti) on Afognak and Raspberry islands, Alaska, USA, 1958–2020. Increasing winter temperatures positively affected elk abundance, supporting the winter severity hypothesis, while a later first fall freeze had a positive effect on elk recruitment, supporting the maternal carry-over hypothesis. Increased brown bear abundance was negatively associated with elk recruitment, supporting the predation hypothesis. Recruitment was unaffected by spring climate conditions or timber harvest. Severe winter weather likely increased elk energy deficits, reducing elk survival and subsequent abundance in the following year. Colder and shorter falls likely reduced late-season forage, resulting in poor maternal condition which limited elk recruitment more than winter severity or late-winter nutritional bottlenecks. Our results additionally demonstrated potential negative effects of brown bears on elk recruitment. The apparent long-term decline in elk recruitment did not result in a decline of abundance, which suggests that less severe winters may increase elk survival and counteract the potential effects of predation on elk abundance.

Mapping Freezing and Thawing Surface State Periods With the CYGNSS Based F/T Seasonal Threshold Algorithm

Freeze/Thaw (F/T) surface state detection and imaging has been demonstrated as a capability of NASA's Cyclone Global Navigation Satellite System (CYGNSS) mission. The objective of the research presented here is the application of an existing F/T retrieval algorithm to multiple target areas over several years in order to demonstrate the ability to provide long term trending of F/T behavior. Metrics are developed and evaluated to characterize F/T trends related to global warming. They include the annual fraction of time spent frozen and thawed as a function of location and year, and the earliest Fall freeze and earliest Spring thaw as a function of location and year. The results demonstrate the capability of Global Navigation Satellite Systems Reflectometry (GNSS-R) to provide long term trending of F/T behavior relevant to studies of climate change.

Methane pathways in winter ice of a thermokarst lake–lagoon–coastal water transect in north Siberia

Abstract. The thermokarst lakes of permafrost regions play a major role in the global carbon cycle. These lakes are sources of methane to the atmosphere although the methane flux is restricted by an ice cover for most of the year. How methane concentrations and fluxes in these waters are affected by the presence of an ice cover is poorly understood. To relate water body morphology, ice formation and methane to each other, we studied the ice of three different water bodies in locations typical of the transition of permafrost from land to ocean in a continuous permafrost coastal region in Siberia. In total, 11 ice cores were analyzed as records of the freezing process and methane composition during the winter season. The three water bodies differed in terms of connectivity to the sea, which affected fall freezing. The first was a bay underlain by submarine permafrost (Tiksi Bay, BY), the second a shallow thermokarst lagoon cut off from the sea in winter (Polar Fox Lagoon, LG) and the third a land-locked freshwater thermokarst lake (Goltsovoye Lake, LK). Ice on all water bodies was mostly methane-supersaturated with respect to atmospheric equilibrium concentration, except for three cores from the isolated lake. In the isolated thermokarst lake, ebullition from actively thawing basin slopes resulted in the localized integration of methane into winter ice. Stable δ13CCH4 isotope signatures indicated that methane in the lagoon ice was oxidized to concentrations close to or below the calculated atmospheric equilibrium concentration. Increasing salinity during winter freezing led to a micro-environment on the lower ice surface where methane oxidation occurred and the lagoon ice functioned as a methane sink. In contrast, the ice of the coastal marine environment was slightly supersaturated with methane, consistent with the brackish water below. Our interdisciplinary process study shows how water body morphology affects ice formation which mitigates methane fluxes to the atmosphere.

Ecosystem Services and Cash Crop Tradeoffs of Summer Cover Crops in Northern Region Organic Vegetable Rotations

Intensive production practices characterizing vegetable farming contribute to high productivity, but often at the expense of supporting and regulating ecosystem services. Diversification with cover crops may support increased resilience through soil organic matter (SOM) contributions and physical soil protection. Vegetable farming often includes spring and fall production, limiting establishment and productive potential of over-wintered cover crops that are more widely used in the USA. In northern climate vegetable systems, warm-season cover crops planted during short summer fallows could be a tool to build resilience via ecosystem service enhancement. This project evaluated summer cover crops in the northern USA (MN and WI) for biomass accumulation, weed suppression, and contribution to fall cash crop yield. Our study included four site years, during which we investigated the effects of four cover crop species treatments, grown for 30 (short duration, SD) or 50 days (long duration, LD) alongside bare fertilized and unfertilized control treatments: buckwheat (Fagopyrum esculentum) and sunn hemp (Crotalaria juncea) monocultures, and biculture of chickling vetch (Lathyrus sativus) or cowpea (Vigna unguiculata) with sorghum-sudangrass (sudex) (Sorghum bicolor x S. bicolor var. Sudanese). To quantify cover crop quantity, quality, and weed suppression capacity, we measured cover crop and weed biomass, and biomass C:N. To quantify effects on cash crops, we measured fall broccoli yield and biomass. Mean total biomass (cover crop + weeds) by site year ranged from 1,890 kg ha−1 in MN Y1 to 5,793 kg ha−1 in WI Y2 and varied among species in Y1 for both the SD and LD treatments. Most cover crops did not outcompete weeds, but treatments with less weeds produced more overall biomass. Data from Y1 show that cover crops were unable to replace fertilizer for fall broccoli yield, and led to reduced fall crop yield. Broccoli in Y2 did not reach maturity due to fall freeze. Summer cover crops, because of their biomass accumulation potential, may be used by farmers in northern climates to fit into cropping system niches that have historically been left as bare soil, but care with timing is necessary to optimize weed suppression and mitigate tradeoffs for cash crop production.

Methane emission from high latitude lakes: methane-centric lake classification and satellite-driven annual cycle of emissions

Methane (CH4) is emitted from lakes by several processes: bubbles released from bottom sediments that reach the atmosphere (ebullition); spring release of CH4 trapped in bubbles in and under the ice during fall freeze (bubble release), and diffusion of CH4 from sediments to the surface. Each of these emission routes is highly variable over space and time, and episodic in the extreme, making reliable measurements difficult to carry out. However, lakes are receiving increasing interest for their important contribution to global CH4 emissions. Their area, distribution and emissions respond to interannual and longer-term climate fluctuations and close to half the world’s lake area is in high northern latitudes that are experiencing rapidly-warming temperatures and lengthening thaw periods. We report on a new spatially-explicit data set of lakes > 50°N, classified with methane-relevant criteria. The seasonality of daily CH4 fluxes is driven with satellite observations of thaw timing and duration. We found that observed thaw seasons are 10–30% shorter than those assumed in previous studies. The area of lakes is 1,095 × 103 km2 and total CH4 emission is 13.8–17.7 Tg CH4 year−1: 11.2–14.4 Tg via diffusion and ebullition and 2.6–3.3 Tg from spring release of CH4 stored in bubbles in winter lake ice. This novel suite of data and methodologies provides a unique framework to model CH4 emission from lakes under current, past and future climates.

Timing and spatial variability of fall soil freezing in boreal forest and its effect on SMAP L-band radiometer measurements

This paper investigates the potential for the Soil Moisture Active Passive (SMAP) L-band radiometer to estimate the percentage of frozen soil inside a pixel during fall periods. To evaluate the spatial and temporal variability of the autumn freeze in northeastern Canada boreal forest, a network of compact and self-recording temperature sensors (iButton) along transects averaging 10 km was deployed on the soil surface at two different sites. Results show important spatial variability in soil freezing timing with maximums, for the two sites, spanning from 7.5 to 9.5 weeks to reach a frozen state. It is known that L-band radiometry such as Soil Moisture Active Passive (SMAP) can monitor freeze/thaw (F/T) seasonality at low spatial resolution (~40 km). However, the possible strong surface variability within a 40 km × 40 km pixel can lead to spatial delay in fall freezing. Simulations of brightness temperature (TB) weighted by spatially distributed iButton F/T records and using the ω-τ vegetation model show good agreement with SMAP satellite observations during the fall transition period with an average RMSE of 3.5 K. A new intra-pixel frozen soil percentage retrieval algorithm using only SMAP observations and air temperature is proposed. The percentage of frozen soil in falls 2015 and 2016 from the new algorithm and the in situ measurements are in good agreement with a coefficient of determination (R2) ranging between 0.63 and 0.88.

Advancing Primocane-fruiting Raspberry Production in the Intermountain West Using Spring Protection

Winter injury to raspberry floricanes commonly limits productivity in cold climates. Primocane-fruiting raspberries avoid winter injury by fruiting on first year canes, but fruit production in the high-elevation valleys of the Intermountain West is later than needed for local markets, and may be limited by early fall freezes. High tunnels were used for early spring heat to advance primocane growth and the fruiting season of two primocane-fruiting red raspberry cultivars. Tunnels were covered with plastic in March and April. Then, they were covered with shade cloth during fruit ripening. Tunnel-covered plots were compared with field plantings for primocane growth rate, fruiting season, yield, and fruit quality over two seasons. High tunnels increased cane growth rate, with the harvest season advanced by 18 to 26 days depending on season and cultivar, but they did not consistently affect the total season yield or fruit size. Low-cost two-season tunnels used in conjunction with early-season primocane-fruiting raspberries may provide a viable method for small acreage producers in harsh climates to reliably supply high-value seasonal raspberry markets.

Projected climate and agronomic implications for corn production in the Northeastern United States.

Corn has been a pillar of American agriculture for decades and continues to receive much attention from the scientific community for its potential to meet the food, feed and fuel needs of a growing human population in a changing climate. By midcentury, global temperature increase is expected to exceed 2°C where local effects on heat, cold and precipitation extremes will vary. The Northeast United States is a major dairy producer, corn consumer, and is cited as the fastest warming region in the contiguous U.S. It is important to understand how key agronomic climate variables affect corn growth and development so that adaptation strategies can be tailored to local climate changes. We analyzed potential local effects of climate change on corn growth and development at three major dairy locations in the Northeast (Syracuse, New York; State College, Pennsylvania and Landisville, Pennsylvania) using downscaled projected climate data (2000–2100) from nine Global Climate Models under two emission pathways (Representative Concentration Pathways (RCP) 4.5 and 8.5). Our analysis indicates that corn near the end of the 21st century will experience fewer spring and fall freezes, faster rate of growing degree day accumulation with a reduction in time required to reach maturity, greater frequencies of daily high temperature ≥35°C during key growth stages such as silking-anthesis and greater water deficit during reproductive (R1-R6) stages. These agronomic anomalies differ between the three locations, illustrating varying impacts of climate change in the more northern regions vs. the southern regions of the Northeast. Management strategies such as shifting the planting dates based on last spring freeze and irrigation during the greatest water deficit stages (R1-R6) will partially offset the projected increase in heat and drought stress. Future research should focus on understanding the effects of global warming at local levels and determining adaptation strategies that meet local needs.

Potential Arctic connections to eastern North American cold winters

Far-field temperature and geopotential height fields associated with eastern North American early winter (DEC-JAN) extreme cold events are documented since 1950. Based on 19 cases of monthly extreme cold events, two large-scale patterns emerge. First, a strong Alaskan Ridge (AR) can develop with higher 700 hPa geopotential heights and positive temperature anomalies from Alaska south along the coastal northeastern Pacific Ocean, and low eastern North American geopotential height anomalies, the well-known North American ridge/trough pattern. A second subset of cases is a Greenland-Baffin Blocking (GBB) pattern that have positive temperature anomalies centered west of Greenland with a cut off tropospheric polar vortex feature over eastern North America; cold temperature anomalies extend from southeastern United States northwestward into central Canada. Both of these historical large-scale patterns associated with eastern North American cold events (AR and GBB) have the potential for future reinforcement by sea ice loss and associated warm Arctic regional temperature anomalies. An example of a GBB case is 15-22 December 2010 and an extreme AR case is in early 4-14 December 2016. In both cases lack of sea ice and warm temperature anomalies were colocated with local maximums in the geopotential height anomaly fields. Future regional delay of fall freeze up in the Chukchi Sea and Baffin Bay regions could reinforce these geopotential height patterns once they occur, but is not likely to initiate AR and GBB type events.

Developing a growing degree day model for North Dakota and Northern Minnesota soybean

Farmers in North Dakota and Northern Minnesota did not have a model to predict when their soybean (Glycine max L. Merr.) crop will be mature. Soybean plants need to be mature before the first fall freeze. The objectives of this study were to estimate needed accumulated growing degree days (AGDD) for adapted soybean maturity groups (MG) to reach maturity (R8). Research was conducted during 2007–2012 at northern, central, and southern North Dakota, to develop a model to predict the soybean maturity date based on accumulated GDDs and to verify the model using field research data from 2013 to 2015. Based on 1816 data points a regression analysis was performed which predicted that 1666, 1862, 2030 AGDD (with a 50°F base temperature) were needed to reach maturity for 00.7, 0.4, and 1.0 MG soybean cultivars, respectively.The predicted values were used to create a model that allows producers to select the MG of their cultivar, date of planting, and the nearest North Dakota Agricultural Weather Network (NDAWN) station. The output provides the predicted maturity date, as well as the average first day of occurrence of select threshold temperatures (36°, 32° and 28°F) at 50% probability level. This model will help growers to judge the risks of certain frost occurring before the selected cultivar's projected maturity date. The projected maturity date is based on two parts. The first part is the observed period from the planting date to the date the model is queried and is based on daily maximum and minimum temperatures. The second part is the forecast period that is based on normal (1981–2010 average) daily maximum and minimum temperatures from the queried date through the end of the select growing season. The growing degree days (GDD) values are accumulated from the planting date through the end of season based on these daily maximum and minimum temperatures. Input into the model should be for adapted MG soybean for the region and for planting dates between May 15 and 31. Farmers and agriculturist in North Dakota and northwest Minnesota can use the model before planting to determine if a MG would mature before the frost with varying temperature thresholds and monitor the accumulation of GDD and development of the crop during the season and estimated date of maturity.

Plot-scale assessment of soil freeze/thaw detection and variability with impedance probes: implications for remote sensing validation networks

Abstract Several large in-situ soil moisture-monitoring networks currently exist over seasonally frozen regions that may have use for the validation of remote sensing soil freeze/thaw (F/T) products. However, further understanding of how the existing network instrumentation responds to changes in near surface soil F/T is recommended. This case study describes the results of a small plot-scale (7 × 7 m) study from November 2013 through April 2014 instrumented with 36 impedance probes. Soil temperature and real dielectric permittivity ϵr' were measured every 15 minutes during F/T transition periods at shallow soil depths (0–10 cm). Categorical soil temperature and real dielectric permittivity techniques were used to define the soil F/T state during these periods. Results demonstrate that both methods for detecting soil F/T have strong agreement (84.7–95.6%) during the fall freeze but weak agreement (53.3–60.9%) during the spring thaw. Bootstrapping results demonstrated both techniques showed a mean difference within ±1.0°C and ±1.4 ϵr' between the standard 5 cm below surface measurement depth and probes at 2, 10 and integrated 0–5.7 cm depths installed within the same study plot. Overall this study demonstrates that the Hydra Probe offers promise for near surface soil F/T detection using existing soil moisture monitoring networks particularly for the fall freeze.

Prior exposure to freezing stress enhances the survival and recovery of Poa pratensis exposed to severe drought.

Both freezing and drought cause cellular dehydration, and they elicit similar increases in protective compounds, which suggests that these stresses could potentially interact. We examined whether the physiological changes that occur in response to freezing in the fall and spring could affect subsequent survival and growth after summer drought. We froze Poa pratensis tillers in the late fall, early spring, or late spring at 0, -5, or -10°C for 3 d and then subjected them to no drought (-0.025 MPa), moderate drought (-0.140 MPa), or severe drought (-0.250 MPa) for 3 wk in the summer. We quantified survival and total biomass after a 3-wk recovery period, and we determined leaf soluble sugar concentrations before and then 0, 30, and 55 d after freezing. For survival and biomass, there were significant interactions between freezing and drought. Spring frozen tillers had the highest biomass and survival following severe drought, whereas fall freezing did not significantly increase the biomass of tillers following the severe drought. Increased drought tolerance after spring freezing did not appear to be associated with increased soluble sugar content, given that the freezing effects on leaf glucose, fructose, and sucrose content were absent 55 d post freezing. Our results demonstrate that multiple stresses that occur over different seasons can interact; this interaction is highly relevant to herbaceous species in northern temperate regions that are experiencing more intense and frequent stress as a result of changes in snow cover and extreme climatic events.

The open-ocean sensible heat flux and its significance for Arctic boundary layer mixing during early fall

Abstract. The increasing ice-free area during late summer has transformed the Arctic to a climate system with more dynamic boundary layer (BL) clouds and seasonal sea ice growth. The open-ocean sensible heat flux, a crucial mechanism of excessive ocean heat loss to the atmosphere during the fall freeze season, is speculated to play an important role in the recently observed cloud cover increase and BL instability. However, lack of observations and understanding of the resilience of the proposed mechanisms, especially in relation to meteorological and interannual variability, has left a poorly constrained BL parameterization scheme in Arctic climate models. In this study, we use multi-year Japanese cruise-ship observations from R/V Mirai over the open Arctic Ocean to characterize the surface sensible heat flux (SSHF) during early fall and investigate its contribution to BL turbulence. It is found that mixing by SSHF is favored during episodes of high surface wind speed and is also influenced by the prevailing cloud regime. The deepest BLs and maximum ocean–atmosphere temperature difference are observed during cold air advection (associated with the stratocumulus regime), yet, contrary to previous speculation, the efficiency of sensible heat exchange is low. On the other hand, the SSHF contributes significantly to BL mixing during the uplift (low pressure) followed by the highly stable (stratus) regime. Overall, it can explain ∼ 10 % of the open-ocean BL height variability, whereas cloud-driven (moisture and radiative) mechanisms appear to be the other dominant source of convective turbulence. Nevertheless, there is strong interannual variability in the relationship between the SSHF and the BL height which can be intensified by the changing occurrence of Arctic climate patterns, such as positive surface wind speed anomalies and more frequent conditions of uplift. This study highlights the need for comprehensive BL observations like the R/V Mirai for better understanding and predicting the dynamic nature of the Arctic climate.

Future Decreases in Freezing Days across North America

Abstract This study used air temperatures from a suite of regional climate models participating in the North American Climate Change Assessment Program (NARCCAP) together with two atmospheric reanalysis datasets to investigate changes in freezing days (defined as days with daily average temperature below freezing) likely to occur between 30-yr baseline (1971–2000) and midcentury (2041–70) periods across most of North America. Changes in NARCCAP ensemble mean winter temperature show a strong gradient with latitude, with warming of over 4°C near Hudson Bay. The decline in freezing days ranges from less than 10 days across north-central Canada to nearly 90 days in the warmest areas of the continent that currently undergo seasonally freezing conditions. The area experiencing freezing days contracts by 0.9–1.0 × 106 km2 (5.7%–6.4% of the total area). Areas with mean annual temperature between 2° and 6°C and a relatively low rate of change in climatological daily temperatures (<0.2°C day−) near the time of spring thaw will encounter the greatest decreases in freezing days. Advances in the timing of spring thaw will exceed the delay in fall freeze across much of the United States, with the reverse pattern likely over most of Canada.

Role of exogenous abscisic acid in adapting of ‘Sultana’ grapevine to low-temperature stress

Abscisic acid (ABA) has a key role in some plant species under low-temperature stress (LTS). In this study the effects of ABA at four concentrations (0, 50, 100 and 200 μΜ) on electrolyte leakage (EL), visual damage index, soluble carbohydrate, proline, total phenolic compound and relative water content (RWC) of leaves of ‘Sultana’ grapevines were studied under LTS (0, 4 and −4 °C). Moreover, to further elucidate the efficiency of foliar ABA on freezing tolerance, some relevant morpho-physiological changes were evaluated at three sampling dates: September, October and November. The increase in cane and leaf EL and the visible symptoms of leaf damage caused by LTS were minimized by the application of ABA. Moreover, in ABA-treated vines, specifically at 200 μM, the contents of soluble carbohydrate, proline and total phenolic increased and the RWC decreased. These alterations in ABA-induced metabolism alleviated the deleterious effects of LTS on ‘Sultana’ grapevine. Freezing tolerance of ABA-treated vines improved within each sampling date in autumn. Foliar application of ABA at 200 μM decreased LT50 in September, October and November by −2.6, −3.6 and −3.9 °C, respectively, as compared to those of control vines. This study also demonstrated that ABA was effective in shoot growth inhibition, leaf abscission and periderm development, indicating that ABA treatments advanced cold acclimation in grapevines. Increased fall freezing tolerance induced by foliar ABA application may be beneficial in nurseries, new established vineyards and for late-bearing cultivars in regions with fall frost events.

Response of double cropping suitability to climate change in the United States

In adapting US agriculture to the climate of the 21st century, a key unknown is whether cropping frequency may increase, helping to offset projected negative yield impacts in major production regions. Combining daily weather data and crop phenology models, we find that cultivated area in the US suited to dryland winter wheat–soybeans, the most common double crop (DC) system, increased by up to 28% from 1988 to 2012. Changes in the observed distribution of DC area over the same period agree well with this suitability increase, evidence consistent with climate change playing a role in recent DC expansion in phenologically constrained states. We then apply the model to projections of future climate under the RCP45 and RCP85 scenarios and estimate an additional 126–239% increase, respectively, in DC area. Sensitivity tests reveal that in most instances, increases in mean temperature are more important than delays in fall freeze in driving increased DC suitability. The results suggest that climate change will relieve phenological constraints on wheat–soy DC systems over much of the United States, though it should be recognized that impacts on corn and soybean yields in this region are expected to be negative and larger in magnitude than the 0.4–0.75% per decade benefits we estimate here for double cropping.

Past and future changes in frost day indices in Catskill Mountain region of New York

AbstractChanges in frost indices in the New York's Catskill Mountains region, the location of water supply reservoirs for New York City, have potentially important implications. Frost day is defined as a day with Tmin < 0 °C. The objective of this study was to investigate past and predicted changes in minimum temperature (Tmin) and six frost indices in the Catskill Mountains covering six reservoir watersheds. Studied frost indices included (1) number of frost days, (2) number of months with frost, (3) last spring freeze date (LSF), (4) first fall freeze date (FFF), (5) growing season length (GSL), and (6) frost season length. Past changes in the frost indices were studied using observed daily Tmin for each watershed for the periods 1960–2008. Future changes in frost indices for the periods (2045–2065 and 2080–2100) were studied for emission scenarios (A1B, A2, and B1) downscaled from global climate models (GCMs). Results indicated a general increase in average Tmin and GSL and a decrease in number of frost days, months with frost, frost season length, earlier LSF, and later FFF from the historical to the future periods, and the magnitude of change varied among the watersheds and GCMs. For the period 1960–2000, in all watersheds (except Cannonsville), LSF occurred earlier by 2.6–4.3 days/decade, FFF occurred later by 2.7–3.2 day/decade, and GSL was longer by 2.4–4 day/decade. Among the scenarios and GCMs, LSF occurred earlier by 4–11 and 4.5–15 days/decade for the periods 2045–2065 and 2081–2100, respectively; FFF occurred later by 1–10 and 4–13 days/decade for the periods 2045–2065 and 2081–2100, respectively; and GSL was longer by 10–25 and 13–40 days/decade for the periods 2045–2065 and 2081–2100, respectively. The increase in GSL is expected to affect hydrologic, ecosystem, and biogeochemical processes with increased net primary productivity and a resulting increase in total annual evapotranspiration. Copyright © 2013 John Wiley & Sons, Ltd.

Long-term spatial and temporal trends in frost indices in Kansas, USA

Frost indices such as number of frost days (nFDs), number of frost-free days (nFFDs), last spring freeze (LSF), first fall freeze (FFF), and growing-season length (GSL) were calculated using daily minimum air temperature (Tmin) from 23 centennial weather stations across Kansas during four time periods (through 1919, 1920-1949, 1950-1979, and 1980-2009). A frost day is defined as a day with Tmin<0 °C. The long- and short-term trends in frost indices were analyzed at monthly, seasonal, and annual timescales. Probability of occurrence of the indices was analyzed at 5 %, 25 %, 50 %, 75 %, and 95 %. Results indicated a general increase in Tmin from 1900 through 2009 causing a decrease in nFDs. LSF and FFF occurred earlier and later than normal in the year, respectively, thereby resulting in an increase in GSL. In general, northwest Kansas recorded the greatest nFD and lowest Tmin, whereas southeast Kansas had the lowest nFD and highest Tmin; however, the magnitude of the trends in these indices varied with location, time period, and time scales. Based on the long-term records in most stations, LSF occurred earlier by 0.1- 1.9 days/decade, FFF occurred later by 0.2-0.9 day/decade, and GSL was longer by 0.1-

Changing freeze‐thaw seasons in northern high latitudes and associated influences on evapotranspiration

AbstractSeasonal frozen states in the northern terrestrial cryosphere limit vegetation photosynthetic activities and evapotranspiration (ET) through cold temperature constraints to biological processes and chemical unavailability of water as a result of being frozen. Seasonal transitions of the landscape between predominantly frozen and thawed conditions are analogous to a biospheric and hydrological on/off switch, with marked differences in ET, vegetation productivity and other biological activity between largely dormant winter and active summer conditions. We investigated changes in freeze–thaw (FT) seasons and ET from 1983 to 2006 and their connections in the northern cryosphere by analyzing independent satellite remote sensing derived FT and ET records. Our findings show that the northern cryosphere (≥ 40°N) has experienced advancing (−2.5 days/decade; P = 0.005) and lengthening (3.5 days/decade; P = 0.007) non‐frozen season trends over the 24‐year period, coinciding with an upward trend (6.4 mm/year/decade; P = 0.014) in regional mean annual ET over the same period. Regional average timing of spring primary thaw and the annual non‐frozen period are highly correlated with regional annual ET (|r| ≥ 0.75; P &lt; 0.001), with corresponding impacts to annual ET of approximately 0.6 and 0.5% per day, respectively. The impact of primary fall freeze timing on ET is relatively minor compared with primary spring thaw timing. Earlier onset of the non‐frozen season generally promotes annual ET in colder areas but appears to suppress summer ET by increasing drought stress in the southernmost parts of the domain where water supply is the leading constraint to ET. The cumulative effect of future freeze‐thaw changes on ET in the region will largely depend on future changes of large‐scale atmosphere circulations and rates of vegetation disturbance and adaptation to continued warming. Copyright © 2011 John Wiley &amp; Sons, Ltd.