George Leshkevich

Rapid and highly variable warming of lake surface waters around the globe

In this first worldwide synthesis of in situ and satellite-derived lake data, we find that lake summer surface water temperatures rose rapidly (global mean = 0.34°C decade−1) between 1985 and 2009. Our analyses show that surface water warming rates are dependent on combinations of climate and local characteristics, rather than just lake location, leading to the counterintuitive result that regional consistency in lake warming is the exception, rather than the rule. The most rapidly warming lakes are widely geographically distributed, and their warming is associated with interactions among different climatic factors—from seasonally ice-covered lakes in areas where temperature and solar radiation are increasing while cloud cover is diminishing (0.72°C decade−1) to ice-free lakes experiencing increases in air temperature and solar radiation (0.53°C decade−1). The pervasive and rapid warming observed here signals the urgent need to incorporate climate impacts into vulnerability assessments and adaptation efforts for lakes.

Development of recurrent coastal plume in Lake Michigan observed for first time

NOAA CoastWatch satellite imagery from early 1996 captured the initiation, development, and decay of a recurrent coastal plume in southern Lake Michigan (Figure 1). For the past 4 years intermittent satellite coverage has revealed a late winter-early spring plume in the lake, a feature also observed by Mortimer [1988]. In 1996, clear weather conditions allowed researchers to observe the plumes development for the first time and they also collected water samples from helicopter and a small boat.

Automated Mapping of Surface Water Temperature in the Great Lakes

Abstract A procedure for producing daily cloud-free maps of surface water temperature in the Great Lakes has been developed. It is based on satellite-derived AVHRR (Advanced Very High Resolution Radiometer) imagery from NOAAs CoastWatch program. The maps have a nominal resolution of 2.6 km and provide as complete as possible coverage of the Great Lakes on a daily basis by using previous imagery to estimate temperatures in cloud covered areas. Surface water temperature estimates derived from this procedure compare well with water temperatures measured at the eight NOAA weather buoys in the lakes. The mean difference between the buoy temperature and the satellite-derived temperature estimates is less than 0.5°C for all buoys. The root mean square differences range from 1.10 to 1.76°C. As one example of the possible applications of this product, the daily surface water temperature maps for 1992 to 1997 were analyzed to produce daily estimates of average surface water temperature for each lake. Results are compared to the long-term (28 year) mean annual cycle of average surface water temperatures. The average surface water temperatures vary from as much as 4°C below climatology in 1993 to 2 to 3°C above climatology in 1995. The new analysis procedure also provides a more realistic depiction of the spatial distribution of temperature in the springtime than the climatological maps.

A global database of lake surface temperatures collected by in situ and satellite methods from 1985–2009

Global environmental change has influenced lake surface temperatures, a key driver of ecosystem structure and function. Recent studies have suggested significant warming of water temperatures in individual lakes across many different regions around the world. However, the spatial and temporal coherence associated with the magnitude of these trends remains unclear. Thus, a global data set of water temperature is required to understand and synthesize global, long-term trends in surface water temperatures of inland bodies of water. We assembled a database of summer lake surface temperatures for 291 lakes collected in situ and/or by satellites for the period 1985–2009. In addition, corresponding climatic drivers (air temperatures, solar radiation, and cloud cover) and geomorphometric characteristics (latitude, longitude, elevation, lake surface area, maximum depth, mean depth, and volume) that influence lake surface temperatures were compiled for each lake. This unique dataset offers an invaluable baseline perspective on global-scale lake thermal conditions as environmental change continues.

Particle Transport, Nutrient Cycling, and Algal Community Structure Associated with a Major Winter-Spring Sediment Resuspension Event in Southern Lake Michigan

Abstract Over the past decade, intermittent satellite imagery revealed the presence of an extensive plume of resuspended sediments in late winter-early spring with a clear offshore projection coinciding with the region of maximum sediment accumulation in the lake. The large scale of the plume implied that this process was important in sediment, and associated constituent, cycling and transport, but it had never been sampled due to severe conditions. The onset of the 1996 event coincided with a major March storm. Within a few days the plume was approximately 10 km wide and over 300 km in length, implying that the source of the reflective materials was widely distributed. An estimate of the total mass of resuspended sediment, 12 days after the storm, was similar to the annual external load of (sand-free) particulate material to the southern basin. The high turbidity plume persisted for over a month, progressing northward along the eastern shore with a major offshore transport feature. Sediment traps within this feature recorded a major mass flux event. The plume was sampled on two occasions and was found to contain 5 to 10 times as much suspended matter as open-lake locations outside the visible plume. Total particulate phosphorus was high within the plume making this episodic process important in sedimentwater exchange. The diatom community structure within the plume was significantly different from outside the plume and was characteristic of more eutrophic waters. Abundance of non-diatom phytoplankton and microbial food web organisms were highest at the plume edge. The episodic nature of this process makes it difficult to sample, but the scale makes it important in designing monitoring programs and massbalance modeling efforts.

Classifying and Forecasting Coastal Upwellings in Lake Michigan Using Satellite Derived Temperature Images and Buoy Data

ABSTRACT Coastal upwellings are common in the Great Lakes but have lacked enumeration and systematic classification of spatial extent, frequency, duration, and magnitude. Near real-time sea surface temperature (SST) images derived from the Advanced Very High Resolution Radiometer (AVHRR) provide indices of upwelling events, but visual inspection of daily images can be tedious. Moreover, the definition of what constitutes an upwelling from AVHRR data is subjective. We developed a semi-automated method to classify upwellings during the period of thermal stratification using daily, cloud-free surface temperature charts from AVHRR SST data. Then we statistically evaluated the location, frequency, magnitude, extent, and duration of upwelling events in Lake Michigan from 1992–2000. Further, we analyzed meteorological data from the National Data Buoy Center buoys in an attempt to improve the reliability of the classification and to provide a means for future forecast of coastal upwelling. Although variable, upwelling events along the western shoreline were preceded by 4 days of southerly and west-to-north-westerly winds, while upwelling events occurring along the eastern shore were preceded by 4 days of northerly winds. Probability of an upwelling event occurring was a function of the direction-weighted wind speed, reaching a 100% probability at direction weighted wind speeds of 11 m s−1 for the western shore. Probability of an upwelling occurrence along the east coast reached 73% at 11 m s−1 and 100% at 13 m s−1. Continuous measurements of wind data with a sufficient temporal resolution are required during the entire upwelling season to improve the predictability of upwellings.

Satellite Measurements of Surface Water Temperature in the Great Lakes: Great Lakes Coastwatch

Abstract This paper describes the NOAA CoastWatch program for the Great Lakes and discusses the applications and limitations of satellite-measured surface water temperature images received as a result of this program in mapping and analyzing physical features of the Great Lakes environment. The initial product of the CoastWatch program is a set of surface water temperature images derived from NOAA AVHRR (Advanced Very High Resolution Radiometer) data. These temperature maps are produced on a routine basis (usually 2–3 sufficiently cloud-free images per week) and are made available within hours of acquisition. The satellite-derived water temperatures from images acquired during the period May 1990 to May 1991 were compared to temperatures measured at NOAA weather buoys and found to be highly correlated. We found the satellite-derived temperatures were consistently 1–1.5C° cooler than buoy temperatures. Root mean square deviations between buoy and satellite temperatures ranged from 0.8 to 1.6C°. There was also a consistent pattern to the geographic registration errors of the images, ranging from 8.4 km westward for the NOAA 10 nighttime pass to 8.3 km northeast for the NOAA 11 nighttime pass. Potential application of the imagery to detection and location of thermal fronts, analysis of circulation patterns, and ice and snow mapping are also discussed.

Development of the Great Lakes Ice-Circulation Model (GLIM): Application to Lake Erie in 2003–2004

ABSTRACT To simulate ice and water circulation in Lake Erie over a yearly cycle, a Great Lakes Ice-circulation Model (GLIM) was developed by applying a Coupled Ice-Ocean Model (CIOM) with a 2-km resolution grid. The hourly surface wind stress and thermodynamic forcings for input into the GLIM are derived from meteorological measurements interpolated onto the 2-km model grids. The seasonal cycles for ice concentration, thickness, velocity, and other variables are well reproduced in the 2003/04 ice season. Satellite measurements of ice cover were used to validate GLIM with a mean bias deviation (MBD) of 7.4%. The seasonal cycle for lake surface temperature is well reproduced in comparison to the satellite measurements with a MBD of 1.5%. Additional sensitivity experiments further confirm the important impacts of ice cover on lake water temperature and water level variations. Furthermore, a period including an extreme cooling (due to a cold air outbreak) and an extreme warming event in February 2004 was examined to test GLIMs response to rapidly-changing synoptic forcing.

Optical characterizations and pursuit of optical closure for the western basin of Lake Erie through in situ measurements

ABSTRACT In situ measurements of inherent (IOPs) and apparent optical properties (AOPs), along with laboratory measurements of optically active constituents, were made at sites (n = 14) in western Lake Erie following a wind event to advance the characterization of the underwater and emergent light fields of these waters and to support related IOP-based model development and testing. Modern instrumentation was used to make spectral (wavelength,&lgr;) measurements of the IOPs of absorption [a(&lgr;)], particulate scattering ]bp(&lgr;)], and particulate backscattering [bbp(&lgr;)] coefficients, and the AOPs of remote sensing reflectance ]Rrs(&lgr;)], and the diffuse attenuation coefficient for downwelling irradiance [Kd(&lgr;)], Optical closure analyses were conducted to demonstrate the credibility of the measurements, by comparing AOP observations to predictions based on radiative transfer expressions that utilized IOP measurements as inputs. Substantial spectral variations in a and its contributing components, and more modest wavelength dependencies for bp and bbp, were documented that are consistent with observations reported for marine case 2 systems. The backscattering ratio, bbp:bp, was strongly positively related to the contribution of minerogenic particles to the overall concentration of suspended particulate material. Major spatial differences in both IOPs and AOPs were observed that were driven by the attendant differences in the concentrations and composition of the optically active constituents, but particularly minerogenic particles, mediated in part by sediment resuspension. Good optical closure between the independently measured IOPs and AOPs was achieved. Direct measurement of bbp(&lgr;) was found to be critical to pursue closure for Rrs(&lgr;) and thereby support related remote sensing initiatives.

Severe Ice Cover on Great Lakes During Winter 2008–2009

The North American Great Lakes contain about 95% of the fresh surface water supply for the United States and 20% for the world. Nearly one eighth of the population of the United States and one third of the population of Canada live within their drainage basins. Because of this concentration of population, the ice cover that forms on the Great Lakes each winter and its year-to-year variability affect the regional economy [Niimi, 1982]. Ice cover also affects the lakes abiotic environment and ecosystems [Vanderploeg et al., 1992] in addition to influencing summer hypoxia, lake effect snow inland, water level variability, and the overall hydrologic cycle of the region [Assel et al., 2004].