Climate change is altering the timing and duration of the vernal window, a period that marks the end of winter and the start of the growing season when rapid transitions in ecosystem energy, water, nutrient, and carbon dynamics take place. Research on this period typically captures only a portion of the ecosystem in transition and focuses largely on the dates by which the system wakes up. Previous work has not addressed lags between transitions that represent delays in energy, water, nutrient, and carbon flows. The objectives of this study were to establish the sequence of physical and biogeochemical transitions and lags during the vernal window period and to understand how climate change may alter them. We synthesized observations from a statewide sensor network in New Hampshire, USA, that concurrently monitored climate, snow, soils, and streams over a three-year period and supplemented these observations with climate reanalysis data, snow data assimilation model output, and satellite spectral data. We found that some of the transitions that occurred within the vernal window were sequential, with air temperatures warming prior to snow melt, which preceded forest canopy closure. Other transitions were simultaneous with one another and had zero-length lags, such as snowpack disappearance, rapid soil warming, and peak stream discharge. We modeled lags as a function of both winter coldness and snow depth, both of which are expected to decline with climate change. Warmer winters with less snow resulted in longer lags and a more protracted vernal window. This lengthening of individual lags and of the entire vernal window carries important consequences for the thermodynamics and biogeochemistry of ecosystems, both during the winter-to-spring transition and throughout the rest of the year.

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The conversion efficiencies and the linewidth of the Stokes components which result from stimulated Raman scattering (SRS) in hydrogen gas pumped with a high power alexandrite laser have been studied. The pump laser was operated at 742 nm giving first and second Stokes components at 1074 nm and 1935 nm respectively. The pump laser could be operated either in the normal Q-switched configuration with a linewidth of the order of 0.2-0.3 nm or with injection seeding which narrowed the linewidth to 1-2 pm. Measurements of the Stokes components under these varying conditions reveal that there is no effect on the conversion efficiency by the narrowing of the pump linewidth and that the linewidth of the first Stokes component is broader than the expected linewidth of the injection seeded pump. Modelling of the conversion from the pump to the Stokes components shows a strong dependence of this conversion on a specific, resonant four wave mixing process. The study gives a complete set of information about the radiation resulting from SRS in molecular hydrogen pumped with a high power, narrow linewidth, tunable solid-state laser.

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We have developed an interactive numerical model, which describes the operational characteristics of multiple diode-pumped laser systems that are Q-switched or Q-switched/cavity-dumped. These are the first computer-generated results obtained using rate equations that incorporate the 'inversion reduction parameter' gamma . The quantity gamma can partially take into account effects of relaxation and thermally driven transfer, which occurs between laser level multiplets. A value of gamma near unity is found for the Nd:YAG lasers, implying considerable thermal relaxation of the lower lasing level, and it may also imply some thermal refreshing of the upper lasing level. A value of gamma =2 is found to fit the Ti:sapphire results, indicating that the lower lasing level is not relaxed during laser pulse development. The interactive nature of the model allows for variation of all the significant parameters required to generate a Q-switched or cavity-dumped pulse. Input information is expressed in terms of standard variables such as stored energy, optical and dissipative losses and stimulated emission and pump absorption cross sections. The effects of additional parameters, including the Gaussian beam profile of the cavity, the induced losses associated with the rise and fall times of the Q-switch and cavity-dump Pockels cell, and the induced thermal lensing effects produced by the pump rate in the gain medium are taken into account. The width and energy of the cavity-dumped laser pulse can also be studied in the model by varying the cavity-dump voltage switch transition time.