Jeffrey Freedman

Boundary Layer Clouds and Vegetation–Atmosphere Feedbacks

Abstract An analysis of boundary layer cumulus clouds and their impact on land surface–atmosphere exchange is presented. Seasonal trends indicate that in response to increasing insolation and sensible heat flux, both the mixed-layer height (zi) and the lifting condensation level (LCL) peak (∼1250 and 1700 m) just before the growing season commences. With the commencement of transpiration, the Bowen ratio falls abruptly in response to the infusion of additional moisture into the boundary layer, and zi and LCL decrease. By late spring, boundary layer cumulus cloud frequency increases sharply, as the mixed layer approaches a new equilibrium, with zi and LCL remaining relatively constant (∼1100 and 1500 m) through the summer. Boundary layer cloud time fraction peaks during the growing season, reaching values greater than 40% over most of the eastern United States by June. At an Automated Surface Observing System (ASOS) station in central Massachusetts, a growing season peak is apparent during 1995–98 but reve...

Growing season water balance at a boreal jack pine forest

Measurements of energy and CO2 fluxes were made over the growing seasons of 1994 and 1996 in a northern jack pine forest as part of the Boreal Ecosystem-Atmosphere Study (BOREAS). Simultaneous measurements of soil water potential and content, rainfall, leaf wetness, and air specific humidity by our group and others allowed us to construct a complete water balance at this site for the 1994 season. The longer-term (spanning weeks) rate of loss of soil moisture in the upper 0.25 m of soil matched the total evapotranspiration, measured by eddy covariance. Evapotranspiration (measured as QE, latent heat flux) was just 24% of the net radiation, a result that has been found in many boreal forest types, regardless of the canopy coverage. Low canopy conductances (typically 2–4 mm s−1) probably represent an adaptation to the extremely low soil moistures and poor nutrient status of the site. Net radiation was the best single-variable predictor of evapotranspiration, having a correlation coefficient of 0.8 with QE for the 1994 season. Afternoons with sustained (>4 continuous hours) cloudless conditions resulted in water stress detectable as reduced QE relative to what would be predicted from the net radiation alone. The open canopy at our site promoted the role of the lichen-covered surface in the overall water vapor exchange; subcanopy QE was 26% of the total.

Histopathologic study of the molteno glaucoma implant in three patients

Three eyes with the Molteno glaucoma implant (one eye with epithelial downgrowth, one eye with iridocorneal endothelial syndrome, and one eye with aphakia and glaucoma) were enucleated two to six years after implantation. Histopathologic examinations disclosed no evidence of erosion of sclera or conjunctiva of the eye by the glaucoma implant device. In the outer layers of the bleb wall, few and mostly degenerated inflammatory cells were present, which represented a minimal inflammatory reaction. Scanning electron microscopy of the tubes in these three patients showed that the tube was intact, patent, and without signs of degradation. The tube entering into the anterior chamber caused no appreciable inflammation and maintained its patency even when downgrowth epithelial cells lined the anterior chamber. The Molteno plate induced little or no inflammatory reaction. Therefore, the Molteno glaucoma implant is a useful device for patients with high risk for failure after surgery for glaucoma.

Supra–Tenon Capsule Placement of Original Molteno vs Molteno 3 Tube Implants in Black Patients With Refractory Glaucoma: A Single-Surgeon Experience

OBJECTIVE To evaluate the efficacy of supra-Tenon capsule placement of original Molteno vs Molteno 3 tube implants (Molteno Ophthalmic, Dunedin, New Zealand) in black patients with refractory glaucoma. METHODS A retrospective study comparing the efficacy of supra-Tenon capsule placement of the original Molteno tube implant (130-mm(2) plate size) with the newer Molteno 3 tube implant (175-mm(2) plate size). Original Molteno tube implants were placed into 17 eyes and Molteno 3 tube implants into 27 eyes. RESULTS Success was defined as an intraocular pressure of 18 mm Hg or less with or without adjuvant medical therapy. The median follow-up periods were 24 months (range, 12-48 months) for the original Molteno-implanted group (hereafter referred to as the original Molteno group) and 18 months (range, 12-48 months) for the Molteno 3-implanted group (hereafter referred to as the Molteno 3 group). The median preoperative intraocular pressures were 30 mm Hg for the original Molteno group and 24 mm Hg for the Molteno 3 group. The median postoperative intraocular pressures were 13 mm Hg for the original Molteno group and 14 mm Hg for the Molteno 3 group. Using Kaplan-Meier analysis, survival percentages were 71% for the original Molteno group and 88% for the Molteno 3 group. Log-rank test for comparison of survival indicated no significant difference between the 2 groups (P > .25). CONCLUSIONS Supra-Tenon capsule placement of single-plate Molteno tube implants of different sizes can adequately control intraocular pressure in a group of patients with refractory glaucoma. Tenon capsule elimination seems to negate the effect of plate size.

Postfrontal Airmass Modification

The northeastern United States is subject to relatively frequent passages of frontal systems during the growing season. After a frontal passage, the newly arrived air mass is gradually modified by the underlying, mostly vegetated landscape. For the 1995‐98 growing seasons, 25 frontal sequences with at least 4 days between frontal passages were identified; 16 had sufficient data continuity for rigorous analysis. A composite of sequences featuring the daily appearance of boundary layer cumulus clouds (BLcu) indicates a diminished role for entrainment and other external forcings because of the daily occurrence of a rapid growth phase in the mixedlayer (ML) diurnal evolution subsequent to day 1. Between frontal passages, net heat and moisture flux convergence in the ML is near zero, but during the warming and moistening phase, the surface flux terms, through a net radiation‐BLcu feedback, are the principal controls on the tendencies of the ML temperature u and specific humidity q. The combination of the u and q tendencies leads to a nearly constant lifting condensation level, relative humidity, and BLcu cloud fraction during the latter part of the sequences. The presence of BLcu enhances water use efficiency and net afternoon carbon uptake throughout the sequence, with day 4 featuring optimal conditions. A multiday box model was used to perform sensitivity studies on subsidence, the lapse rate g uy above the ML, cloud mass flux, and the regional surface Bowen ratio breg. The effects of subsidence and g uy on ML processes are most conspicuous on day 1; during subsequent days, the rapid growth phase dominates the ML growth equation and reduces the impact of these external terms. Increasing breg to 3.5 reduces BLcu fraction to less than 20% and produces little net moistening of the ML, whereas reducing breg by 30% increases sequence BLcu coverage by 30%‐80%. In sum, the presence of a net radiation‐BLcu feedback allows for the establishment of an equilibrium in the ML heat and moisture tendencies and ensures the appearance of BLcu on each day of the sequence, thus sustaining favorable conditions for forest‐atmosphere exchange (i.e., carbon uptake).

Simulating Impacts of Real-World Wind Farms on Land Surface Temperature Using the WRF Model: Validation with Observations

AbstractThis study simulates the impacts of real-world wind farms on land surface temperature (LST) using the Weather Research and Forecasting (WRF) Model driven by realistic initial and boundary conditions. The simulated wind farm impacts are compared with the observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) and the first Wind Forecast Improvement Project (WFIP) field campaign. Simulations are performed over west-central Texas for the month of July throughout 7 years (2003–04 and 2010–14). Two groups of experiments are conducted: 1) direct validations of the simulated LST changes between the preturbine period (2003–04) and postturbine period (2010–14) validated against the MODIS observations; and 2) a model sensitivity test of LST to the wind turbine parameterization by examining LST differences with and without the wind turbines for the postturbine period. Overall, the WRF Model is moderately successful at reproducing the observed spatiotemporal variations of the background LS...