Global Drought and Flood. Группа авторов

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Global Drought and Flood - Группа авторов


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f( ϕ ) is the fraction of the sensor view angle occupied by vegetation when viewed at an angle ϕ from nadir (Norman & Becker, 1995). For a canopy with a random distribution of leaves, a spherical distribution of leaf angles, and a leaf area index F,

      (2.2)equation

      The net balance of energy at the Earth’s surface can be represented by

      (2.3)equation

      where R n is the net radiation above the vegetated surface, and H, LE, and G are the net fluxes of sensible, latent, and ground conduction heating, respectively.

Schematic illustrations of (a) the surface-layer component of the ALEXI model. (b) The surface-layer model component is applied at times t1 and t2 during the morning hours, returning instantaneous sensible heat flux estimates.

      (Source: From Mecikalski, J. M., G. R. Diak, M. C. Anderson & J. M. Norman (1999). Estimating fluxes on continental scales using remotely sensed data in an atmosphere–land exchange model. Journal of Applied Meteorology, 38, 1352–1369. © American Meteorological Society.)

      (2.4)equation

      The ABL component of ALEXI is a simple slab model which describes the dynamics of the atmospheric boundary layer and is used as a closure technique to evaluate the morning evolution of air temperature, Ta, in the surface layer. It is assumed that all the air within the mixed layer is at a uniform potential temperature, and this value is related to the surface air temperature by

      (2.5)equation

      where p is the atmospheric pressure (in kPa) at the surface and R/c p = 0.286 (Anderson et al., 1997). Tennekes (1973) showed that the height of the convective boundary layer at any time is uniquely defined by the current surface air temperature and a morning temperature sounding. McNaughton and Spriggs (1986) presented a simplified conservation equation describing the growth of a convective boundary layer over time, assuming no subsidence and horizontal advection:

      (2.6)equation

Data source Specifications Resolution Format Example file names Size (MB)
GOES East and West Band 02 4 km McIDAS 1350954623.goes13.2014.001.061520.BAND_02 ~25
GOES East and West Band 04 4 km McIDAS 1350954623.goes13.2014.001.061520.BAND_04 ~25
GSIP L2 product 4 km NetCDF gsipL2_goes13_GENHEM_2014198_1145.nc.gz gsipL2_goes15_GWNHEM_2014198_1400.nc.gz ~20–30
VIIRS Global NDVI and EVI 375 m HDF5 GVF‐ASEVI‐P2_s20120726_e20120801_h00v01.h5 ~800
IMS Daily Northern Hemisphere Snow and Ice Analysis 24 km ASCII ims2014017_24km.asc ~1

      2.3.2. Extrapolation from Instantaneous to Hourly and Daily Fluxes

      A common technique for extrapolating instantaneous satellite‐based flux estimates to daily totals is to assume that the evaporative fraction (EF), given by the ratio of latent heat to the available energy, is constant during daylight hours for a given day (Gurney & Hsu, 1990; Shuttleworth et al., 1989; Sugita & Brutsaert, 1991). Given the value of EF determined at the ALEXI modeling time (t 2) along with hourly estimates of R n and G at times t i, which can be obtained from GOES, hourly values of system sensible and latent heating can be computed for days with clear mornings as (Anderson et al., 1997):

      (2.7)equation

      (2.8)Скачать книгу