Craig A.
Mattocks, University of Miami/CIMAS, NOAA/AOML/Hurricane
Research Division
Miami, Florida; Paul Trimble, Matthew Hinton,
Beheen Trimble, Marie Pietrucha, South Florida Water Management District, West
Palm Beach, Florida
This project directly addresses the first
central question articulated in the Strategic Plan of the Interagency Florida
Bay Science Program, namely: "How, and at what rates, do persistent and/or
catastrophic storms alter freshwater input (via their associated local
evaporation/precipitation patterns), thereby inducing changes in the
circulation and salinity/nutrient content of Florida Bay?"
The approach is to employ the Center for
Analysis and Prediction of Storms' Advanced Regional Prediction System (ARPS) cloud-/mesoscale
atmospheric numerical weather prediction model, coupled to the SFWMD's
hydrologic models, to simulate persistent, locally-forced weather regimes
(land/lake/urban heat island breeze circulations) which generate thunderstorm
complexes over the Everglades and coastal areas that account for roughly
one-third of Florida's annual rainfall. Besides providing precipitation and
high-resolution surface winds for use as boundary conditions/forcing in bay and
ocean circulation models, the atmospheric model's surface energy
parameterization allows the prediction of evaporation - at the ground surface,
from the fraction of foliage covered by intercepted rainfall, and from
transpiration by leaves.
Numerous meteorological modeling studies have
pointed out that land use modifications (including commercial development,
agriculture, and water management practices) may have a significant impact on
the spatial and temporal distribution of regional scale rainfall for selected
synoptic scale meteorological conditions. In particular, small scale
heterogeneities in the soil moisture, surface albedo and thermal inertia are
the most dominant controlling factors involved in altering the speed and
intensity of the sea breeze convergence zone and, hence, the location of
attendant thunderstorm formation over the Florida peninsula. However, until
recently, such numerical studies were severely limited by computer capacity, so
the simulations could not include these feedback mechanisms between the
hydrologic and atmospheric systems. With the advent of powerful desktop
computer workstations, the simulation of important coupled system processes is
now possible.
An exact, transparent coupling of the ARPS
mesoscale meteorological model with SFWMD's regional scale hydrologic models
would involve an extensive redevelopment effort to match their complex surface
energy budget formulations. This is beyond the scope of the current project.
Nevertheless, it is possible to reconfigure the models so they have consistent
grid domains and run them in a "loosely coupled" mode - exchanging
boundary conditions and fields off-line, rather than in exact lockstep sequence
- while integrating the models over a period of a few months. It is then
possible to run land use sensitivity experiments (such as present day
development vs. pre-colonized natural system conditions) and examine periods
which exhibit correlations between differences in ponded water and the spatial
and temporal distribution of rainfall. One of the most intriguing potential
applications of this coupled model approach is the prediction of the
environment's response to "What if ...?" anthropogenic impact
assessment scenarios, such as the development/urbanization of pristine areas or
the restoration of natural habitats.
In earlier coarse-mesh ARPS model simulations
of an August 1975 Florida Area Cumulus Experiment (FACE) sea breeze case, the
atmosphere's response to incorporating realistic, modern-day horizontal
gradients in the land use were striking. Enhanced diurnal heating over heavily
developed areas, such as Naples-Fort Meyers and Tampa, induced strong urban
heat island circulations which doubled the amount of simulated rainfall over
these heavily populated regions. Rapid evaporation/drainage and heating of the
porous, cultivated land south of Lake Okeechobee caused abrupt divergent
deflections of the surface winds over the lake and generated a thunderstorm
complex similar to the convective cells diagnosed in the real data. The arc-shaped
band of maximum rainfall, associated with the lake breeze, shifted from east of
Lake Okeechobee to a more realistic location at its southern shore.
The evaporation pattern correlated well with
the strongest divergent and initially driest surface wind fields, in the
vicinity of the greatest surface-to-air temperature/humidity differences.
Moisture was picked up by the atmospheric flow over Lake Okeechobee and by the
organized offshore downdrafts associated with the west coast sea breeze
circulation, while the Florida Everglades "muck" soil in the interior
of the state tended to resist evaporation due to its high water retention and
strong capillary forces. Thus, a significant north-south mesoscale gradient in
evaporation was simulated across Florida Bay.
This research is currently in its third year.
In the first phase of development of a
hydro-meteorological coupled model, the ARPS model grid has been completely reconfigured
to closely match the grid of the Natural System Model (NSM) and the South
Florida Water Management Model (SFWMM). The horizontal resolution of ARPS has
been tripled, from 9 km to 3.22 km (2 miles) on a Mercator projection. High-resolution GIS soil/vegetation, land
cover/use surface characteristics, and terrain elevation data from SFWMD has
been reprocessed using GIS ARC-INFO "fishnet" area-weighted polygon
interpolation techniques and incorporated into ARPS. This allows the simulation
of local weather regimes driven by micro-scale features in the surface
properties.
The first step is to rerun the
climatologically representative sea breeze simulation at the higher resolution,
incorporating the SFWMD's hydrologic model soil, vegetation, land cover and
terrain elevation datasets. The results will be compared/calibrated against
composites of the real data measurements to assess model performance. This
realistic "present day" atmospheric response will then be contrasted
against a simulation initialized with surface property databases that portray
the more homogeneous pre-colonized "natural system". Through direct
comparisons of the simulations and by selectively reverting isolated areas of
urbanization and drainage to their natural state, any distinctive microclimates
(urban heat islands, associated shifts in the horizontal rainfall distribution,
and modulations in the amount of rainfall) which have emerged over the past
century can be identified.
These improvements in horizontal resolution,
the specification of surface characteristics, and the inclusion of feedback
mechanisms between the hydrologic and atmospheric systems should help to
resolve details in the shape of thunderstorm convective cells, rectify previous
underpredictions of rainfall over the Florida peninsula, and provide more
reliable estimates of total freshwater input from the atmosphere into the
ground surface/bay/ocean below. It will then be possible to quantify the
effects that heavy or persistent rain episodes have on the salinity/nutrient
composition of Florida Bay, determine the extent of sewage system overflows,
and assess the degree of eutrophication by fertilizer/pesticide/contaminant
runoff from agricultural and industrial areas. This work also lays a foundation
for the development of more closely coupled versions of a hydro-meteorological
model in the near future.