Simulations of Regional Climatic Patterns Which Impact the Florida Bay Water Cycle

 

Topical Area: Meteorology and Hydrology

 

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

 

Principal Objective or Hypothesis of Project

 

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.

 

Summary of Methods

 

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.

 

Summary of Results to Date

 

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.

 

Project Duration

 

This research is currently in its third year.

 

Details of Methodology

 

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.

 

Outlook for Remaining Work

 

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.