South Florida Migratory Drought

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Resolution is a term used by mappers to define the relationship of data to dimension. South Florida Water Management District has a network of rainfall gauges that blanket Collier County. Many of these gauges record data automatically and post results to a website. Some of the gauges have been collecting data since the early 1940’s.

The inset map is the result of a spreadsheet analysis of some field data using a 5 year moving average, and indicates a substantial drop in annual rainfall over the last 30 years. The moving average technique smoothes the graph of the data, so that patterns in the data are more discernable. The decent of coastal annual rainfall volume hasn’t been constant, but it has stepped down in ten year cycles.

The effect of this type of dramatic shortfall in rain would normally present itself as a more pronounced fire season. There have been substantial burns in the everglades, but the extensive burns in the urban estates like the ones in the early 80’s have not been an issue. A keener fire awareness of the population, a better structural capability of the first responders, and better roads and bridges are all factors.

Then there is the spring rainfall anomaly. The severity of the fire season is constrained by the regularity of spring rains. Even though the Corkscrew rain gauge has not seen above average rainfall since 1994, recent years have been relatively fire disaster free, because of early spring rains.

The Collier coastal rain gauges indicate a regular 10 year pattern with peak rainfall on the 5’s. The 2011 gauge reading was lower than both the 2009 and the 2010 reading. While it is probable that 2012 is a better rainfall year than 2011, the 30 year trend is of lower high rainfall years and lower low rainfall years. That trend will not be broken with a bounce in the rain volume for a single year.

This aspect of climate change doesn’t seem to be indicated in the Climate Prediction Center's historic rainfall graphics. The reason could be that those graphics are built from satellite accumulated rainfall data instead of rain gauge data. The Problem could be resolution. The National Weather Service has rain gauges at the major reporting stations sprinkled around the state, and the National Oceanic and Atmospheric Administration just assimilated NWS. It could be that CPC which is a branch of NOAA just hasn’t gotten around to integrating the data.

The national drought pattern has in the past been from west to east up to about 2007 when a long term extreme Southwest Florida drought eased, and gave way to shorter dry season and a dryer wet season. Now the drought migration in Southwest Florida seems to be from season to season.

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http://www.srh.noaa.gov/mrx/research/precip/precip.php

4) Gage data adjustment techniques

This field embraces a large array of procedures, several originating as far back as the 1970's. Most are statistical, but there are wide variations in analysis mode. The sampling problems addressed in Section 3a affect all of them. Whichever one is used in the bias adjustment algorithm should minimize these problems as much as possible for estimation of heavier rainfall, which is of primary concern in flood forecasting.

The bias adjustment algorithm will eventually be activated in the WSR-88D. It has recently been tested in a few cases and was shown not to degrade the Rr field, even with input from as little as three gages (Seo et al. 1995). It will continue to be evaluated in the field, however, and might be changed or simplified if problems are found. If its benefits do not outweigh its liabilities, the scheme should be overhauled, augmented, or replaced. A few prototypes for potential augmentation or replacement follow.

Modification of adjustment for specific meteorological regimes or physical factors has been done by Collier et al. (1983) and Austin (1987). Classification of a precipitation system as stratiform or convective should be feasible at the very least.

A good candidate for classification might be "tropical" systems. There is much evidence that the PPS underestimates rainfall with these systems (Natural Disaster Survey Report 1995; Ruthi et al. 1993; Hitchens et al. 1993, Woods et al. 1995). This may result from their high precipitation efficiencies, DSD's weighted toward small sizes or, less likely, from a hail threshold that is too small for these cases. In response to the evidence, the OSF recently submitted a new relation, for use in the field during "tropical rain" events. This relation, Z=250 R 1.2 (from Rosenfeld et al. 1993), provided good Rr estimates in four tropical rainstorms in Texas and Florida. It will be interesting to follow the performance of this relation upon a larger sample of tropical systems, across the country.

Finally, Rosenfeld et al. (1994) proposed that Rr be adjusted locally, in small windows surrounding a gage. This requires more computation than for a single bias, but not enough to tax the workstations of today. A bigger obstacle is a sparsity of gages under many WSR-88D umbrellas.