Archive for May, 2012

Daily Rainfall At WPBIA

May 31, 2012

Lately things have been quite wet around here in my part of Florida. This is about what one would expect if we are heading into an El Nino year, but El Nino is not yet official, I think. Anyway, I was curious about what the typical rainfall in our area tends to be from day to day. The nearest climate station is apparently up at the West Palm Beach  International Airport. It has (somewhat incomplete) data from July of 1938 until roughly the present (data available at KNMI run up until the 16th as of now) and the daily rainfall history looks like this:

Figure 1, Daily Precipitation Totals in inches, WPBIA

You probably notice that point near the beginning that is higher than any other point: specifically, according to the data the rainfall total for April 17 1942 was more than fifteen inches! Well, I’m not sure about that particular data point since it is so exceptional, but I’m going to assume it, and the rest of the data, are accurate. It does not appear there is any obvious trend within the daily data, though of course this mostly because it is just so noisy. What does the annual variation look like?

Figure 2, Daily Precipitation Totals plotted against “Fraction of the Year”

The above plot shows rainfall totals plotted against the “Fraction of the Year” which here is taken as the sum of the month minus one divided by twelve and the day of the month minus one divided by thirty one divided by twelve. That plot has 26420 data points. So I decided to see what the 1321 point center averages of that as a series is, each point of which representing the average daily precipitation in approximately one twentieth of the year. But first, I put a second version of the above series at the end of it, so that I could then get a full annual cycle in a smooth. So here is a full cycle of one twentieth of the year centered averages, plotted against ay of the year (well, almost, it’s actual “Fraction of the Year” times 365.2425, rounded down to the nearest integer, plus one):

Figure 3, one twentieth year centered averages of the Daily Precipitation Totals

Interestingly, the annual cycle in precipitation at WPBIA has not one wet season peak, but two. The wet “half” of the year (the above curve’s first and last points above it’s overall average) lasts from roughly mid to late May to the end of October. The two wet peaks occur during the above average parts of the wet “half” of the year (In the same manner as as the dates for the above average half year were determined, only this time with the average as the average of points above the average for all points)¬† from the end of May to till just after Independence Day (ie early July) and from mid-August to mid-October, with the sharp peak of the latter period occurring in in mid to late September. Now, obviously this is just on average, and the edges of these periods are fuzzy even in the smoothed data by a few days. But the double peak is an interesting feature of our local seasonal weather variation. Anyone have any idea why this may occur?

Revisting Category 4&5 Tropical Cyclones…Again

May 19, 2012

A little less than a year ago, I updated my work examining whether there has been any increase in the number of category four and five tropical cyclones, first in the Northern Hemisphere and then, more cautiously, the entire world since 1987 (that is, when aircraft reconnaissance ceased in the Northern West Pacific Basin) (in the case of the Southern Hemisphere, this means the 1987-88 season at the start). Well I figure now is as good a time as any to update to include the year 2011 in the analysis (the 2010-2011 season for the Southern Hemisphere.

First, there is the Northern Hemisphere data:

As you can see, 2011 was a slightly above average year in terms of the number of category four and five storms in the Northern Hemisphere (in terms of overall activity it was still below average). Nevertheless, the trend remains ever so slightly negative which is contrary to the hypothesis that the number of such storms would increase with higher sea surface temperatures. But what about the world? Well, as mentioned before, because the Southern Hemisphere seasons occur during the end of one year and the beginning of the next, it is difficult to combine seasons. So I did it two ways: in one time series I added the Southern Hemisphere seasons to the Northern Hemisphere totals in the year associated with the beginning of their season, while in the other added them to end year time series.

Here they are, in green and red, respectively:

The conclusion with respect to the global data is even stronger now than last year: there are no trends! The slight positive trend in the green time series has vanished, in fact it appears it is now very, very slightly negative; the red trend is still very slightly negative. So I remain confident in my initial assessment of this data: there is no evidence that higher sea surface temperatures are increasing the number of strong tropical cyclones.