Past IEM Features tagged: preciptablewater
This week the daily features have been looking at precipitation climatologies for Iowa. In general, there is a northwest to southeast pattern with intensities increasing the further south you travel. One of the reasons for this gradient is the featured map for today. It shows a climatology from one of the main weather forecast models in this country for a variable known as preciptable water. This term is simply the depth of liquid water in the atmosphere above our heads if all of the water was condensed. The amount of water in the atmosphere is important to consider when looking at rainfall rates. The water coming as rain has to come from somewhere. So while the differences shown on this plot may not look large, on average a storm in southern Iowa will have about 10% more water (primarily water vapor) to work with than a storm over northern Iowa. These difference add up over the course of a year and help explain the differences in amount of rain. The observant reader may wonder why then there is not a more NW to SE gradient? Generally, that is likely more to do with the common storm track paths, which is a proxy to jet stream orientation.
One of the reasons for our recent lack of rainfall has been the dearth of precipitable water. Precipitable water is a measure of depth of water in a column of air if all phases found were converted to liquid. The featured chart presents the combination of preciptable water analyzed by a weather forecast model and the total rainfall for that day. The upper chart presents the frequency of having rainfall on a given May day partitioned by preciptable water value. Increasing water content in the atmosphere increases our chances of rain. The lower chart presents the combination of daily precip observations against the precipitable water value. The one to one line shows that often the preciptable water value provides an upper bound to the amount of rainfall we may receive. Advection processes help to replenish atmospheric water content so we can get rainfall totals greater than the amount of water in the column of air ("in this house we obey the conservation of mass law").
Tags: preciptablewater may