Analysis of Percentile Frequency Change Maps
Flood Analysis
Annual:
In the >95 percentile maps, areas across the general northern and eastern part of
the region show increases in frequencies between the control and 4x CO2 data. In the
general southern and western part of the region are minor decreases in frequencies.
Physically, this implies a greater frequency of > 95 percentile flood events in the positive
regions, and smaller frequency in the negative regions. As the period length becomes
longer, the intensity of frequency increase across the north and especially northeast
increases. Some parts of the south and southwest begin to see a lessening and breakdown
of negative values as the period length lengthens, while other areas, such as near point
(10, 7) see intensification of the frequency decrease.
The 1 day >98 and >99 percentile maps show generally the same relative pattern
of values, but with intensification of the frequency increases and positive values creeping
into the southwest. There is also a modest breakdown of negative values. The 2 day
maps also show intensification and spreading of positive values as the percentile bin
narrows and moves closer to the upper tail. The 3 day maps do the same thing but with
more emphasis on the southwest spreading in the >99 percentile version. The 7 day maps
also follow this pattern, and the southwest emergence of positive values becomes
noticeably intense as the numbers in this region begin to creep over 50%. Finally, the 30
day maps show the same pattern. This map is characterized by above 100% values across
a majority of the north and east, west, and southwest, with significant negative values
emerging near the point (10, 7).
Summer:
The >95 percentile 1 day map for summer shows an area of modest positive
values across a sizeable chunk of the north and east, and modest negative values across
the southwest. As the period length becomes longer, the pattern remains unchanged but
both areas of positive and negative values intensify. Generally, the same trend occurs for
all period lengths when the size of the wet extreme tail becomes smaller, but with more
emphasis on positive intensification.
Winter:
The >95 percentile 1 day map for winter shows frequency increases across the
general northern part of the region, with relative areas of maximum increase in regions of
the interior northwest and northeast. There are fewer areas of near zero or negative
change lying across the southern part of the region from west to east. This pattern
remains consistent with positive values intensifying and spreading as the period length
becomes longer. Negative values across the south become less frequent and/or intense as
the period length lengthens. A similar intensification and spreading of positive values
occurs as the wet extreme tail becomes smaller for all period lengths. For the >99
percentile 30 day map, most of the region is positive, where many values exceed 100%.
Drought Analysis
Annual:
In the <5 percentile 30 day maps, there are generally frequency increases present
in a majority of the northern and eastern part of the region, with the highest values in the
upper northeast. Parts of the southwest and west are dotted with negative values, while
the extreme southwest shows positive values. Physically, this would imply an increase in
the frequency of <5 percentile droughts in the positive areas, and a decrease in the
negative areas. However, the soil moisture change map indicates that areas of the
southwest will lose soil moisture in a warmer world. Because the soil is dry to begin with
due to the inherently arid climate in this region, the decrease in frequency of <5
percentile events is likely due to the depletion of available moisture to evaporate, not a
decreased frequency of droughts. The intensity of increase or decrease varies with
location. As the period length changes from 30 to 90 days, the relative pattern of values
does not change much. Some positive values intensify while some negative values also
intensify. Overall, there is no meaningful or descriptive pattern of change. When the
period length lengthens to 180 and 360 days, there is a noticeable breakdown of positive
values in the east, and an emergence of positive values in the west. The general northern
and eastern part of the region sees many locations with significant negative values, while
values above 100% take over a great portion of the west. Physically, this pattern shows
an opposite change in the frequency of longer term droughts (with respect to shorter term
droughts) in many parts of the northeast and west.
The 30 day <2 and <1 percentile maps show the same general pattern of positive
and negative values as the <5 percentile map, but with great intensification of the positive
values in the east, and some intensification of many negative values elsewhere. The 60
day <2 and <1 percentile maps also show the same pattern as the 60 day <5 percentile
map, but with more intensification of positive than negative values. The 90 day narrower
tail maps show the same pattern as the <5 percentile map, with intensification of positive
values everywhere and negative values in the north. The smaller tail 180 day maps show
the same relative pattern of values as the <5 percentile map, with intensification of both
positive and negative values. Finally, the smaller tail 360 day maps show a similar
relative pattern of values as the <5 percentile map, but with intensification occurring
mainly in the positive areas. The general southeastern part of the region becomes
noticeably redder as the tail narrows.
Summer:
The 30 day <5 percentile map for summer shows a pronounced area of positive
values across the northern and eastern part of the United States and a negative area in the
southwest. Again, the negative values in the southwest probably have to do with
decreased soil moisture. As the period length becomes longer, the positive area across
the northeast becomes less intense. For all period lengths, as the dry extreme tail
becomes smaller, both positive and negative values intensify in the pronounced areas
described above.
Winter:
The 30 day <5 percentile map for winter shows positive values across much of the
western part of the region, and a cluster of negative values across the interior northeastern
part of the region. When the period length becomes longer, negative values grow in the
northeast, begin to emerge in the northwest, and the positive areas intensify but shrink in
size. As the extreme dry tail becomes narrower, positive values tend to intensify and
grow for all period lengths.