August 2011: A Month to Remember - Not for Air Quality but for Abundant Rainfall, Flooding, a Hurricane and a rare Earthquake!

VISUALIZATION

Note: Graphic was prepared for August 2011 Quality of Air Summaries. Click here to see full-resolution version.
Top-left: Deviation from normal precipitation for August 2011 in inches.
Top-right: 24-hour rainfall total ending at 8 AM EDT August 28, 2011 in inches. Precipitation data is overlaid on Terra MODIS Visible Satellite imagery on August 27, 2011 along with Hurricane Irene's track.
Bottom-left: Map showing the number of rainy days for a typical August using long-term NCDC Comparative Climate Data. Small inset map for August 2011 is provided as a comparison to long-term data.
Bottom-right: Annotated upper-level pressure anomaly for August 2011 showing a persistent area of an upper-level trough of low pressure over New England.

THE STORY

Source: Maryland Department of the Environment (MDE)'s Quality of Air Summaries. (It is copied and pasted directly here.)

Record breaking temperatures for this summer ended during the first week of August in the Mid-Atlantic and Northeast. However, other weather conditions took center stage for the remainder of the month. August 2011 will be remembered for abundant rainfall, flooding, a hurricane and even a rare earthquake that originated near central Virginia (not shown in maps) on the 23^rd. Not to mention a wildfire burning throughout the month in the Great Dismal Swamp (Lateral West Fire), south of Suffolk, VA also threatened the air quality in parts of the region. The fire caused reduced visibility, a strong smoke odor and possibly very elevated particle pollution levels (i.e. health concerns) in the Delmarva Peninsula. However, it was not possible to determine the fire’s pollutant impact due to lack of PM monitors in those areas. Going back to other meteorological conditions, precipitation data from the NWS Advanced Hydrologic Prediction Service (AHPS) shows above normal rainfall over the Northeast and parts of the Mid-Atlantic. In fact, some localized areas observed 18-25 inches above normal rainfall when compared to a typical August. Much of the rainfall fell on the 27^th-28^th due to the influence of Hurricane Irene. In the top right panel is the estimated 24-hour rainfall ending at 8AM EDT on August 28, 2011 overlaid on a visible satellite imagery along with Hurricane Irene’s track. Irene caused record rainfall and flooding in many cities in New England (NCDC). Needless to say, these weather conditions brought generally Good air quality across the region during the last week of August. However, the effect of Hurricane Irene was not sufficient to result in one of the lowest monthly median AQI [Air Quality Index] being recorded in many areas across the region, including all four forecast regions in Maryland. The low daily median AQI was driven by a persistent upper-level trough of low pressure located over New England (bottom right map by NCDC). This was associated with a jet-stream that directed storms into the region frequently and caused more days with unsettled weather conditions. It was clearly evident in the number of rainy days (days with 0.01" or more precipitation, also referred to as measurable rainfall) over the same region. Based on the long-term average, most of the Eastern U.S. usually experiences about 9-10 rainy days during the month of August (bottom left map). The exceptions are located along the Southeast Coast and mountainous areas over the Northeast Appalachians where precipitation is enhanced due to topographic (orographic) lifting. During August, due to the presence of the jet-stream and the upper-level trough of low pressure, many areas in New England observed as many as 18 rainy days. When compared to the long term average (bottom left map) the result was roughly one additional week of rain and non-conducive weather for ozone formation in August. This combined with the continuous decline in pollutant emissions resulted in many areas in the region recording one of their lowest monthly median AQI.

DISCUSSION OF VISUALS AND ANALYSIS TECHNIQUE

It is important to re-state the fact that many natural "disasters" took center stage during August 2011. These occurrences (abundant rainfall, flooding, and more frequent rainy days) were meteorologically driven and directly resulted in improved air quality conditions across the Maryland and New England as a whole. This air quality trend in Maryland is provided in the MDE's Quality of Air Summaries for four forecast regions using a time-series of daily maximum AQI presented in the form of a simple box-plot (low-median-high). For readers in other areas in the New England, you can view air quality maps from EPA AIRNow for a quick inspection. These maps highlighted areas observing Moderate (Code Yellow) AQI or higher. For areas with no shading indicated Good air quality conditions. There were only several bad air days (shaded Orange for Unhealthy for Sensitive Groups). From these maps, it is fair to conclude that air quality was mostly Good and Moderate in New England during August 2011.

Since air quality was not very "interesting" for August 2011, the goal was to highlight the meteorological conditions (i.e. abundant rainfall, flooding and more frequent rainy days). I typically start an analysis with a top-down approach (examine the big picture and then drill down to the details as needed). For this reason, I typically start out by searching for weather highlights. The National Climatic Data Center (NCDC) State of the Climate page is a good place to start. Other resources such as weather providers and newspapers are available sooner than NCDC products. For this reason they can sometimes be more useful. For this edition, residing in the East as well as visiting the NCDC State of the Climate for August 2011 provided me with an excellent summary. I identified flooding, excess precipitation and Hurricane Irene to be important factors. This led to the top two figures highlighted in the VISUALIZATION section.

• The NWS Advanced Hydrologic Prediction Service (AHPS) is an excellent resource that provides archived precipitation data. August 2011 monthly departure from normal precipitation map is provided directly on this blog for discussion.
• This map quickly highlights the
  
  Northeast and parts of the Mid-Atlantic as area of interest for observing excess precipitation. Note that this observation is biased since the air quality story is for Maryland and the Mid-Atlantic. If you live in other parts of the county, the deficit precipitation over most of the country should be more interesting.
• The color scale was designed fairly well to highlight areas of deficit/excess precipitation with "hot/cool" colors. However, it's important to note that it can be improved using one of the recommended diverging color schemes on the ColorBrewer2.org website by Cynthia Brewer, et al.

• For publication purpose, I decided not to use the map for two reasons: (a) high-resolution version is not available; and (b) color scheme doesn't match with the template designed for the MDE's Quality of Air Summaries. As a result, data were extracted from the NWS AHPS  website and plotted using ArcMap with Spatial Analyst extension.
• The color scale follows a generic hot/cool color scheme to denote deficit and excess. Note that the NASA Blue Marble background image is provided as a visual attraction but un-intentionally it somewhat competes with the data (shades of blue to denote excess precipitation over the New England). This should be avoided if possible.

The next step was to investigate areas of excess precipitation (i.e. New England). According to NCDC State of the Climate for August 2011, Hurricane Irene made landfall near Cape Lookout, North Carolina on August 27^th, and tracked north along the Eastern Seaboard through August 29^th. Hurricane Irene brought record rainfall and caused flooding in many cities in the East. This was enough clue to suggest that the excess precipitation was likely caused by Hurricane Irene. For this, I began to drill down into daily precipitation map on the NWS AHPS website.

• The 24-hour precipitation ending 8AM EDT August 28, 2011 proves that the excess rainfall is caused by Hurricane Irene. Note that data is provided based on a hydrologic day not a calendar day. The map is once again provided for discussion.
• Pros: Reveals spatial pattern quickly with relative magnitude.
• Cons: Gives a false impression to areas with warm colors (orange, red, pink are much more intense in comparison to yellow. Shaded color relief map to show topography in background sometimes compete with data. In addition, contour intervals are not the same at low levels. As a meteorologist, I suspect that it is intentionally designed to highlight areas with drizzle, light/heavy rain, etc.
• This map can be improved with the following: (a) shaded relief map with subdued colors such that they won't compete with data; (b) use a discrete color ramp with increasing intensity, also known as sequential color schemes on the ColorBrewer2.org website. Four versions of this precipitation map are genenerated to back up my recommendation. In all four versions, it is easy for us to quickly describe the spatial pattern of the data. HOWEVER, there is only version that allows us to describe the intensity quantitatively between each color hue EVEN without a color legend and the associated contour intervals.

For publication purpose, the map on the right is used. The note that the contour interval was intentionally broken into 4 categories for areas observing below 2 inches. The purpose was to highlight areas trace amount of precipitation, drizzle, light/moderate rain intensity. Additional data layers were also overlaid to provide supporting information. In the background, a true-color imagery from Terra satellite showed the beautiful cloud structure and areas impacted by Hurricane Irene on the morning of August 27, 2011 (courtessy of UW MODIS Today website). In addition, Hurricane Irene's track (from Hurricane and Storm Tracking website) provided a partial history of its impact over New England.

Going back to the air quality story, the precipitation maps discussed thus far only show that New England was impacted by Hurricane Irene during the last week of August 2011. Needless to say, weather conditions associated with Hurricane Irene brought generally Good air quality across the region during the same period. However, its effect was not sufficient to result in mostly Good to Moderate air quality conditions across the region (refer to air quality maps from EPA AIRNow for a quick inspection). To answer this question, I visited the NCDC National Temperature and Precipitation Maps website for a quick look. A 500 Millibar Heights and Anomalies map is also featured on this page at the bottom of the selection list. I personally think the map was designed exceptionally well (color scheme, contour interval and how elements in the map do not compete with one another). For this reason, I decided to use the map for publication. I only had to annotate the map with a couple of features (trough line and a jet-stream) to help non-technical readers understand the map better. The map indicated a persistent upper-level trough of low pressure located over New England. This was associated with a jet-stream that directed storms into the region frequently and caused more days with unsettled weather conditions. This map helped fill a big gap in the air quality story to help anwer why air quality in the region was mostly Good to Moderate.

At this point, I only needed to find data to prove that there were more frequent rainy days to relate them back to the jet-stream and a persistent trought line. For this Comparative Climate Data from NCDC were downloaded, matched with a master surface station list (NCDC Surface Inventories) and plotted. This was not a fun task! Perhaps, if there is interest, I can provide a help document at a later time. The comparative climate data was used to create a contour map showing the number of rainy days for a typical August (based on long-term average). The main goal is to examine large spatial patterns, not any specific details. Based on the contour map, it's clear that most of the Eastern U.S. usually experiences about 9-10 rainy days during the month of August. The exceptions are located along the Southeast Coast and mountainous areas over the Northeast Appalachians where precipitation is enhanced due to topographic (orographic) lifting.

The remaining task was to compare August 2011 data to the long term average. Due to the nature of the data, I had to manually obtain the number of rainy days (.01+ inches) for selected stations over the Mid-Atlantic and Northeast using preliminary monthly climate data (CF6) from the local NWS offices. Note that I only obtained selected station data and were not complete as compared to the long term average data set. For this reason, it was logical for to create a point map instead of a countour map. By the way, this was also the reason why I couldn't create a departure from normal map similar to others shown above. The point map showing the number of rainy days during August 2011 is provided as a insert map on the bottom-right corner of the contour map. Using the discrete color scheme alone, it was clear to conclude that many areas in New England observed as many as 18 rainy days. When compared to the long term average (contour map) the result was roughly one additional week of rain and non-conducive weather for ozone formation in August. This observation helped explain the reason why air quality in New England was mostly Good to Moderate during August 2011. This made the air quality story complete.

I hope that the discussion of the visuals used to communicate this particular air quality was sufficient.  I hope that it provides enough meaningful comparisons to show how abundant rainfall, flooding and requent rainy days resulted in mostly Good to Moderate air quality conditions in New England.

SHORT TAKE-HOME MESSAGE

In this edition, the main element used to communicate this particular air quality story was map and I repeatedly emphasize the use of color schemes. I personally think creating a readable and usable map to convey the message spatially is not difficult but will take practice. If you are new to this and you are asking for tips. Well I recommend Stephen Few’s Visual Business Intelligence Newsletter titled “Cartographic Malpractice” for further reading. Knowing what to avoid doesn't create good "cartographers" but will reduce our chance of making another horrible map!!!
What are your thoughts in regards to the visuals?  Please participate in the discussion to help promote thoughtful presentation of air quality information. This is a learning process.


DISCLAIMER: Any statements, postings, shares, likes, follows on the blog should be considered personal. They do not represent any political or official endorsement from my employer.