Tag Archives: distribution

Tropical Cyclones and Precipitation Distribution

Leave a reply
Tropical Storm Cristobal ~June 6th, 2020

Flooding and storm surge are the two most deadly elements of a tropical cyclone (TC). In this post, I am going to explore the forecasting of heavy rain and flooding within TCs. To do this, I have read “Precipitation Distribution Associated with Landfalling Tropical Cyclones over the Eastern United States” by Eyad Atallah, Lance F. Bosart and Anantha R. Aiyyer. I plan to summarize what I believe to be the key findings from this paper, and will organize them here so they can be utilized in the operational setting.

INTRO

Before they enter their study, they talk in brief about extratropical transition (ET), how it occurs, its varying definitions depending on frameworks and what determines its strength. They note, [from Atallah and Bosart (2003)] that the distribution and intensity of rainfall depend on surrounding synoptic features (troughs/ridges) and the TC’s transition to a extratropical cyclone (EC). So when ET occurs is very important in addition to the surrounding synoptic features. ET is a complicated process and as mentioned above, it has varying definitions depending on different frameworks (QG, PV, cross sections, surface frontal boundaries). There are a few debated definitions of how ET occurs. First, changes in the low-level thickness field and mid-tropospheric thermal wind; second and more simply, look at the system and once you can identify frontal and thermal structures ET has occurred. It should be noted that while it is important to know when ET will occur, it matters little the strength of the extratropical cyclone as heavy rain events have been recorded out both weak [Hurricane Camille (1969)] and strong [Irene (1999)] ECs. As to whether the ECs will be strong or not depends mostly on whether a negatively titled trough is located upstream of a TC [Hart et al. (2006)]. If the trough is positively tilted, than a weaker EC is preferred. However, “the main focus of this paper is to understand the synoptic-scale dynamics modulating the precipitation distribution associated with landfalling TCs” (pg. 2187).

STORMS STUDIED

The 32 storms that were compiled were storms which had all of the following:

  • Landfall was along either the Gulf or Atlantic coasts of the United States.
  • The storm had to display a poleward component in its track.
  • The storm was far enough inland to permit rain measurements in all quadrants of the storm.

Some of the storms that the team analyzed are listed below with LOC meaning “left of center” and ROC meaning “right of center”. You will note that some fall under both of the LOC and the ROC composite. The times listed are when the cyclone started exhibiting that particular bias.

A list of TCs included in the study. The dates and times listed represent the initial time that a storm was included in each of the composites. The numbers in parentheses show the maximum sustained wind (kt) of the cyclone at the time listed as taken from the best-track data.

RESULTS

The storm tracks which produced the LOC and ROC composites are below.

The tracks of the tropical cyclones (based on the National Hurricane Center best-track data) for (a) storms with precipitation to the left of track and (b) storms with precipitation to the right of track.

With the LOC tracks, notice that there is a pretty broad spectrum of tracks with respect to spacial area covered. This is to say that while the Appalachians and cold air damming (CAD) will influence precipitation distribution, it is not necessary for a LOC event to occur as they occur to the east and west of the Apps. At the same time however, do note that the LOC tracks which make landfall farther north align with the LOC composite rather than the ROC tracks. This is to say that storm tracks which landfall farther north will be more heavily influenced by CAD and mid-level troughs which make heavy precipitation LOC more likely. With the ROC tracks, notice that all storms that made landfall on the east coast which exhibited ROC characteristics made landfall between Georgia and South Carolina. The rest of the tracks made landfall across the Gulf. Note that tracks which push farther into the Mississippi Valley are more likely to be ROC while most of the LOC tracks are farther east.

SYNOPTIC OVERVIEW

The 1000–500-hPa thickness (dashed black lines, contoured every 60 m), 1000-hPa geopotential height (solid black lines, contoured every 30 m), and the 850–200-hPa wind shear (shaded, contoured every 5 m s−1, starting at 20 m s−1) for (a) LOC at t = 0, (b) LOC at t = 12, (c) LOC at t = 24, (d) ROC at t = 0, (e) ROC at t = 12, and (f) ROC at t = 24. Thick solid lines denote thermal trough axes. The thick arrows in (a) and (d) depict the composite storm tracks.

The above figure has 1000-500 mb thickness in dashed lines, 1000 mb heights in solid lines and 850-200 mb shear in m/s filled. The LOC is on the left while the ROC composite is on the right.

In short…

LOC – Look for a trough approaching the TC from the west, which will also negatively tilt into the vicinity of the TC. This will in turn effectively turn the TC into a EC. If you utilize a cross section analysis of this composite (see Fig. 7 below ), you will see strong easterly isentropic ascent to the west of the center of the cyclone, which “is consistent with a redistribution of precipitation to the northwest quadrant of the storm” (pg. 2197).

ROC – “Storms that exhibit an ROC precipitation distribution tend to be the ones that weakly interact with midlatitude troughs in comparison to the LOC storms. ROC storms are in general characterized by small circulation centers at the time of landfall. As the circulation center approaches a zonally oriented baroclinic zone to the north, the storm tilts downshear (see Fig. 7d below) causing the precipitation to become displaced to the east of the cyclone.”

Cross sections of PVU (shaded, starting at 0.4 PVU as given by the color bar), isentropes (thin solid lines, contoured every 3 K), and relative humidity (dashed lines, contoured every 10%) for (a) LOC at t = 0, (b) LOC at t = 12, (c) LOC at t = 24, (d) ROC at t = 0, (e) ROC at t = 12, and (f) ROC at t = 24. Note that the thick black line denotes the 1.5-PVU surface, taken to be the DT. Thick solid lines indicate the position of the warm core associated with the TC.

Below is the schematic of landfalling TCs with LOC portrayed by (a) and ROC portrayed by (b). The solid black lines are 250 mb flow, the motion and shear arrows portray respective vectors as well as relative magnitude, the green line is the parcel path starting at the surface in the warm sector and ending in the mid to upper troposphere in the cold sector and the grey area is the location of the heaviest precipitation.

Schematics for landfalling tropical cyclones for (a) the LOC composite and (b) the ROC composite. The curved black lines represent streamlines of the upper-tropospheric (i.e., 250 hPa) flow. Arrows represent motion and deep tropospheric shear with the relative magnitudes given by the length of the arrow. The curved green line represents the trajectory of a parcel starting near the surface in the warm sector and ending in the mid- to upper troposphere in the cool sector. The gray shaded area represents regions of precipitation and pluses and minuses represent the local PV tendency resulting from a combination of advection and the diabatic redistribution of PV.

This post was triggered by Tropical Storm Cristobal which occurred in early June of 2020. It was a ROC storm and while the models were incredibly consistent, the rain forecast was tricky, specifically on the right flank. Below is the observed rainfall, with landfall having occurred in south-central Louisiana. As an update to this post or separate one, I hope to tackle a quick analysis of why Cristobal was a ROC precipitation event with respect to the findings presented in this paper. Namely that surrounding synoptic features (troughs/ridges) and the TC’s transition to a extratropical cyclone (EC) impact the distribution and intensity of rainfall.

Reference

Atallah, E., L. F. Bosart, and A. R. Aiyyer, 2007: Precipitation Distribution Associated with Landfalling Tropical Cyclones over the Eastern United States. Mon. Wea. Rev., 135, 2185–2206, https://doi.org/10.1175/MWR3382.1.