Calculating and Visualising Event Categories

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Robert W Schlegel

2021-10-26

Category	Description
I Moderate	Events that have been detected, but with a maximum intensity that does not double the distance between the seasonal climatology and the threshold value. These are common and not terribly worrisome.
II Strong	Events with a maximum intensity that doubles the distance from the seasonal climatology to the threshold, but does not triple it. These are not uncommon, but have yet to be shown to cause any long term biological or ecological damage.
III Severe	Thankfully these are relatively uncommon as they have been linked to damaging events. The 2003 Mediterranean MHW was this category.
IV Extreme	Events with a maximum intensity that is four times or greater than the aforementioned distance. These events are currently rare, but are projected to increase with frequency. This is troubling as events in this category are now well documented as causing widespread and lasting ecological damage. The 2011 Western Australia MHW was this category. It is also the logo of this package.

Calculating MHW categories

The categories of MHWs under the Hobday et al. (2018) naming scheme may be calculated with the heatwaveR package using the category() function on the output of the detect_event() function. By default this function will order events from most to least intense. Note that one may control the output for the names of the events by providing ones own character string for the name argument. Because we have calculated MHWs on the Western Australia data, we provide the name “WA” below:

# Load libraries
library(dplyr)
library(tidyr)
library(ggplot2)
library(heatwaveR)

# Calculate events
ts <- ts2clm(sst_WA, climatologyPeriod = c("1982-01-01", "2011-12-31"))
MHW <- detect_event(ts)
MHW_cat <- category(MHW, S = TRUE, name = "WA")

# Look at the top few events
tail(MHW_cat)

## # A tibble: 6 × 11
##   event_no event_name peak_date  category   i_max duration p_moderate p_strong
##      <int> <fct>      <date>     <chr>      <dbl>    <dbl>      <dbl>    <dbl>
## 1       60 WA 2012b   2012-12-31 II Strong   3.42       14         64       36
## 2       29 WA 1999    1999-05-22 II Strong   3.64       95         63       37
## 3       47 WA 2009    2009-03-25 II Strong   2.38        7         57       43
## 4       72 WA 2015    2015-10-02 II Strong   2.46        7         57       43
## 5       41 WA 2008a   2008-04-14 III Severe  3.83       35         57       23
## 6       52 WA 2011a   2011-02-28 IV Extreme  6.58      105         52       27
## # … with 3 more variables: p_severe <dbl>, p_extreme <dbl>, season <chr>

Note that this functions expects the data to have been collected in the southern hemisphere, hence the argument S = TRUE. If they were not, one must set S = FALSE as seen in the example below. This ensures that the correct seasons are attributed to the event.

res_Med <- detect_event(ts2clm(sst_Med, climatologyPeriod = c("1982-01-01", "2011-12-31")))
res_Med_cat <- category(res_Med, S = FALSE, name = "Med")
tail(res_Med_cat)

## # A tibble: 6 × 11
##   event_no event_name peak_date  category   i_max duration p_moderate p_strong
##      <int> <fct>      <date>     <chr>      <dbl>    <dbl>      <dbl>    <dbl>
## 1       30 Med 2003a  2003-06-20 II Strong   5.05       30         53       47
## 2       98 Med 2018b  2018-08-04 II Strong   4.75       44         52       48
## 3       67 Med 2012b  2012-08-20 II Strong   4.32       18         44       56
## 4       46 Med 2007c  2007-04-25 III Severe  4.05       19         42       53
## 5       75 Med 2014   2014-10-18 II Strong   3.34      144         39       60
## 6       96 Med 2018a  2018-04-28 II Strong   3.32       11         27       73
## # … with 3 more variables: p_severe <dbl>, p_extreme <dbl>, season <chr>

Visualising MHW categories

Default MHW category visuals

A quick and easy visualisation of the categories of a MHW may be accomplished with event_line() by setting the category argument to TRUE.

event_line(MHW, spread = 100, start_date = "2010-11-01", end_date = "2011-06-30", category = TRUE)

Custom MHW category visuals

Were one to want to visualise the categories of a MHW ‘by hand,’ the following code will provide a good starting point.

# Create category breaks and select slice of data.frame
clim_cat <- MHW$clim %>%
  dplyr::mutate(diff = thresh - seas,
                thresh_2x = thresh + diff,
                thresh_3x = thresh_2x + diff,
                thresh_4x = thresh_3x + diff) %>% 
  dplyr::slice(10580:10690)

# Set line colours
lineColCat <- c(
  "Temperature" = "black",
  "Climatology" = "gray20",
  "Threshold" = "darkgreen",
  "2x Threshold" = "darkgreen",
  "3x Threshold" = "darkgreen",
  "4x Threshold" = "darkgreen"
  )

# Set category fill colours
fillColCat <- c(
  "Moderate" = "#ffc866",
  "Strong" = "#ff6900",
  "Severe" = "#9e0000",
  "Extreme" = "#2d0000"
  )

ggplot(data = clim_cat, aes(x = t, y = temp)) +
  geom_flame(aes(y2 = thresh, fill = "Moderate")) +
  geom_flame(aes(y2 = thresh_2x, fill = "Strong")) +
  geom_flame(aes(y2 = thresh_3x, fill = "Severe")) +
  geom_flame(aes(y2 = thresh_4x, fill = "Extreme")) +
  geom_line(aes(y = thresh_2x, col = "2x Threshold"), size = 0.7, linetype = "dashed") +
  geom_line(aes(y = thresh_3x, col = "3x Threshold"), size = 0.7, linetype = "dotdash") +
  geom_line(aes(y = thresh_4x, col = "4x Threshold"), size = 0.7, linetype = "dotted") +
  geom_line(aes(y = seas, col = "Climatology"), size = 0.7) +
  geom_line(aes(y = thresh, col = "Threshold"), size = 0.7) +
  geom_line(aes(y = temp, col = "Temperature"), size = 0.6) +
  scale_colour_manual(name = NULL, values = lineColCat,
                      breaks = c("Temperature", "Climatology", "Threshold",
                                 "2x Threshold", "3x Threshold", "4x Threshold")) +
  scale_fill_manual(name = NULL, values = fillColCat, guide = FALSE) +
  scale_x_date(date_labels = "%b %Y") +
  guides(colour = guide_legend(override.aes = list(linetype = c("solid", "solid", "solid",
                                                                "dashed", "dotdash", "dotted"),
                                                   size = c(0.6, 0.7, 0.7, 0.7, 0.7, 0.7)))) +
  labs(y = "Temperature [°C]", x = NULL)

Calculating MCS categories

MCSs are calculated the same as for MHWs. The category() function will automagically detect if it has been fed MHWs or MCSs so no additional arguments are required. For the sake of clarity the following code chunks demonstrates how to calculate MCS categories.

# Calculate events
ts_MCS <- ts2clm(sst_WA, climatologyPeriod = c("1982-01-01", "2011-12-31"), pctile = 10)
MCS <- detect_event(ts_MCS, coldSpells = T)
MCS_cat <- category(MCS, S = TRUE, name = "WA")

# Look at the top few events
tail(MCS_cat)

## # A tibble: 6 × 11
##   event_no event_name peak_date  category  i_max duration p_moderate p_strong
##      <int> <fct>      <date>     <chr>     <dbl>    <dbl>      <dbl>    <dbl>
## 1       77 WA 2018a   2018-08-02 II Strong -2.43       46         67       33
## 2       40 WA 2000    2000-08-13 II Strong -2.27       11         64       36
## 3       15 WA 1990    1990-05-11 II Strong -3.19       76         62       38
## 4       53 WA 2005    2005-10-16 II Strong -1.86       13         62       38
## 5       83 WA 2020    2020-05-25 II Strong -3.14       41         61       39
## 6       11 WA 1987    1987-12-10 II Strong -2.50        9         44       56
## # … with 3 more variables: p_severe <dbl>, p_extreme <dbl>, season <chr>

Visualising MCS categories

Default MCS category visuals

The event_line() function also works for visualising MCS categories. The function will automagically detect that it is being fed MCSs so we do not need to provide it with any new arguments. Note that the colour palette for MCS does have four colours, same as for MHWs, but none of the demo time series that come packaged with heatwaveR have MCSs that intense so we are not able to demonstrate the full colour palette here.

event_line(MCS, spread = 100, start_date = "1989-11-01", end_date = "1990-06-30", category = TRUE)

Custom MCS category visuals

The following code chunk demonstrates how to manually create a figure showing the MCS categories.

# Create category breaks and select slice of data.frame
MCS_clim_cat <- MCS$clim %>%
  dplyr::mutate(diff = thresh - seas,
                thresh_2x = thresh + diff,
                thresh_3x = thresh_2x + diff,
                thresh_4x = thresh_3x + diff) %>% 
  dplyr::slice(2910:3150)

# Set line colours
lineColCat <- c(
  "Temperature" = "black",
  "Climatology" = "grey40",
  "Threshold" = "darkorchid",
  "2x Threshold" = "darkorchid",
  "3x Threshold" = "darkorchid",
  "4x Threshold" = "darkorchid"
  )

# Set category fill colours
fillColCat <- c(
  "Moderate" = "#C7ECF2",
  "Strong" = "#85B7CC",
  "Severe" = "#4A6A94",
  "Extreme" = "#111433"
  )

# Create plot
ggplot(data = MCS_clim_cat, aes(x = t, y = temp)) +
  geom_flame(aes(y = thresh, y2 = temp, fill = "Moderate")) +
  geom_flame(aes(y = thresh_2x, y2 = temp, fill = "Strong")) +
  geom_flame(aes(y = thresh_3x, y2 = temp, fill = "Severe")) +
  geom_flame(aes(y = thresh_4x, y2 = temp, fill = "Extreme")) +
  geom_line(aes(y = thresh_2x, col = "2x Threshold"), size = 0.7, linetype = "dashed") +
  geom_line(aes(y = thresh_3x, col = "3x Threshold"), size = 0.7, linetype = "dotdash") +
  geom_line(aes(y = thresh_4x, col = "4x Threshold"), size = 0.7, linetype = "dotted") +
  geom_line(aes(y = seas, col = "Climatology"), size = 0.7) +
  geom_line(aes(y = thresh, col = "Threshold"), size = 0.7) +
  geom_line(aes(y = temp, col = "Temperature"), size = 0.6) +
  scale_colour_manual(name = NULL, values = lineColCat,
                      breaks = c("Temperature", "Climatology", "Threshold",
                                 "2x Threshold", "3x Threshold", "4x Threshold")) +
  scale_fill_manual(name = NULL, values = fillColCat, guide = FALSE) +
  scale_x_date(date_labels = "%b %Y") +
  guides(colour = guide_legend(override.aes = list(linetype = c("solid", "solid", "solid",
                                                                "dashed", "dotdash", "dotted"),
                                                   size = c(0.6, 0.7, 0.7, 0.7, 0.7, 0.7)))) +
  labs(y = "Temperature [°C]", x = NULL)

Category colour palettes

For the sake of convenience the MHW and MCS colour palettes are provided below with a figure showing the direct comparison.

# The MCS colour palette
MCS_colours <- c(
  "Moderate" = "#C7ECF2",
  "Strong" = "#85B7CC",
  "Severe" = "#4A6A94",
  "Extreme" = "#111433"
)

# The MHW colour palette
MHW_colours <- c(
  "Moderate" = "#ffc866",
  "Strong" = "#ff6900",
  "Severe" = "#9e0000",
  "Extreme" = "#2d0000"
)

# Create the colour palette for plotting by itself
colour_palette <- data.frame(category = factor(c("I Moderate", "II Strong", "III Severe", "IV Extreme"),
                                               levels = c("I Moderate", "II Strong", "III Severe", "IV Extreme")),
                             MHW = c(MHW_colours[1], MHW_colours[2], MHW_colours[3], MHW_colours[4]),
                             MCS = c(MCS_colours[1], MCS_colours[2], MCS_colours[3], MCS_colours[4])) %>% 
  pivot_longer(cols = c(MHW, MCS), names_to = "event", values_to = "colour")

# Show the palettes side-by-side
ggplot(data = colour_palette, aes(x = category, y = event)) +
  geom_tile(fill = colour_palette$colour) +
  coord_cartesian(expand = F) +
  labs(x = NULL, y = NULL)

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