Summary of Fog Occurrence at Tampico and Veracruz, Mexico Jerome Fast, PNNL August 29, 2005

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1 Summary of Fog Occurrence at Tampico and Veracruz, Mexico Jerome Fast, PNNL August 29, Motivation Surface observations from Tampico and Veracruz were analyzed to determine how frequently fog occurs along the Gulf of Mexico, since these conditions will impact flight operations during MILAGRO. Periods of low cloud ceiling, low visibility, and overcast skies were characterized. Summary Fog occurs at Veracruz.6-3.9% of the time during February and March, depending on the year, and it usually forms between 18 and 2 LST. Visibility less than 1 mile was reported to occur between.6 and 2.9% of the time, usually between 18 and 2 LST. The data suggests that while aircraft are likely to be able to take off from Veracruz during the day, there may be problems in landing the aircraft after 18 LST on a few days. Therefore, forecasts for aircraft operations should include the likelihood for fog formation as guidance for the principal investigators. Other conditions that impact aircraft operations but probably do not lead to closing of the Veracruz airport include low cloud ceilings that occurred % of the time, visibilities less than 3 miles that occurred % of the time, and overcast conditions that occurred % of the time. Haze and smoke (most likely from biomass burning) often reduces the visibility at Veracruz. Smoke may be a bigger issue than fog in terms of flight operations. Note that some of the surface reports do not make any sense. For example, there are periods when fog is reported at the same time visibility is high, the ceiling is above ground level, or when the relative humidity is much lower than 1%.. Methodology Data was obtained from a National Climatic Data Center (NCDC) web site that claims that many global stations report only at 3-hour intervals. Hourly and 3-hour data were available from Veracruz and Tampico, respectively. Data availability was poor in Tampico with 2 64% of the data record missing, depending on the year. In contrast, only 3 1% of the data data record was missing at Veracruz. In the first part of this study, observations at Tampico between 1997 and during the months of February and March were analyzed first because the period of record encompassed recent years in which field campaigns were conducted in the vicinity of Mexico City (1997, 23). Data from Veracruz was available only up to These analyses assume that the meteorological conditions of Tampico and Veracruz would be similar. To test this hypothesis, the second part of this study compared the meteorology between the two sites when there was an overlap in the data records between 1991 and Parameters that were analyzed included fog, ceiling, visibility, sky cover, relative humidity, wind speed, and. Only those periods in which cloud ceilings were less than 1 ft, visibility was less than 3 miles, and sky cover was overcast were examined. Tables and Figures The frequency of occurrence of fog, ceiling < 1 feet, visibility < 1 miles, visibility < 3 miles, and overcast sky cover is summarized in Tables 1-4. The histograms depicted in Figs. 1 associate periods of fog, ceiling < 1 feet, visibility < 1 miles, visibility < 3 miles, and overcast sky cover with other parameters. Figures 1 5 are based on data obtained from Tampico between 1997 and. Figures 6 directly compare the results from Tampico and Veracruz using data between 1991 and Conclusions from the tables and figures are relatively straight-forward; therefore, only the findings on fog occurrence are summarized next. Fog Occurrence In general, fog does not form frequently at Tampico as indicated by Table 1. The of occurrences of fog range from 1 (23) to, () that comprises.5-7% of the available data record. The year-to-year variations in the fog occurrence show no trend. When Tampico and Veracruz are compared in Tables 2 and 3, one finds that the occurrence of fog is actually much lower in Veracruz, that ranged between.6-3.9%, than in Tampico, that

2 ranged between %. Because of the large fraction of missing data, the occurrence of fog may be underreported at Tampico. A complication in the interpretation of the surface reports is that fog events at Veracruz are frequently accompanied by reports of haze and smoke. This combination of fog with haze and smoke was not reported at Tampico. Table 4 includes the reports of haze, smoke and fog which increases the frequency of occurrence by.1 to 4.5%. Recent satellite images suggest that the smoke is likely the result of biomass burning that is transported over Veracruz. Figure 1 indicates that fog in Tampico between 1997 and occurred mainly between 18 and 21 LST. As expected, the fog events are accompanied by low wind speeds and high relative humidity. While fog generally has low visibility, surprisingly there are many cases during fog events in which visibility is reported to be greater than five miles and ceilings to be greater than 1 ft. Between 1991 and 1994, the characteristics of fog between Tampico and Verzcruz were similar for some parameters and different for others. As with Tampico, fog at Veracruz formed most frequently between 18 and 2 LST as shown in Figs. 6 and 7. Visibility and wind speed during fog conditions tended to be higher in Veracruz than at Tampico. Fog events at Tampico and Verracruz did not necessarily coincide (not shown in figures), indicating fog is not necessarily wide-spread along the Gulf of Mexico. Next Step The meteorological conditions associated with fog formation and reduced visibility in Veracruz has yet to be determined. In the near future, I will be examining synoptic conditions and satellite images associated with fog conditions.

3 Table 1 Frequency of various surface conditions observed at Tampico, Mexico during February and March of Observations obtained from NCDC reported at 3-h intervals with a total possible of observations of 472 and 48 during non-leap and leap years (2, 24), respectively. observations fog ceiling < 1 ft visibility < 1 mi visibility < 3 mi sky cover = overcast year # % # % # % # % # % # % Table 2 Frequency of various surface conditions observed at Tampico, Mexico during February and March of Observations obtained from NCDC reported at 3-h intervals with a total possible of observations of 472 and 48 during non-leap and leap years (1992), respectively. observations fog ceiling < 1 ft visibility < 1 mi visibility < 3 mi sky cover = overcast year # % # % # % # % # % # % Table 3 Frequency of various surface conditions observed at Veracruz, Mexico during February and March of Observations obtained from NCDC reported at 3-h intervals with a total possible of observations of 1416 and 144 during non-leap and leap years (1992), respectively. observations fog ceiling < 1 ft visibility < 1 mi visibility < 3 mi sky cover = overcast year # % # % # % # % # % # % Table 4 Frequency of various fog conditions/combinations observed at Veracruz, Mexico during February and March of Observations obtained from NCDC reported at 3-h intervals with a total possible of observations of 1416 and 144 during non-leap and leap years (1992), respectively. observations fog only haze, smoke, and fog total fog year # % # % # % # %

4 < > NR > <.5 NR > < NR > Figure 1. The occurrence of various meteorological conditions at Tampico, Mexico for periods of fog during February and March of < > NR > < NR > Figure 2. The occurrence of various meteorological conditions at Tampico, Mexico for periods when the ceiling is less than 1 ft during February and March of

5 < > NR > < NR > Figure 3. The occurrence of various meteorological conditions at Tampico, Mexico for periods when the visibility is less than 1 mile during February and March of < > NR > < NR > Figure 4. The occurrence of various meteorological conditions at Tampico, Mexico for periods when the visibility is less than 3 miles during February and March of

6 cloud level NR L L/M L/H L/M/H M M/H H NR > <.5 NR > < NR > Figure 5. The occurrence of various meteorological conditions at Tampico, Mexico for periods when the sky cover is overcast during February and March of

7 < > NR > <.5 NR > < NR > Figure 6. The occurrence of various meteorological conditions at Tampico, Mexico for periods of fog during February and March of NR < > NR > <.5 NR > < NR > Figure 7. The occurrence of various meteorological conditions at Veracruz, Mexico for periods of fog during February and March of

8 NR < > NR > <.5 NR > < NR > Figure 8. The occurrence of various meteorological conditions at Veracruz, Mexico for periods of haze, smoke, and fog during February and March of Note: there are no hazy and smoke reports at Tampico for the entire period of record.

9 < > NR > < NR > Figure 9. The occurrence of various meteorological conditions at Tampico, Mexico for periods when the ceiling is less than 1 ft during February and March of < > NR > < NR > Figure 1. The occurrence of various meteorological conditions at Veracruz, Mexico for periods when the ceiling is less than 1 ft during February and March of

10 < > NR > < NR > Figure 11. The occurrence of various meteorological conditions at Tampico, Mexico for periods when the visibility is less than 1 mile during February and March of < > NR > < NR > Figure 12. The occurrence of various meteorological conditions at Veracruz, Mexico for periods when the visibility is less than 1 mile during February and March of

11 < > NR > < NR > Figure 13. The occurrence of various meteorological conditions at Tampico, Mexico for periods when the visibility is less than 3 miles during February and March of < > NR > < NR > Figure 13. The occurrence of various meteorological conditions at Veracruz, Mexico for periods when the visibility is less than 3 miles during February and March of

12 cloud level NR L L/M L/H L/M/H M M/H H NR > <.5 NR > < NR > Figure 14. The occurrence of various meteorological conditions at Tampico, Mexico for periods when the sky cover is overcast during February and March of cloud level NR L L/M L/H L/M/H M M/H H NR > <.5 NR > < NR > Figure. The occurrence of various meteorological conditions at Veracruz, Mexico for periods when the sky cover is overcast during February and March of