Elementos de la Controversia Pública

The preceding section presents an overview of the magnitude of the event in terms of A06 without showing how the event evolved. This section is dedicated to presenting results from sub-daily evolution of the event spatially and temporally. Since TRMM, CMORPH, SEVIRI and METAR reports have temporal resolutions higher than 24 hours, they were deployed to investigate the sub-daily characteristics of the event. In addition, satellite overpasses of TRMM, AMSU/MHS and A-train (CALIPSO and CloudSat) have been utilised as well.

Hours leading up to the event on 14 May 2007 reflected an active troposphere. Figure 7.4 shows evolution of the event between 06 UTC 13 May to 06 UTC 14 May 2007. On average, moderate to deep convective clouds were found to characterise the troposphere intermittently between 06 UTC 13 May and 06 UTC 14 May 2007. Note that low SEVIRI Infrared brightness temperatures at 10.8µ (BT) are a proxy for deep convection as they infer the height of a cloud. Shallow to moderate convection was found to develop between 06–15 UTC 13 May where BT gradually decreased from about 250 K to just below 240 K6. This was also reflected in the 3-hourly accumulated precipitation from CMORPH and

TRMM where precipitation gradually increased on average between 09 and 12 UTC 13 May. A TRMM-2A12 overpass recorded the initial stages of the event at around 0542 UTC 13 May showing moderate to heavy showers with the northern part of the Coastal Lowlands reporting showers of between 4 and 7mm as shown in figure 7.5a. An A-train overpass between 1104 UTC and 1117 UTC May 13 (polar orbital path shown in figure 7.7a) pointed to a developing convective system. The vertical depth of the system during this time when A-train satellites scanned the region was about 15 Km. The depth was characterised primarily by cirrus clouds between 14 and 15 Km above ground and a relatively deep convective system between 5 and 14 Km in the vertical captured by CALIPSO lidar as shown in figure 7.7b. The rest of the vertical extent appeared transparent to the lidar beam with no much activity below 5 Km. However, CloudSat - a twin sister of CALIPSO - reflected a brightband7 between 4 and 7 Km above ground as shown in figure 7.7c. A

brightband is usually a clear reflection of stratiform precipitation as argued by Houze et al. (2015).

Between 15–18 UTC, the moderately deep clouds dissipated as BT rose to about 260 K. Formation of new cloud cells ensued after the dissipation of the former, but this time, at a faster rate between 18 UTC and midnight. BT went to as low as 235 K at 00 UTC reflecting a tentatively deep system. During this period, an AMSU-B instrument overflew the Coastal Lowlands. The 150 GHz microwave channel of AMSU-B reported brightness temperatures of as low as 160 K near the center of the system less than an average of 10 miles off the coastline as shown in figure 7.5b. At these temperatures, the microwave channel interpretation is a deep convection laden with hydrometeors due to a reduction of brightness temperatures by scattering of large ice particles e.g. Redl et al. (2015). This was also the window period when precipitation peaked to reach highs of 23 mm in 3 hours between 18–21 UTC as reflected by the spatially-averaged TRMM precipitation in figure 7.4. Within the same window period, CMORPH showed a somewhat similar precipitation signal with the highest amount of 18 mm in three hours recorded at just about midnight.

Availability of a METAR report at the Moi International Airport in Mombasa as-

6BT brightness temperatures below this value reflect deep convection e.g. Rempel et al. (2017); Feidas

and Giannakos (2012); Tobin et al. (2012)

06UTC 09UTC 12UTC 15UTC 18UTC 21UTC 00UTC 03UTC 06UTC

13May 13May 13May 13May 13May 13May 14May 14May 14May

2007 2007 2007 2007 2007 2007 2007 2007 2007 0 5 10 15 20 25 30 35 40 0 5 10 15 20 25 30 35 40 mm mm K ● ● HS RA CB TCU BR

METAR REPORT FROM MOMBASA (MOI INT. AIRPORT)

8 12 0 7 10 10 8 16 9 7 10 11 11 10 8 8 6 3230 5 10 235 240 245 250 255 260 265 270 275 280 285 290 CMORPH TRMM SEVIRI Area A verage

36°E 38°E 40°E 5°S 4°S 3°S 2°S 1°S Mombasa ● ●

Figure 7.4: Three-hourly evolution of precipitation and convection between 06UTC 13 May to 06 UTC 14

May 2007. Shown are the spatially averaged precipitation from CMORPH (red, mm) and TRMM (black, mm) as well as the spatial mean of BT from SEVIRI (pink, Kelvin). Symbols represent the METAR report for Moi international airport, Mombasa between 13 and 14 May 2007. The Key is as follows: HS: Heavy showers, RA: Moderate to heavy rain, CB: Cumulonimbus clouds, TCU: Towering cumulus clouds, BR: Broken clouds. Arrow heads represent wind vectors which show wind direction from the true north while

annotated numbers are the wind speeds in m s-1. Blue region in the inset map shows the Coastal Lowlands

- the area where the spatial averaging was done. Geographical location of the Moi International Airport, Mombasa where METAR report was available is shown by a ring in the inset map.

certained the situation leading to the event of 14 May 2007 as reported by other data sources. Symbols in figure 7.4 represent the evolution of weather as reported at the airport at irregular hourly intervals. The METAR report differed slightly on sub-daily scale with the satellite-based products because, the latter is spatially averaged over the entire Coastal Lowlands with the bulk of severe weather located towards the north-eastern part whereas the station whose METAR report was analysed here is located towards the south-western part of the region as shown in the inset map of figure 7.4. Nevertheless, the station reported intermittent heavy showers from broken clouds in the early hours of 13 May alternating with moderate to heavy rain with towering cumulus clouds. TRMM and CMORPH also showed signs of moderate rains during these dawn hours too. The winds were predomi- nantly westerly between 06 UTC and 08 UTC but later turned south-westerly throughout the day leading to the morning of 14 May 2007. The wind speeds were moderate during the morning hours rising to a maximum of about 10 m s-1_{at 09 UTC 13 May. There were}

no METAR records during the mid-morning hours (10 - 14 UTC). However, moderate to heavy precipitation was reported over the station between 15 – 18 UTC even though this is the window period when there was not much activity as observed from the satellite-based products namely, CMORPH, TRMM and SEVIRI. Wind speeds were the highest here compared to the rest of the day ranging between 8 and 16m s-1_{. Even though no METAR}

reports were available between 18 – 20 UTC, the available reports at 21 UTC confirms the occurrence of a heavy storm alternating with showery convection. Towering cumulus and broken clouds were reported during the late hours of the night going into midnight.

Diagnosis of extreme precipitation events in Kenya

Figure 7.5: Selected overpasses of TRMM level 2 product (2A12), AMSU-B and MHS instruments. a)

is the surface precipitation rate from TRMM 2A12 product at 0542 UTC, b) shows BT at the microwave channel of 150GHz from the AMSU-B instrument at 1918 UTC while c) depicts BT at 157GHz from the MHS instrument at 2315 UTC. Annotated numbers show the actual time when the overpass was flying over the Coastal Lowlands.)

Indeed, 15-min BT loops over the Coastal Lowlands (only a few selected images from the loops are listed here) showed shallow clouds beginning to form offshore at 18 UTC 12 May 2007 (figure 7.6a). The shallow clouds intensified as they moved onshore and became a moderate twin system approaching the shore 6 hours later (figure 7.6b). This dual system was already over the region at 0415 UTC (figure 7.6c) and had even shifted a few hundred miles inland at 1515 UTC. During this time, a deep system was already offshore a few kilometers from the mainland while another fully-developed system was trailing it further offshore (figure 7.6d). The CLAAS algorithm confirmed the presence of an already deep system during this hour some miles offshore (stippling in figure 7.6d). The deep system observed further offshore at 1515 UTC rapidly shifted onshore towards the Kenyan coastline and enhanced the intensity of the already moderately deep system near the coastline and together created an even deeper system with cloud top temperatures below 200 K. This was also the period when the region a few miles offshore and over a wide area was characterised by overshooting towers of convection whose cloud top temperatures were in the order of 180 to 190 K (figure 7.6e).

In fact, the same convective organisation was reported as well by the 150 GHz AMSU- B microwave channel overpass at around 1918 UTC (figure 7.5b). This deep8 system is

hypothesized to have resulted to the heavy precipitation episode between 18 UTC 13 May and midnight as has been shown previously by the spatially averaged TRMM and CMORPH precipitation (figure 7.4). The deep system was then observed to retreat offshore towards the Somali coast (figure 7.6f) where overshooting towers of convection as well as the low brightness temperatures were observed to retreat offshore towards the coast of Somalia. Indeed, a 157 GHz microwave channel of the MHS at 2315 UTC showed the leading edge of the system retreating offshore towards the Somali coast while part of the system trailed over the northern part of the Coastal Lowlands contributing to enhanced precipitation as reported by TRMM and CMORPH between 21 UTC and midnight. The system eventually dissipated as it encroached further into the Indian Ocean as observed

8This is inferred from the low BT and CLAAS algorithm on cloud types. The lower the temperatures,

the higher is the cloud top and hence the deeper the system. CLAAS algorithm has been implemented by Stengel et al. (2014).

Figure 7.6: Spatial evolution of convection from SEVIRI BT (filled contours, K) and cloud type (blue stipples, no units) over the Coastal Lowlands for selected hours namely a) 1800 UTC 12 May b) 0000 UTC 13 May, c) 0415 UTC 13 May, d) 1500 UTC 13 May, e) 1800 UTC 13 May and f) 1445 UTC 14

May. BT above 230 K have been suppressed while only the overshooting convectioncloud type from

the CLAAS algorithm has been rendered to emphasize deep convective areas. (Full 15-min IR brightness

temperature loops between 0000 UTC 12 May 2007 to 2345 UTC 14 May 2007 can be accessed at:https:

//drive.google.com/file/d/1DkRmCImAvk2Fz674fQUliFdHJkbKiOiY/view?usp=sharing).

Diagnosis of extreme precipitation events in Kenya

Figure 7.7: a) shows the A-train polar-orbital path (available on cloudSat website). The

spacecraft was overhead the region of study at 1110 UTC May 13 2007 (white box). b)

shows the Cloud subtype derived from the vertical feature mask of CALIPSO Lidar (im-

age url: https://www-calipso.larc.nasa.gov/products/lidar/browse_images/show_date.

php?s=production& v=V3-01& browse_date=2007-05-13) and c) shows Radar reflectivity (dBze) de-