Radio Occultation Bending Angle Anomalies During Tropical Cyclones

The tropical deep convection affects the radiation balance of th The tropical deep convection affects the radiation balance of the e atmosphere changing the water vapor mixing ratio and the tempera atmosphere changing the water vapor mixing ratio and the temperature of ture of the upper troposphere and lower stratosphere (UT/LS). The aim of the upper troposphere and lower stratosphere (UT/LS). The aim of this this work is to better understand these processes and to investigate work is to better understand these processes and to investigate if severe if severe storms leave a significant signature in radio occultation profil storms leave a significant signature in radio occultation profiles in the es in the tropical tropical tropopausetropopause layer.layer. phase and

The tropical deep convection affects the radiation balance of th The tropical deep convection affects the radiation balance of the e atmosphere changing the water vapor mixing ratio and the tempera atmosphere changing the water vapor mixing ratio and the temperature of ture of the upper troposphere and lower stratosphere (UT/LS). The aim of the upper troposphere and lower stratosphere (UT/LS). The aim of this this work is to better understand these processes and to investigate work is to better understand these processes and to investigate if severe if severe storms leave a significant signature in radio occultation profil storms leave a significant signature in radio occultation profiles in the es in the tropical tropical tropopause tropopause layer. layer.
The GPS receiver, measures the phase and amplitude The GPS receiver, measures the phase and amplitude of two L of two L--band signals. The bending angle is band signals. The bending angle is calculated from the phase and amplitude and calculated from the phase and amplitude and refractivity is computed using Abel inversion. refractivity is computed using Abel inversion. Temperature, pressure and water vapour are derived Temperature, pressure and water vapour are derived via a one via a one--dimensional variational (1Dvar) approach dimensional variational (1Dvar) approach involving the refractivity and the ECMWF model. involving the refractivity and the ECMWF model. N=77.6*P/T+3.73*10 N=77.6*P/T+3.73*10 5 5 *p *p w w /T /T 2 2 (N is refractivity, T is the temperature, P is pressure, p (N is refractivity, T is the temperature, P is pressure, p w w is water vapour pressure) is water vapour pressure)  Bending angle anomaly profile shows a clear TC signature in the Bending angle anomaly profile shows a clear TC signature in the UT/LS UT/LS  Bending angle anomaly sensitivity is higher than sensitivity of Bending angle anomaly sensitivity is higher than sensitivity of the other the other parameters parameters  Bending angle could be used Bending angle could be used as an indicator of convective towers as an indicator of convective towers

Objective Objective
Case selection Case selection

Analyses Analyses --Bill 2009 Bill 2009
During During TCs TCs the bending angle anomaly always shows a positive spike the bending angle anomaly always shows a positive spike between 14 and 18 km of altitude (97% of the storms and 100% of between 14 and 18 km of altitude (97% of the storms and 100% of the TC). the TC). Almost the 90% of Almost the 90% of TCs TCs has a positive mean bending angle anomaly has a positive mean bending angle anomaly between 14 km and 18 km, while in no between 14 km and 18 km, while in no--TC conditions the probability to get TC conditions the probability to get positive or negative mean anomaly is evenly distributed positive or negative mean anomaly is evenly distributed. . We selected the tropical cyclone (TC) tracks and they were compa We selected the tropical cyclone (TC) tracks and they were compared with red with GPS/MET, SAC GPS/MET, SAC--C, CHAMP, GRACE and COSMIC Radio C, CHAMP, GRACE and COSMIC Radio Occultations Occultations ( (ROs ROs). 1194 coincidences were found between ). 1194 coincidences were found between ROs ROs and and TCs TCs considering a considering a time window of 3h and a space window of 300km from the eye of th time window of 3h and a space window of 300km from the eye of the TC, e TC, 77% of coincidences are coming from COSMIC project. Then we sele 77% of coincidences are coming from COSMIC project. Then we selected cted all the cases with available co all the cases with available co--located located radiosondes radiosondes. .

GPS RO GPS RO
From GOES, we used the brightness temperatures From GOES, we used the brightness temperatures at 6.8 microns (water vapour channel) and the 10.7 at 6.8 microns (water vapour channel) and the 10.7 microns. In clear sky conditions (and low level microns. In clear sky conditions (and low level clouds) the brightness temperature in the water clouds) the brightness temperature in the water vapour channel is smaller than the brightness vapour channel is smaller than the brightness temperature at 10.7 microns. When the overshooting temperature at 10.7 microns. When the overshooting is present, there is an inversion and the brightness is present, there is an inversion and the brightness temperature and 6.8 microns becomes larger by a temperature and 6.8 microns becomes larger by a few degrees. few degrees.

From the From the radiosondes radiosondes ( (RAOBs RAOBs) we used the ) we used the temperatures and water vapour profiles. temperatures and water vapour profiles. A double
A double-peak (two local minima) in temperature, one at the peak (two local minima) in temperature, one at the tropopause tropopause, and one a couple of km higher , and one a couple of km higher corresponding to an increase of water corresponding to an increase of water vapour vapour, , suggests the presence of overshoot ( suggests the presence of overshoot (Danielsen Danielsen, , 1982

This radio occultation was acquired on 22 This radio occultation was acquired on 22 nd nd of of August 2009 at 8.27 UTC, in the Atlantic Ocean August 2009 at 8.27 UTC, in the Atlantic Ocean when the hurricane Bill was weakening from when the hurricane Bill was weakening from category 3 to category 2 category 3 to category 2. . The distance between
The distance between the RO and the RAOB was 98.32 km. The RAOB the RO and the RAOB was 98.32 km. The RAOB was acquired at 11 UTC. was acquired at 11 UTC. This figure shows the difference of brightness This figure shows the difference of brightness temperature at 6.8 microns and the brightness temperature at 6.8 microns and the brightness temperature at 10.7 microns on the RO area from GOES temperature at 10.7 microns on the RO area from GOES 11 (the RO is the red line) 11 (the RO is the red line). A . An inversion from 3 hours n inversion from 3 hours before the RO until 1 hour later on the same area is before the RO until 1 hour later on the same area is present, consequently some overshoot should be present, consequently some overshoot should be expected. expected.

The
The radiosonde radiosonde shows an shows an increase of water vapour increase of water vapour mixing ratio just below 17 km mixing ratio just below 17 km of altitude and the of altitude and the temperature profiles with two temperature profiles with two minima, fully corroborating minima, fully corroborating the hypothesis of the hypothesis of Danielsen Danielsen. . On the other hand we can not see the same On the other hand we can not see the same variations in the RO profiles. The bending angle variations in the RO profiles. The bending angle anomaly, as usual during anomaly, as usual during TCs TCs, has a local , has a local maximum above the maximum above the tropopause tropopause and the amplitude and the amplitude of the anomaly is 10% larger than the climatology of the anomaly is 10% larger than the climatology in the same area. From the bending angle anomaly in the same area. From the bending angle anomaly we can clearly recognize the variation in we can clearly recognize the variation in temperature but the temperature but the double double--peak peak from the RO from the RO temperature anomaly profile is less evident. temperature anomaly profile is less evident. The distance between the RO and the RAOB was 193 km. The RAOB was launched at 6 UTC. the RO and the RAOB was 193 km. The RAOB was launched at 6 UTC. GOES data are not available. GOES data are not available.

This radio occultation was acquired on 29
Also in this case, the Also in this case, the radiosonde radiosonde shows an shows an increase of water vapour increase of water vapour mixing ratio just above the mixing ratio just above the tropopause tropopause and the and the temperature profiles with two temperature profiles with two minima, but the RO profiles minima, but the RO profiles do not confirm it. do not confirm it. The bending angle anomaly, however, shows the The bending angle anomaly, however, shows the double peak which is not evident in the RO double peak which is not evident in the RO temperature anomaly profile. The bending angle temperature anomaly profile. The bending angle variation is probably due to the temperature variation is probably due to the temperature variation, but the sensitivity of the bending angle variation, but the sensitivity of the bending angle is higher and it can recognize also small is higher and it can recognize also small variations, otherwise not detectable. variations, otherwise not detectable.